# Double 結構

## 定義

``public value class Double : IComparable, IComparable<double>, IConvertible, IEquatable<double>, IFormattable``
``````[System.Runtime.InteropServices.ComVisible(true)]
[System.Serializable]
public struct Double : IComparable, IComparable<double>, IConvertible, IEquatable<double>, IFormattable``````
``````type double = struct
interface IFormattable
interface IConvertible``````
``````Public Structure Double
Implements IComparable, IComparable(Of Double), IConvertible, IEquatable(Of Double), IFormattable``````

Double

## 範例

``````// The Temperature class stores the temperature as a Double
// and delegates most of the functionality to the Double
// implementation.
public ref class Temperature: public IComparable, public IFormattable
{
// IComparable.CompareTo implementation.
public:
virtual int CompareTo( Object^ obj )
{
if (obj == nullptr) return 1;

if (dynamic_cast<Temperature^>(obj) )
{
Temperature^ temp = (Temperature^)(obj);
return m_value.CompareTo( temp->m_value );
}
throw gcnew ArgumentException( "object is not a Temperature" );
}

// IFormattable.ToString implementation.
virtual String^ ToString( String^ format, IFormatProvider^ provider )
{
if ( format != nullptr )
{
if ( format->Equals( "F" ) )
{
return String::Format( "{0}'F", this->Value.ToString() );
}

if ( format->Equals( "C" ) )
{
return String::Format( "{0}'C", this->Celsius.ToString() );
}
}
return m_value.ToString( format, provider );
}

// Parses the temperature from a string in the form
// [ws][sign]digits['F|'C][ws]
static Temperature^ Parse( String^ s, NumberStyles styles, IFormatProvider^ provider )
{
Temperature^ temp = gcnew Temperature;

if ( s->TrimEnd(nullptr)->EndsWith( "'F" ) )
{
temp->Value = Double::Parse( s->Remove( s->LastIndexOf( '\'' ), 2 ), styles, provider );
}
else
if ( s->TrimEnd(nullptr)->EndsWith( "'C" ) )
{
temp->Celsius = Double::Parse( s->Remove( s->LastIndexOf( '\'' ), 2 ), styles, provider );
}
else
{
temp->Value = Double::Parse( s, styles, provider );
}
return temp;
}

protected:
double m_value;

public:
property double Value
{
double get()
{
return m_value;
}

void set( double value )
{
m_value = value;
}
}

property double Celsius
{
double get()
{
return (m_value - 32.0) / 1.8;
}

void set( double value )
{
m_value = 1.8 * value + 32.0;
}
}
};
``````
``````// The Temperature class stores the temperature as a Double
// and delegates most of the functionality to the Double
// implementation.
public class Temperature : IComparable, IFormattable
{
// IComparable.CompareTo implementation.
public int CompareTo(object obj) {
if (obj == null) return 1;

Temperature temp = obj as Temperature;
if (obj != null)
return m_value.CompareTo(temp.m_value);
else
throw new ArgumentException("object is not a Temperature");
}

// IFormattable.ToString implementation.
public string ToString(string format, IFormatProvider provider) {
if( format != null ) {
if( format.Equals("F") ) {
return String.Format("{0}'F", this.Value.ToString());
}
if( format.Equals("C") ) {
return String.Format("{0}'C", this.Celsius.ToString());
}
}

return m_value.ToString(format, provider);
}

// Parses the temperature from a string in the form
// [ws][sign]digits['F|'C][ws]
public static Temperature Parse(string s, NumberStyles styles, IFormatProvider provider) {
Temperature temp = new Temperature();

if( s.TrimEnd(null).EndsWith("'F") ) {
temp.Value = Double.Parse( s.Remove(s.LastIndexOf('\''), 2), styles, provider);
}
else if( s.TrimEnd(null).EndsWith("'C") ) {
temp.Celsius = Double.Parse( s.Remove(s.LastIndexOf('\''), 2), styles, provider);
}
else {
temp.Value = Double.Parse(s, styles, provider);
}

return temp;
}

// The value holder
protected double m_value;

public double Value {
get {
return m_value;
}
set {
m_value = value;
}
}

public double Celsius {
get {
return (m_value-32.0)/1.8;
}
set {
m_value = 1.8*value+32.0;
}
}
}
``````
``````' Temperature class stores the value as Double
' and delegates most of the functionality
' to the Double implementation.
Public Class Temperature
Implements IComparable, IFormattable

Public Overloads Function CompareTo(ByVal obj As Object) As Integer _
Implements IComparable.CompareTo

If TypeOf obj Is Temperature Then
Dim temp As Temperature = CType(obj, Temperature)

Return m_value.CompareTo(temp.m_value)
End If

Throw New ArgumentException("object is not a Temperature")
End Function

Public Overloads Function ToString(ByVal format As String, ByVal provider As IFormatProvider) As String _
Implements IFormattable.ToString

If Not (format Is Nothing) Then
If format.Equals("F") Then
Return [String].Format("{0}'F", Me.Value.ToString())
End If
If format.Equals("C") Then
Return [String].Format("{0}'C", Me.Celsius.ToString())
End If
End If

Return m_value.ToString(format, provider)
End Function

' Parses the temperature from a string in form
' [ws][sign]digits['F|'C][ws]
Public Shared Function Parse(ByVal s As String, ByVal styles As NumberStyles, ByVal provider As IFormatProvider) As Temperature
Dim temp As New Temperature()

If s.TrimEnd(Nothing).EndsWith("'F") Then
temp.Value = Double.Parse(s.Remove(s.LastIndexOf("'"c), 2), styles, provider)
Else
If s.TrimEnd(Nothing).EndsWith("'C") Then
temp.Celsius = Double.Parse(s.Remove(s.LastIndexOf("'"c), 2), styles, provider)
Else
temp.Value = Double.Parse(s, styles, provider)
End If
End If
Return temp
End Function

' The value holder
Protected m_value As Double

Public Property Value() As Double
Get
Return m_value
End Get
Set(ByVal Value As Double)
m_value = Value
End Set
End Property

Public Property Celsius() As Double
Get
Return (m_value - 32) / 1.8
End Get
Set(ByVal Value As Double)
m_value = Value * 1.8 + 32
End Set
End Property
End Class
``````

## 備註

Double 數值型別代表雙精確度64位數值，其值的範圍從負數 1.79769313486232 e 308 到正的 1.79769313486232 e 308，以及正數或負零、PositiveInfinityNegativeInfinity，而不是數位（NaN）。The Double value type represents a double-precision 64-bit number with values ranging from negative 1.79769313486232e308 to positive 1.79769313486232e308, as well as positive or negative zero, PositiveInfinity, NegativeInfinity, and not a number (NaN). 它的目的是要代表非常大的值（例如，行星或 galaxies 之間的距離），或非常小的（物質的分子大量，以千克計算），而且通常不精確（例如從地球到另一個日光系統的距離），Double 類型符合適用于二進位浮點算術的 IEC 60559:1989 （IEEE 754）標準。It is intended to represent values that are extremely large (such as distances between planets or galaxies) or extremely small (the molecular mass of a substance in kilograms) and that often are imprecise (such as the distance from earth to another solar system), The Double type complies with the IEC 60559:1989 (IEEE 754) standard for binary floating-point arithmetic.

### 浮點表示和有效位數Floating-Point Representation and Precision

Double 資料類型會以64位的二進位格式儲存雙精確度浮點值，如下表所示：The Double data type stores double-precision floating-point values in a 64-bit binary format, as shown in the following table:

ExponentExponent 52-6252-62
Sign （0 = 正數，1 = 負數）Sign (0 = Positive, 1 = Negative) 6363

``````using System;

public class Example
{
public static void Main()
{
Double value = .1;
Double result1 = value * 10;
Double result2 = 0;
for (int ctr = 1; ctr <= 10; ctr++)
result2 += value;

Console.WriteLine(".1 * 10:           {0:R}", result1);
Console.WriteLine(".1 Added 10 times: {0:R}", result2);
}
}
// The example displays the following output:
//       .1 * 10:           1
//       .1 Added 10 times: 0.99999999999999989
``````
``````Module Example
Public Sub Main()
Dim value As Double = .1
Dim result1 As Double = value * 10
Dim result2 As Double
For ctr As Integer = 1 To 10
result2 += value
Next
Console.WriteLine(".1 * 10:           {0:R}", result1)
Console.WriteLine(".1 Added 10 times: {0:R}", result2)
End Sub
End Module
' The example displays the following output:
'       .1 * 10:           1
'       .1 Added 10 times: 0.99999999999999989
``````

``````using System;

public class Example
{
public static void Main()
{
Double value = 123456789012.34567;
Double additional = Double.Epsilon * 1e15;
Console.WriteLine("{0} + {1} = {2}", value, additional,
}
}
// The example displays the following output:
//    123456789012.346 + 4.94065645841247E-309 = 123456789012.346
``````
``````Module Example
Public Sub Main()
Dim value As Double = 123456789012.34567
Dim additional As Double = Double.Epsilon * 1e15
Console.WriteLine("{0} + {1} = {2}", value, additional,
End Sub
End Module
' The example displays the following output:
'   123456789012.346 + 4.94065645841247E-309 = 123456789012.346
``````

• 針對特定有效位數顯示為相等的兩個浮點數，可能不會比較相等，因為它們的最小值不相同。Two floating-point numbers that appear equal for a particular precision might not compare equal because their least significant digits are different. 在下列範例中，會將一連串的數位相加，並將其總計與預期的總計進行比較。In the following example, a series of numbers are added together, and their total is compared with their expected total. 雖然兩個值看起來是相同的，但呼叫 `Equals` 方法會指出它們不是。Although the two values appear to be the same, a call to the `Equals` method indicates that they are not.

``````using System;

public class Example
{
public static void Main()
{
Double[] values = { 10.0, 2.88, 2.88, 2.88, 9.0 };
Double result = 27.64;
Double total = 0;
foreach (var value in values)
total += value;

if (total.Equals(result))
Console.WriteLine("The sum of the values equals the total.");
else
Console.WriteLine("The sum of the values ({0}) does not equal the total ({1}).",
total, result);
}
}
// The example displays the following output:
//      The sum of the values (36.64) does not equal the total (36.64).
//
// If the index items in the Console.WriteLine statement are changed to {0:R},
// the example displays the following output:
//       The sum of the values (27.639999999999997) does not equal the total (27.64).
``````
``````Module Example
Public Sub Main()
Dim values() As Double = { 10.0, 2.88, 2.88, 2.88, 9.0 }
Dim result As Double = 27.64
Dim total As Double
For Each value In values
total += value
Next
If total.Equals(result) Then
Console.WriteLine("The sum of the values equals the total.")
Else
Console.WriteLine("The sum of the values ({0}) does not equal the total ({1}).",
total, result)
End If
End Sub
End Module
' The example displays the following output:
'      The sum of the values (36.64) does not equal the total (36.64).
'
' If the index items in the Console.WriteLine statement are changed to {0:R},
' the example displays the following output:
'       The sum of the values (27.639999999999997) does not equal the total (27.64).
``````

如果您將 Console.WriteLine(String, Object, Object) 語句中的格式專案從 `{0}``{1}` 變更為 `{0:R}`，並 `{1:R}` 顯示兩個 Double 值的所有有效位數，則表示兩個值不相等，因為精確度遺失在新增作業期間。If you change the format items in the Console.WriteLine(String, Object, Object) statement from `{0}` and `{1}` to `{0:R}` and `{1:R}` to display all significant digits of the two Double values, it is clear that the two values are unequal because of a loss of precision during the addition operations. 在此情況下，您可以藉由呼叫 Math.Round(Double, Int32) 方法，將 Double 值舍入為所需的精確度，然後再執行比較，來解決此問題。In this case, the issue can be resolved by calling the Math.Round(Double, Int32) method to round the Double values to the desired precision before performing the comparison.

• 如果使用十進位數，使用浮點數的數學或比較運算可能不會產生相同的結果，因為二進位浮點數可能不等於十進位數。A mathematical or comparison operation that uses a floating-point number might not yield the same result if a decimal number is used, because the binary floating-point number might not equal the decimal number. 先前的範例會顯示將 .1 乘以10並加上1次的結果，以說明這一點。A previous example illustrated this by displaying the result of multiplying .1 by 10 and adding .1 times.

當數值運算中具有小數值的精確度很重要時，您可以使用 Decimal，而不是 Double 類型。When accuracy in numeric operations with fractional values is important, you can use the Decimal rather than the Double type. 如果數值運算中的整數值在 Int64UInt64 類型的範圍外，其精確度很重要，請使用 BigInteger 類型。When accuracy in numeric operations with integral values beyond the range of the Int64 or UInt64 types is important, use the BigInteger type.

• 如果涉及浮點數，值可能不會往返。A value might not round-trip if a floating-point number is involved. 如果作業將原始浮點數轉換成另一種形式，則會將值稱為來回行程，反向運算會將轉換後的表單轉換回浮點數，而最後的浮點數則不等於原始的浮點數字。A value is said to round-trip if an operation converts an original floating-point number to another form, an inverse operation transforms the converted form back to a floating-point number, and the final floating-point number is not equal to the original floating-point number. 來回行程可能會失敗，因為轉換中遺失或變更了一或多個最不重要的數位。The round trip might fail because one or more least significant digits are lost or changed in a conversion. 在下列範例中，會將三個 Double 值轉換成字串，並儲存在檔案中。In the following example, three Double values are converted to strings and saved in a file. 不過，如輸出所示，雖然值似乎相同，但還原的值不等於原始值。As the output shows, however, even though the values appear to be identical, the restored values are not equal to the original values.

``````using System;
using System.IO;

public class Example
{
public static void Main()
{
StreamWriter sw = new StreamWriter(@".\Doubles.dat");
Double[] values = { 2.2/1.01, 1.0/3, Math.PI };
for (int ctr = 0; ctr < values.Length; ctr++) {
sw.Write(values[ctr].ToString());
if (ctr != values.Length - 1)
sw.Write("|");
}
sw.Close();

Double[] restoredValues = new Double[values.Length];
string temp = sr.ReadToEnd();
string[] tempStrings = temp.Split('|');
for (int ctr = 0; ctr < tempStrings.Length; ctr++)
restoredValues[ctr] = Double.Parse(tempStrings[ctr]);

for (int ctr = 0; ctr < values.Length; ctr++)
Console.WriteLine("{0} {2} {1}", values[ctr],
restoredValues[ctr],
values[ctr].Equals(restoredValues[ctr]) ? "=" : "<>");
}
}
// The example displays the following output:
//       2.17821782178218 <> 2.17821782178218
//       0.333333333333333 <> 0.333333333333333
//       3.14159265358979 <> 3.14159265358979
``````
``````Imports System.IO

Module Example
Public Sub Main()
Dim sw As New StreamWriter(".\Doubles.dat")
Dim values() As Double = { 2.2/1.01, 1.0/3, Math.PI }
For ctr As Integer = 0 To values.Length - 1
sw.Write(values(ctr).ToString())
If ctr <> values.Length - 1 Then sw.Write("|")
Next
sw.Close()

Dim restoredValues(values.Length - 1) As Double
Dim sr As New StreamReader(".\Doubles.dat")
Dim temp As String = sr.ReadToEnd()
Dim tempStrings() As String = temp.Split("|"c)
For ctr As Integer = 0 To tempStrings.Length - 1
restoredValues(ctr) = Double.Parse(tempStrings(ctr))
Next

For ctr As Integer = 0 To values.Length - 1
Console.WriteLine("{0} {2} {1}", values(ctr),
restoredValues(ctr),
If(values(ctr).Equals(restoredValues(ctr)), "=", "<>"))
Next
End Sub
End Module
' The example displays the following output:
'       2.17821782178218 <> 2.17821782178218
'       0.333333333333333 <> 0.333333333333333
'       3.14159265358979 <> 3.14159265358979
``````

在此情況下，您可以使用 "G17"標準數值格式字串來保存完整的 Double 值精確度，以成功地進行迴圈，如下列範例所示。In this case, the values can be successfully round-tripped by using the "G17" standard numeric format string to preserve the full precision of Double values, as the following example shows.

``````using System;
using System.IO;

public class Example
{
public static void Main()
{
StreamWriter sw = new StreamWriter(@".\Doubles.dat");
Double[] values = { 2.2/1.01, 1.0/3, Math.PI };
for (int ctr = 0; ctr < values.Length; ctr++)
sw.Write("{0:G17}{1}", values[ctr], ctr < values.Length - 1 ? "|" : "" );

sw.Close();

Double[] restoredValues = new Double[values.Length];
string temp = sr.ReadToEnd();
string[] tempStrings = temp.Split('|');
for (int ctr = 0; ctr < tempStrings.Length; ctr++)
restoredValues[ctr] = Double.Parse(tempStrings[ctr]);

for (int ctr = 0; ctr < values.Length; ctr++)
Console.WriteLine("{0} {2} {1}", values[ctr],
restoredValues[ctr],
values[ctr].Equals(restoredValues[ctr]) ? "=" : "<>");
}
}
// The example displays the following output:
//       2.17821782178218 = 2.17821782178218
//       0.333333333333333 = 0.333333333333333
//       3.14159265358979 = 3.14159265358979
``````
``````Imports System.IO

Module Example
Public Sub Main()
Dim sw As New StreamWriter(".\Doubles.dat")
Dim values() As Double = { 2.2/1.01, 1.0/3, Math.PI }
For ctr As Integer = 0 To values.Length - 1
sw.Write("{0:G17}{1}", values(ctr),
If(ctr < values.Length - 1, "|", ""))
Next
sw.Close()

Dim restoredValues(values.Length - 1) As Double
Dim sr As New StreamReader(".\Doubles.dat")
Dim temp As String = sr.ReadToEnd()
Dim tempStrings() As String = temp.Split("|"c)
For ctr As Integer = 0 To tempStrings.Length - 1
restoredValues(ctr) = Double.Parse(tempStrings(ctr))
Next

For ctr As Integer = 0 To values.Length - 1
Console.WriteLine("{0} {2} {1}", values(ctr),
restoredValues(ctr),
If(values(ctr).Equals(restoredValues(ctr)), "=", "<>"))
Next
End Sub
End Module
' The example displays the following output:
'       2.17821782178218 = 2.17821782178218
'       0.333333333333333 = 0.333333333333333
'       3.14159265358979 = 3.14159265358979
``````

Double 值搭配使用時，在某些情況下，"R" 格式規範無法成功地反復存取原始值。When used with a Double value, the "R" format specifier in some cases fails to successfully round-trip the original value. 若要確保 Double 值成功地往返，請使用 "G17" 格式規範。To ensure that Double values successfully round-trip, use the "G17" format specifier.

• Single 值的精確度比 Double 的值少。Single values have less precision than Double values. 轉換成看似相等 DoubleSingle 值通常不等於 Double 值，因為有效位數的差異。A Single value that is converted to a seemingly equivalent Double often does not equal the Double value because of differences in precision. 在下列範例中，相同除法運算的結果會指派給 DoubleSingle 值。In the following example, the result of identical division operations is assigned to a Double and a Single value. Single 值轉換成 Double之後，這兩個值的比較會顯示兩者不相等。After the Single value is cast to a Double, a comparison of the two values shows that they are unequal.

``````using System;

public class Example
{
public static void Main()
{
Double value1 = 1/3.0;
Single sValue2 = 1/3.0f;
Double value2 = (Double) sValue2;
Console.WriteLine("{0:R} = {1:R}: {2}", value1, value2,
value1.Equals(value2));
}
}
// The example displays the following output:
//        0.33333333333333331 = 0.3333333432674408: False
``````
``````Module Example
Public Sub Main()
Dim value1 As Double = 1/3
Dim sValue2 As Single = 1/3
Dim value2 As Double = CDbl(sValue2)
Console.WriteLine("{0} = {1}: {2}", value1, value2, value1.Equals(value2))
End Sub
End Module
' The example displays the following output:
'       0.33333333333333331 = 0.3333333432674408: False
``````

若要避免這個問題，請使用 Double 來取代 Single 資料類型，或使用 Round 方法，讓這兩個值具有相同的有效位數。To avoid this problem, use either the Double in place of the Single data type, or use the Round method so that both values have the same precision.

``````double value = -4.42330604244772E-305;

double fromLiteral = -4.42330604244772E-305;
double fromVariable = value;
double fromParse = Double.Parse("-4.42330604244772E-305");

Console.WriteLine("Double value from literal: {0,29:R}", fromLiteral);
Console.WriteLine("Double value from variable: {0,28:R}", fromVariable);
Console.WriteLine("Double value from Parse method: {0,24:R}", fromParse);
// On 32-bit versions of the .NET Framework, the output is:
//    Double value from literal:        -4.42330604244772E-305
//    Double value from variable:       -4.42330604244772E-305
//    Double value from Parse method:   -4.42330604244772E-305
//
// On other versions of the .NET Framework, the output is:
//    Double value from literal:      -4.4233060424477198E-305
//    Double value from variable:     -4.4233060424477198E-305
//    Double value from Parse method:   -4.42330604244772E-305
``````
``````Dim value As Double = -4.42330604244772E-305

Dim fromLiteral As Double = -4.42330604244772E-305
Dim fromVariable As Double = value
Dim fromParse As Double = Double.Parse("-4.42330604244772E-305")

Console.WriteLine("Double value from literal: {0,29:R}", fromLiteral)
Console.WriteLine("Double value from variable: {0,28:R}", fromVariable)
Console.WriteLine("Double value from Parse method: {0,24:R}", fromParse)
' On 32-bit versions of the .NET Framework, the output is:
'    Double value from literal:        -4.42330604244772E-305
'    Double value from variable:       -4.42330604244772E-305
'    Double value from Parse method:   -4.42330604244772E-305
'
' On other versions of the .NET Framework, the output is:
'    Double value from literal:        -4.4233060424477198E-305
'    Double value from variable:       -4.4233060424477198E-305
'    Double value from Parse method:     -4.42330604244772E-305
``````

### 測試是否相等Testing for Equality

``````using System;

public class Example
{
public static void Main()
{
double value1 = .333333333333333;
double value2 = 1.0/3;
Console.WriteLine("{0:R} = {1:R}: {2}", value1, value2, value1.Equals(value2));
}
}
// The example displays the following output:
//        0.333333333333333 = 0.33333333333333331: False
``````
``````Module Example
Public Sub Main()
Dim value1 As Double = .333333333333333
Dim value2 As Double = 1/3
Console.WriteLine("{0:R} = {1:R}: {2}", value1, value2, value1.Equals(value2))
End Sub
End Module
' The example displays the following output:
'       0.333333333333333 = 0.33333333333333331: False
``````

``````using System;

public class Example
{
public static void Main()
{
double value1 = 100.10142;
value1 = Math.Sqrt(Math.Pow(value1, 2));
double value2 = Math.Pow(value1 * 3.51, 2);
value2 = Math.Sqrt(value2) / 3.51;
Console.WriteLine("{0} = {1}: {2}\n",
value1, value2, value1.Equals(value2));
Console.WriteLine("{0:R} = {1:R}", value1, value2);
}
}
// The example displays the following output:
//    100.10142 = 100.10142: False
//
//    100.10142 = 100.10141999999999
``````
``````Module Example
Public Sub Main()
Dim value1 As Double = 100.10142
value1 = Math.Sqrt(Math.Pow(value1, 2))
Dim value2 As Double = Math.Pow(value1 * 3.51, 2)
value2 = Math.Sqrt(value2) / 3.51
Console.WriteLine("{0} = {1}: {2}",
value1, value2, value1.Equals(value2))
Console.WriteLine()
Console.WriteLine("{0:R} = {1:R}", value1, value2)
End Sub
End Module
' The example displays the following output:
'    100.10142 = 100.10142: False
'
'    100.10142 = 100.10141999999999
``````

• 呼叫 Math.Round 方法，以確保這兩個值具有相同的有效位數。Call the Math.Round method to ensure that both values have the same precision. 下列範例會修改前一個範例，以使用此方法，讓兩個小數值相等。The following example modifies a previous example to use this approach so that two fractional values are equivalent.

``````using System;

public class Example
{
public static void Main()
{
double value1 = .333333333333333;
double value2 = 1.0/3;
int precision = 7;
value1 = Math.Round(value1, precision);
value2 = Math.Round(value2, precision);
Console.WriteLine("{0:R} = {1:R}: {2}", value1, value2, value1.Equals(value2));
}
}
// The example displays the following output:
//        0.3333333 = 0.3333333: True
``````
``````Module Example
Public Sub Main()
Dim value1 As Double = .333333333333333
Dim value2 As Double = 1/3
Dim precision As Integer = 7
value1 = Math.Round(value1, precision)
value2 = Math.Round(value2, precision)
Console.WriteLine("{0:R} = {1:R}: {2}", value1, value2, value1.Equals(value2))
End Sub
End Module
' The example displays the following output:
'       0.3333333 = 0.3333333: True
``````

不過要注意的是，精確度的問題仍然適用于中間值的圓角。Note, though, that the problem of precision still applies to rounding of midpoint values. 如需詳細資訊，請參閱 Math.Round(Double, Int32, MidpointRounding) 方法。For more information, see the Math.Round(Double, Int32, MidpointRounding) method.

• 測試近似相等，而不是相等。Test for approximate equality rather than equality. 這會要求您定義一個絕對值，讓這兩個值可以不同但仍然相等，或者您定義較小值可以與較大值分離的相對量。This requires that you define either an absolute amount by which the two values can differ but still be equal, or that you define a relative amount by which the smaller value can diverge from the larger value.

警告

在測試是否相等時，Double.Epsilon 有時會當做兩個 Double 值之間距離的絕對量值。Double.Epsilon is sometimes used as an absolute measure of the distance between two Double values when testing for equality. 不過，Double.Epsilon 會測量其值為零的 Double 可新增或減去的最小可能值。However, Double.Epsilon measures the smallest possible value that can be added to, or subtracted from, a Double whose value is zero. 對於大部分的正和負 Double 值而言，Double.Epsilon 的值太小，無法偵測出來。For most positive and negative Double values, the value of Double.Epsilon is too small to be detected. 因此，除了零以外的值，我們不建議在測試中使用它來進行相等。Therefore, except for values that are zero, we do not recommend its use in tests for equality.

下列範例會使用第二個方法來定義 `IsApproximatelyEqual` 方法，以測試兩個值之間的相對差異。The following example uses the latter approach to define an `IsApproximatelyEqual` method that tests the relative difference between two values. 它也會對比呼叫 `IsApproximatelyEqual` 方法和 Equals(Double) 方法的結果。It also contrasts the result of calls to the `IsApproximatelyEqual` method and the Equals(Double) method.

``````using System;

public class Example
{
public static void Main()
{
double one1 = .1 * 10;
double one2 = 0;
for (int ctr = 1; ctr <= 10; ctr++)
one2 += .1;

Console.WriteLine("{0:R} = {1:R}: {2}", one1, one2, one1.Equals(one2));
Console.WriteLine("{0:R} is approximately equal to {1:R}: {2}",
one1, one2,
IsApproximatelyEqual(one1, one2, .000000001));
}

static bool IsApproximatelyEqual(double value1, double value2, double epsilon)
{
// If they are equal anyway, just return True.
if (value1.Equals(value2))
return true;

// Handle NaN, Infinity.
if (Double.IsInfinity(value1) | Double.IsNaN(value1))
return value1.Equals(value2);
else if (Double.IsInfinity(value2) | Double.IsNaN(value2))
return value1.Equals(value2);

// Handle zero to avoid division by zero
double divisor = Math.Max(value1, value2);
if (divisor.Equals(0))
divisor = Math.Min(value1, value2);

return Math.Abs((value1 - value2) / divisor) <= epsilon;
}
}
// The example displays the following output:
//       1 = 0.99999999999999989: False
//       1 is approximately equal to 0.99999999999999989: True
``````
``````Module Example
Public Sub Main()
Dim one1 As Double = .1 * 10
Dim one2 As Double = 0
For ctr As Integer = 1 To 10
one2 += .1
Next
Console.WriteLine("{0:R} = {1:R}: {2}", one1, one2, one1.Equals(one2))
Console.WriteLine("{0:R} is approximately equal to {1:R}: {2}",
one1, one2,
IsApproximatelyEqual(one1, one2, .000000001))
End Sub

Function IsApproximatelyEqual(value1 As Double, value2 As Double,
epsilon As Double) As Boolean
' If they are equal anyway, just return True.
If value1.Equals(value2) Then Return True

' Handle NaN, Infinity.
If Double.IsInfinity(value1) Or Double.IsNaN(value1) Then
Return value1.Equals(value2)
Else If Double.IsInfinity(value2) Or Double.IsNaN(value2)
Return value1.Equals(value2)
End If

' Handle zero to avoid division by zero
Dim divisor As Double = Math.Max(value1, value2)
If divisor.Equals(0) Then
divisor = Math.Min(value1, value2)
End If

Return Math.Abs((value1 - value2) / divisor) <= epsilon
End Function
End Module
' The example displays the following output:
'       1 = 0.99999999999999989: False
'       1 is approximately equal to 0.99999999999999989: True
``````

### 浮點值和例外狀況Floating-Point Values and Exceptions

• 如果浮點運算的結果太小而無法用於目的地格式，則結果會是零。If the result of a floating-point operation is too small for the destination format, the result is zero. 當兩個非常小的數位相乘時，就會發生這種情況，如下列範例所示。This can occur when two very small numbers are multiplied, as the following example shows.

``````using System;

public class Example
{
public static void Main()
{
Double value1 = 1.1632875981534209e-225;
Double value2 = 9.1642346778e-175;
Double result = value1 * value2;
Console.WriteLine("{0} * {1} = {2}", value1, value2, result);
Console.WriteLine("{0} = 0: {1}", result, result.Equals(0.0));
}
}
// The example displays the following output:
//       1.16328759815342E-225 * 9.1642346778E-175 = 0
//       0 = 0: True
``````
``````Module Example
Public Sub Main()
Dim value1 As Double = 1.1632875981534209e-225
Dim value2 As Double = 9.1642346778e-175
Dim result As Double = value1 * value2
Console.WriteLine("{0} * {1} = {2}", value1, value2, result)
Console.WriteLine("{0} = 0: {1}", result, result.Equals(0.0))
End Sub
End Module
' The example displays the following output:
'       1.16328759815342E-225 * 9.1642346778E-175 = 0
'       0 = 0: True
``````
• 如果浮點運算的結果量超過目的地格式的範圍，則作業的結果會 PositiveInfinityNegativeInfinity（適用于結果的正負號）。If the magnitude of the result of a floating-point operation exceeds the range of the destination format, the result of the operation is PositiveInfinity or NegativeInfinity, as appropriate for the sign of the result. PositiveInfinity溢位 Double.MaxValue 的運算結果，而且會 NegativeInfinity溢位 Double.MinValue 的運算結果，如下列範例所示。The result of an operation that overflows Double.MaxValue is PositiveInfinity, and the result of an operation that overflows Double.MinValue is NegativeInfinity, as the following example shows.

``````using System;

public class Example
{
public static void Main()
{
Double value1 = 4.565e153;
Double value2 = 6.9375e172;
Double result = value1 * value2;
Console.WriteLine("PositiveInfinity: {0}",
Double.IsPositiveInfinity(result));
Console.WriteLine("NegativeInfinity: {0}\n",
Double.IsNegativeInfinity(result));

value1 = -value1;
result = value1 * value2;
Console.WriteLine("PositiveInfinity: {0}",
Double.IsPositiveInfinity(result));
Console.WriteLine("NegativeInfinity: {0}",
Double.IsNegativeInfinity(result));
}
}

// The example displays the following output:
//       PositiveInfinity: True
//       NegativeInfinity: False
//
//       PositiveInfinity: False
//       NegativeInfinity: True
``````
``````Module Example
Public Sub Main()
Dim value1 As Double = 4.565e153
Dim value2 As Double = 6.9375e172
Dim result As Double = value1 * value2
Console.WriteLine("PositiveInfinity: {0}",
Double.IsPositiveInfinity(result))
Console.WriteLine("NegativeInfinity: {0}",
Double.IsNegativeInfinity(result))
Console.WriteLine()
value1 = -value1
result = value1 * value2
Console.WriteLine("PositiveInfinity: {0}",
Double.IsPositiveInfinity(result))
Console.WriteLine("NegativeInfinity: {0}",
Double.IsNegativeInfinity(result))
End Sub
End Module
' The example displays the following output:
'       PositiveInfinity: True
'       NegativeInfinity: False
'
'       PositiveInfinity: False
'       NegativeInfinity: True
``````

PositiveInfinity 也會由零除和正被除數所造成的結果，並 NegativeInfinity 除數為零的結果，並被除數為負。PositiveInfinity also results from a division by zero with a positive dividend, and NegativeInfinity results from a division by zero with a negative dividend.

• 如果浮點運算無效，則會 NaN作業的結果。If a floating-point operation is invalid, the result of the operation is NaN. 例如，NaN 來自下列作業的結果：For example, NaN results from the following operations:

• 任何具有無效輸入的浮點運算。Any floating-point operation with an invalid input. 例如，以負值呼叫 Math.Sqrt 方法會傳回 NaN，如同呼叫 Math.Acos 方法，其值大於1或小於負一。For example, calling the Math.Sqrt method with a negative value returns NaN, as does calling the Math.Acos method with a value that is greater than one or less than negative one.

• 具有 Double.NaN值之引數的任何運算。Any operation with an argument whose value is Double.NaN.

### 類型轉換和雙重結構Type conversions and the Double structure

Double 結構不會定義任何明確或隱含的轉換運算子;相反地，轉換是由編譯器來執行。The Double structure does not define any explicit or implicit conversion operators; instead, conversions are implemented by the compiler.

``````using System;

public class Example
{
public static void Main()
{
dynamic[] values = { Byte.MinValue, Byte.MaxValue, Decimal.MinValue,
Decimal.MaxValue, Int16.MinValue, Int16.MaxValue,
Int32.MinValue, Int32.MaxValue, Int64.MinValue,
Int64.MaxValue, SByte.MinValue, SByte.MaxValue,
Single.MinValue, Single.MaxValue, UInt16.MinValue,
UInt16.MaxValue, UInt32.MinValue, UInt32.MaxValue,
UInt64.MinValue, UInt64.MaxValue };
double dblValue;
foreach (var value in values) {
if (value.GetType() == typeof(Decimal))
dblValue = (Double) value;
else
dblValue = value;
Console.WriteLine("{0} ({1}) --> {2:R} ({3})",
value, value.GetType().Name,
dblValue, dblValue.GetType().Name);
}
}
}
// The example displays the following output:
//    0 (Byte) --> 0 (Double)
//    255 (Byte) --> 255 (Double)
//    -79228162514264337593543950335 (Decimal) --> -7.9228162514264338E+28 (Double)
//    79228162514264337593543950335 (Decimal) --> 7.9228162514264338E+28 (Double)
//    -32768 (Int16) --> -32768 (Double)
//    32767 (Int16) --> 32767 (Double)
//    -2147483648 (Int32) --> -2147483648 (Double)
//    2147483647 (Int32) --> 2147483647 (Double)
//    -9223372036854775808 (Int64) --> -9.2233720368547758E+18 (Double)
//    9223372036854775807 (Int64) --> 9.2233720368547758E+18 (Double)
//    -128 (SByte) --> -128 (Double)
//    127 (SByte) --> 127 (Double)
//    -3.402823E+38 (Single) --> -3.4028234663852886E+38 (Double)
//    3.402823E+38 (Single) --> 3.4028234663852886E+38 (Double)
//    0 (UInt16) --> 0 (Double)
//    65535 (UInt16) --> 65535 (Double)
//    0 (UInt32) --> 0 (Double)
//    4294967295 (UInt32) --> 4294967295 (Double)
//    0 (UInt64) --> 0 (Double)
//    18446744073709551615 (UInt64) --> 1.8446744073709552E+19 (Double)
``````
``````Module Example
Public Sub Main()
Dim values() As Object = { Byte.MinValue, Byte.MaxValue, Decimal.MinValue,
Decimal.MaxValue, Int16.MinValue, Int16.MaxValue,
Int32.MinValue, Int32.MaxValue, Int64.MinValue,
Int64.MaxValue, SByte.MinValue, SByte.MaxValue,
Single.MinValue, Single.MaxValue, UInt16.MinValue,
UInt16.MaxValue, UInt32.MinValue, UInt32.MaxValue,
UInt64.MinValue, UInt64.MaxValue }
Dim dblValue As Double
For Each value In values
dblValue = value
Console.WriteLine("{0} ({1}) --> {2:R} ({3})",
value, value.GetType().Name,
dblValue, dblValue.GetType().Name)
Next
End Sub
End Module
' The example displays the following output:
'    0 (Byte) --> 0 (Double)
'    255 (Byte) --> 255 (Double)
'    -79228162514264337593543950335 (Decimal) --> -7.9228162514264338E+28 (Double)
'    79228162514264337593543950335 (Decimal) --> 7.9228162514264338E+28 (Double)
'    -32768 (Int16) --> -32768 (Double)
'    32767 (Int16) --> 32767 (Double)
'    -2147483648 (Int32) --> -2147483648 (Double)
'    2147483647 (Int32) --> 2147483647 (Double)
'    -9223372036854775808 (Int64) --> -9.2233720368547758E+18 (Double)
'    9223372036854775807 (Int64) --> 9.2233720368547758E+18 (Double)
'    -128 (SByte) --> -128 (Double)
'    127 (SByte) --> 127 (Double)
'    -3.402823E+38 (Single) --> -3.4028234663852886E+38 (Double)
'    3.402823E+38 (Single) --> 3.4028234663852886E+38 (Double)
'    0 (UInt16) --> 0 (Double)
'    65535 (UInt16) --> 65535 (Double)
'    0 (UInt32) --> 0 (Double)
'    4294967295 (UInt32) --> 4294967295 (Double)
'    0 (UInt64) --> 0 (Double)
'    18446744073709551615 (UInt64) --> 1.8446744073709552E+19 (Double)
``````

In addition, the Single values Single.NaN, Single.PositiveInfinity, and Single.NegativeInfinity covert to Double.NaN, Double.PositiveInfinity, and Double.NegativeInfinity, respectively.

Double 值轉換為任何其他基本數值資料類型的值是縮小轉換，而且需要轉換運算子（在中C#為）、轉換方法（在 Visual Basic 中），或對 Convert 方法的呼叫。The conversion of a Double value to a value of any other primitive numeric data type is a narrowing conversion and requires a cast operator (in C#), a conversion method (in Visual Basic), or a call to a Convert method. 目標資料類型的範圍以外的值（由目標型別的 `MinValue``MaxValue` 屬性所定義），其行為如下表所示。Values that are outside the range of the target data type, which are defined by the target type's `MinValue` and `MaxValue` properties, behave as shown in the following table.

Decimal OverflowException 例外狀況。An OverflowException exception.
Single Single.NegativeInfinity for negative values.

Single.PositiveInfinity for positive values.

``````using System;

public class Example
{
public static void Main()
{
Double[] values = { Double.MinValue, -67890.1234, -12345.6789,
12345.6789, 67890.1234, Double.MaxValue,
Double.NaN, Double.PositiveInfinity,
Double.NegativeInfinity };
checked {
foreach (var value in values) {
try {
Int64 lValue = (long) value;
Console.WriteLine("{0} ({1}) --> {2} (0x{2:X16}) ({3})",
value, value.GetType().Name,
lValue, lValue.GetType().Name);
}
catch (OverflowException) {
Console.WriteLine("Unable to convert {0} to Int64.", value);
}
try {
UInt64 ulValue = (ulong) value;
Console.WriteLine("{0} ({1}) --> {2} (0x{2:X16}) ({3})",
value, value.GetType().Name,
ulValue, ulValue.GetType().Name);
}
catch (OverflowException) {
Console.WriteLine("Unable to convert {0} to UInt64.", value);
}
try {
Decimal dValue = (decimal) value;
Console.WriteLine("{0} ({1}) --> {2} ({3})",
value, value.GetType().Name,
dValue, dValue.GetType().Name);
}
catch (OverflowException) {
Console.WriteLine("Unable to convert {0} to Decimal.", value);
}
try {
Single sValue = (float) value;
Console.WriteLine("{0} ({1}) --> {2} ({3})",
value, value.GetType().Name,
sValue, sValue.GetType().Name);
}
catch (OverflowException) {
Console.WriteLine("Unable to convert {0} to Single.", value);
}
Console.WriteLine();
}
}
}
}
// The example displays the following output for conversions performed
// in a checked context:
//       Unable to convert -1.79769313486232E+308 to Int64.
//       Unable to convert -1.79769313486232E+308 to UInt64.
//       Unable to convert -1.79769313486232E+308 to Decimal.
//       -1.79769313486232E+308 (Double) --> -Infinity (Single)
//
//       -67890.1234 (Double) --> -67890 (0xFFFFFFFFFFFEF6CE) (Int64)
//       Unable to convert -67890.1234 to UInt64.
//       -67890.1234 (Double) --> -67890.1234 (Decimal)
//       -67890.1234 (Double) --> -67890.13 (Single)
//
//       -12345.6789 (Double) --> -12345 (0xFFFFFFFFFFFFCFC7) (Int64)
//       Unable to convert -12345.6789 to UInt64.
//       -12345.6789 (Double) --> -12345.6789 (Decimal)
//       -12345.6789 (Double) --> -12345.68 (Single)
//
//       12345.6789 (Double) --> 12345 (0x0000000000003039) (Int64)
//       12345.6789 (Double) --> 12345 (0x0000000000003039) (UInt64)
//       12345.6789 (Double) --> 12345.6789 (Decimal)
//       12345.6789 (Double) --> 12345.68 (Single)
//
//       67890.1234 (Double) --> 67890 (0x0000000000010932) (Int64)
//       67890.1234 (Double) --> 67890 (0x0000000000010932) (UInt64)
//       67890.1234 (Double) --> 67890.1234 (Decimal)
//       67890.1234 (Double) --> 67890.13 (Single)
//
//       Unable to convert 1.79769313486232E+308 to Int64.
//       Unable to convert 1.79769313486232E+308 to UInt64.
//       Unable to convert 1.79769313486232E+308 to Decimal.
//       1.79769313486232E+308 (Double) --> Infinity (Single)
//
//       Unable to convert NaN to Int64.
//       Unable to convert NaN to UInt64.
//       Unable to convert NaN to Decimal.
//       NaN (Double) --> NaN (Single)
//
//       Unable to convert Infinity to Int64.
//       Unable to convert Infinity to UInt64.
//       Unable to convert Infinity to Decimal.
//       Infinity (Double) --> Infinity (Single)
//
//       Unable to convert -Infinity to Int64.
//       Unable to convert -Infinity to UInt64.
//       Unable to convert -Infinity to Decimal.
//       -Infinity (Double) --> -Infinity (Single)
// The example displays the following output for conversions performed
// in an unchecked context:
//       -1.79769313486232E+308 (Double) --> -9223372036854775808 (0x8000000000000000) (Int64)
//       -1.79769313486232E+308 (Double) --> 9223372036854775808 (0x8000000000000000) (UInt64)
//       Unable to convert -1.79769313486232E+308 to Decimal.
//       -1.79769313486232E+308 (Double) --> -Infinity (Single)
//
//       -67890.1234 (Double) --> -67890 (0xFFFFFFFFFFFEF6CE) (Int64)
//       -67890.1234 (Double) --> 18446744073709483726 (0xFFFFFFFFFFFEF6CE) (UInt64)
//       -67890.1234 (Double) --> -67890.1234 (Decimal)
//       -67890.1234 (Double) --> -67890.13 (Single)
//
//       -12345.6789 (Double) --> -12345 (0xFFFFFFFFFFFFCFC7) (Int64)
//       -12345.6789 (Double) --> 18446744073709539271 (0xFFFFFFFFFFFFCFC7) (UInt64)
//       -12345.6789 (Double) --> -12345.6789 (Decimal)
//       -12345.6789 (Double) --> -12345.68 (Single)
//
//       12345.6789 (Double) --> 12345 (0x0000000000003039) (Int64)
//       12345.6789 (Double) --> 12345 (0x0000000000003039) (UInt64)
//       12345.6789 (Double) --> 12345.6789 (Decimal)
//       12345.6789 (Double) --> 12345.68 (Single)
//
//       67890.1234 (Double) --> 67890 (0x0000000000010932) (Int64)
//       67890.1234 (Double) --> 67890 (0x0000000000010932) (UInt64)
//       67890.1234 (Double) --> 67890.1234 (Decimal)
//       67890.1234 (Double) --> 67890.13 (Single)
//
//       1.79769313486232E+308 (Double) --> -9223372036854775808 (0x8000000000000000) (Int64)
//       1.79769313486232E+308 (Double) --> 0 (0x0000000000000000) (UInt64)
//       Unable to convert 1.79769313486232E+308 to Decimal.
//       1.79769313486232E+308 (Double) --> Infinity (Single)
//
//       NaN (Double) --> -9223372036854775808 (0x8000000000000000) (Int64)
//       NaN (Double) --> 0 (0x0000000000000000) (UInt64)
//       Unable to convert NaN to Decimal.
//       NaN (Double) --> NaN (Single)
//
//       Infinity (Double) --> -9223372036854775808 (0x8000000000000000) (Int64)
//       Infinity (Double) --> 0 (0x0000000000000000) (UInt64)
//       Unable to convert Infinity to Decimal.
//       Infinity (Double) --> Infinity (Single)
//
//       -Infinity (Double) --> -9223372036854775808 (0x8000000000000000) (Int64)
//       -Infinity (Double) --> 9223372036854775808 (0x8000000000000000) (UInt64)
//       Unable to convert -Infinity to Decimal.
//       -Infinity (Double) --> -Infinity (Single)
``````
``````Module Example
Public Sub Main()
Dim values() As Double = { Double.MinValue, -67890.1234, -12345.6789,
12345.6789, 67890.1234, Double.MaxValue,
Double.NaN, Double.PositiveInfinity,
Double.NegativeInfinity }
For Each value In values
Try
Dim lValue As Int64 = CLng(value)
Console.WriteLine("{0} ({1}) --> {2} (0x{2:X16}) ({3})",
value, value.GetType().Name,
lValue, lValue.GetType().Name)
Catch e As OverflowException
Console.WriteLine("Unable to convert {0} to Int64.", value)
End Try
Try
Dim ulValue As UInt64 = CULng(value)
Console.WriteLine("{0} ({1}) --> {2} (0x{2:X16}) ({3})",
value, value.GetType().Name,
ulValue, ulValue.GetType().Name)
Catch e As OverflowException
Console.WriteLine("Unable to convert {0} to UInt64.", value)
End Try
Try
Dim dValue As Decimal = CDec(value)
Console.WriteLine("{0} ({1}) --> {2} ({3})",
value, value.GetType().Name,
dValue, dValue.GetType().Name)
Catch e As OverflowException
Console.WriteLine("Unable to convert {0} to Decimal.", value)
End Try
Try
Dim sValue As Single = CSng(value)
Console.WriteLine("{0} ({1}) --> {2} ({3})",
value, value.GetType().Name,
sValue, sValue.GetType().Name)
Catch e As OverflowException
Console.WriteLine("Unable to convert {0} to Single.", value)
End Try
Console.WriteLine()
Next
End Sub
End Module
' The example displays the following output for conversions performed
' in a checked context:
'       Unable to convert -1.79769313486232E+308 to Int64.
'       Unable to convert -1.79769313486232E+308 to UInt64.
'       Unable to convert -1.79769313486232E+308 to Decimal.
'       -1.79769313486232E+308 (Double) --> -Infinity (Single)
'
'       -67890.1234 (Double) --> -67890 (0xFFFFFFFFFFFEF6CE) (Int64)
'       Unable to convert -67890.1234 to UInt64.
'       -67890.1234 (Double) --> -67890.1234 (Decimal)
'       -67890.1234 (Double) --> -67890.13 (Single)
'
'       -12345.6789 (Double) --> -12346 (0xFFFFFFFFFFFFCFC6) (Int64)
'       Unable to convert -12345.6789 to UInt64.
'       -12345.6789 (Double) --> -12345.6789 (Decimal)
'       -12345.6789 (Double) --> -12345.68 (Single)
'
'       12345.6789 (Double) --> 12346 (0x000000000000303A) (Int64)
'       12345.6789 (Double) --> 12346 (0x000000000000303A) (UInt64)
'       12345.6789 (Double) --> 12345.6789 (Decimal)
'       12345.6789 (Double) --> 12345.68 (Single)
'
'       67890.1234 (Double) --> 67890 (0x0000000000010932) (Int64)
'       67890.1234 (Double) --> 67890 (0x0000000000010932) (UInt64)
'       67890.1234 (Double) --> 67890.1234 (Decimal)
'       67890.1234 (Double) --> 67890.13 (Single)
'
'       Unable to convert 1.79769313486232E+308 to Int64.
'       Unable to convert 1.79769313486232E+308 to UInt64.
'       Unable to convert 1.79769313486232E+308 to Decimal.
'       1.79769313486232E+308 (Double) --> Infinity (Single)
'
'       Unable to convert NaN to Int64.
'       Unable to convert NaN to UInt64.
'       Unable to convert NaN to Decimal.
'       NaN (Double) --> NaN (Single)
'
'       Unable to convert Infinity to Int64.
'       Unable to convert Infinity to UInt64.
'       Unable to convert Infinity to Decimal.
'       Infinity (Double) --> Infinity (Single)
'
'       Unable to convert -Infinity to Int64.
'       Unable to convert -Infinity to UInt64.
'       Unable to convert -Infinity to Decimal.
'       -Infinity (Double) --> -Infinity (Single)
' The example displays the following output for conversions performed
' in an unchecked context:
'       -1.79769313486232E+308 (Double) --> -9223372036854775808 (0x8000000000000000) (Int64)
'       -1.79769313486232E+308 (Double) --> 9223372036854775808 (0x8000000000000000) (UInt64)
'       Unable to convert -1.79769313486232E+308 to Decimal.
'       -1.79769313486232E+308 (Double) --> -Infinity (Single)
'
'       -67890.1234 (Double) --> -67890 (0xFFFFFFFFFFFEF6CE) (Int64)
'       -67890.1234 (Double) --> 18446744073709483726 (0xFFFFFFFFFFFEF6CE) (UInt64)
'       -67890.1234 (Double) --> -67890.1234 (Decimal)
'       -67890.1234 (Double) --> -67890.13 (Single)
'
'       -12345.6789 (Double) --> -12346 (0xFFFFFFFFFFFFCFC6) (Int64)
'       -12345.6789 (Double) --> 18446744073709539270 (0xFFFFFFFFFFFFCFC6) (UInt64)
'       -12345.6789 (Double) --> -12345.6789 (Decimal)
'       -12345.6789 (Double) --> -12345.68 (Single)
'
'       12345.6789 (Double) --> 12346 (0x000000000000303A) (Int64)
'       12345.6789 (Double) --> 12346 (0x000000000000303A) (UInt64)
'       12345.6789 (Double) --> 12345.6789 (Decimal)
'       12345.6789 (Double) --> 12345.68 (Single)
'
'       67890.1234 (Double) --> 67890 (0x0000000000010932) (Int64)
'       67890.1234 (Double) --> 67890 (0x0000000000010932) (UInt64)
'       67890.1234 (Double) --> 67890.1234 (Decimal)
'       67890.1234 (Double) --> 67890.13 (Single)
'
'       1.79769313486232E+308 (Double) --> -9223372036854775808 (0x8000000000000000) (Int64)
'       1.79769313486232E+308 (Double) --> 0 (0x0000000000000000) (UInt64)
'       Unable to convert 1.79769313486232E+308 to Decimal.
'       1.79769313486232E+308 (Double) --> Infinity (Single)
'
'       NaN (Double) --> -9223372036854775808 (0x8000000000000000) (Int64)
'       NaN (Double) --> 0 (0x0000000000000000) (UInt64)
'       Unable to convert NaN to Decimal.
'       NaN (Double) --> NaN (Single)
'
'       Infinity (Double) --> -9223372036854775808 (0x8000000000000000) (Int64)
'       Infinity (Double) --> 0 (0x0000000000000000) (UInt64)
'       Unable to convert Infinity to Decimal.
'       Infinity (Double) --> Infinity (Single)
'
'       -Infinity (Double) --> -9223372036854775808 (0x8000000000000000) (Int64)
'       -Infinity (Double) --> 9223372036854775808 (0x8000000000000000) (UInt64)
'       Unable to convert -Infinity to Decimal.
'       -Infinity (Double) --> -Infinity (Single)
``````

### 浮點功能Floating-Point Functionality

Double 結構和相關類型提供在下欄區域執行作業的方法：The Double structure and related types provide methods to perform operations in the following areas:

• 值的比較Comparison of values. 您可以呼叫 Equals 方法來判斷兩個 Double 值是否相等，或 CompareTo 方法來判斷兩個值之間的關聯性。You can call the Equals method to determine whether two Double values are equal, or the CompareTo method to determine the relationship between two values.

Double 結構也支援一組完整的比較運算子。The Double structure also supports a complete set of comparison operators. 例如，您可以測試是否相等或不等比較，或判斷某個值是否大於或等於另一個值。For example, you can test for equality or inequality, or determine whether one value is greater than or equal to another. 如果其中一個運算元是 Double以外的數數值型別，則會先將它轉換成 Double，然後再執行比較。If one of the operands is a numeric type other than a Double, it is converted to a Double before performing the comparison.

警告

因為精確度的差異，您預期相等的兩個 Double 值可能會變成不相等，這會影響比較的結果。Because of differences in precision, two Double values that you expect to be equal may turn out to be unequal, which affects the result of the comparison. 如需比較兩個 Double 值的詳細資訊，請參閱測試相等一節。See the Testing for Equality section for more information about comparing two Double values.

您也可以呼叫 IsNaNIsInfinityIsPositiveInfinityIsNegativeInfinity 方法來測試這些特殊值。You can also call the IsNaN, IsInfinity, IsPositiveInfinity, and IsNegativeInfinity methods to test for these special values.

• 數學運算Mathematical operations. 一般算數運算（例如加法、減法、乘法和除法）是由語言編譯器和一般中繼語言（CIL）指示來執行，而不是透過 Double 方法。Common arithmetic operations, such as addition, subtraction, multiplication, and division, are implemented by language compilers and Common Intermediate Language (CIL) instructions, rather than by Double methods. 如果數學運算中的其中一個運算元是 Double以外的數數值型別，則會先將它轉換成 Double，然後再執行作業。If one of the operands in a mathematical operation is a numeric type other than a Double, it is converted to a Double before performing the operation. 運算的結果也是 Double 值。The result of the operation is also a Double value.

藉由呼叫 System.Math 類別中的 `static``Shared` Visual Basic）方法，可以執行其他數學運算。Other mathematical operations can be performed by calling `static` (`Shared` in Visual Basic) methods in the System.Math class. 其中包含常用於算術的其他方法（例如 Math.AbsMath.SignMath.Sqrt）、geometry （例如 Math.CosMath.Sin）和微積分（例如 Math.Log）。It includes additional methods commonly used for arithmetic (such as Math.Abs, Math.Sign, and Math.Sqrt), geometry (such as Math.Cos and Math.Sin), and calculus (such as Math.Log).

您也可以在 Double 值中操作個別位。You can also manipulate the individual bits in a Double value. BitConverter.DoubleToInt64Bits 方法會在64位整數中保留 Double 值的位模式。The BitConverter.DoubleToInt64Bits method preserves a Double value's bit pattern in a 64-bit integer. BitConverter.GetBytes(Double) 方法會在位元組陣列中傳回其位模式。The BitConverter.GetBytes(Double) method returns its bit pattern in a byte array.

• 進位Rounding. 進位通常用來做為減少因浮點標記法和精確度問題而造成之值差異的技術。Rounding is often used as a technique for reducing the impact of differences between values caused by problems of floating-point representation and precision. 您可以藉由呼叫 Math.Round 方法來四捨五入 Double 值。You can round a Double value by calling the Math.Round method.

• 格式Formatting. 您可以藉由呼叫 ToString 方法或使用複合格式功能，將 Double 值轉換為其字串表示。You can convert a Double value to its string representation by calling the ToString method or by using the composite formatting feature. 如需格式字串如何控制浮點值的字串標記法的詳細資訊，請參閱標準數值格式字串自訂數值格式字串主題。For information about how format strings control the string representation of floating-point values, see the Standard Numeric Format Strings and Custom Numeric Format Strings topics.

• 剖析字串Parsing strings. 您可以藉由呼叫 ParseTryParse 方法，將浮點值的字串表示轉換為 Double 值。You can convert the string representation of a floating-point value to a Double value by calling either the Parse or TryParse method. 如果剖析作業失敗，Parse 方法會擲回例外狀況，而 TryParse 方法會傳回 `false`If the parse operation fails, the Parse method throws an exception, whereas the TryParse method returns `false`.

• 類型轉換Type conversion. Double 結構會針對 IConvertible 介面提供明確的介面實作為，支援在任何兩個標準 .NET Framework 資料類型之間進行轉換。The Double structure provides an explicit interface implementation for the IConvertible interface, which supports conversion between any two standard .NET Framework data types. 語言編譯器也支援將所有其他標準數數值型別的值隱含轉換成 Double 值。Language compilers also support the implicit conversion of values of all other standard numeric types to Double values. 將任何標準數數值型別的值轉換為 Double 是擴輾轉換，而且不需要轉換運算子或轉換方法的使用者。Conversion of a value of any standard numeric type to a Double is a widening conversion and does not require the user of a casting operator or conversion method,

不過，Int64Single 值的轉換可能會遺失有效位數。However, conversion of Int64 and Single values can involve a loss of precision. 下表列出每種類型的精確度差異：The following table lists the differences in precision for each of these types:

類型Type 最大有效位數Maximum precision 內部有效位數Internal precision
Double 1515 1717
Int64 19個小數位數19 decimal digits 19個小數位數19 decimal digits
Single 7個小數位數7 decimal digits 9個小數位數9 decimal digits

有效位數的問題最常影響轉換成 DoubleSingle 值。The problem of precision most frequently affects Single values that are converted to Double values. 在下列範例中，相同除法運算所產生的兩個值不相等，因為其中一個值是轉換成 Double的單精確度浮點值。In the following example, two values produced by identical division operations are unequal because one of the values is a single-precision floating point value converted to a Double.

``````using System;

public class Example
{
public static void Main()
{
Double value = .1;
Double result1 = value * 10;
Double result2 = 0;
for (int ctr = 1; ctr <= 10; ctr++)
result2 += value;

Console.WriteLine(".1 * 10:           {0:R}", result1);
Console.WriteLine(".1 Added 10 times: {0:R}", result2);
}
}
// The example displays the following output:
//       .1 * 10:           1
//       .1 Added 10 times: 0.99999999999999989
``````
``````Module Example
Public Sub Main()
Dim value As Double = .1
Dim result1 As Double = value * 10
Dim result2 As Double
For ctr As Integer = 1 To 10
result2 += value
Next
Console.WriteLine(".1 * 10:           {0:R}", result1)
Console.WriteLine(".1 Added 10 times: {0:R}", result2)
End Sub
End Module
' The example displays the following output:
'       .1 * 10:           1
'       .1 Added 10 times: 0.99999999999999989
``````

## 欄位

 代表大於零的最小正 Double 值。Represents the smallest positive Double value that is greater than zero. 這個欄位是常數。This field is constant. 表示 Double 最大的可能值。Represents the largest possible value of a Double. 這個欄位是常數。This field is constant. 表示 Double 最小的可能值。Represents the smallest possible value of a Double. 這個欄位是常數。This field is constant. 代表不是數字 (`NaN`) 的值。Represents a value that is not a number (`NaN`). 這個欄位是常數。This field is constant. 表示負無限大。Represents negative infinity. 這個欄位是常數。This field is constant. 表示正無限大。Represents positive infinity. 這個欄位是常數。This field is constant.

## 方法

 比較這個執行個體與雙精確度的浮點數值，並且傳回一個整數，指出這個執行個體的值是小於、等於或大於特定的雙精確度浮點數值。Compares this instance to a specified double-precision floating-point number and returns an integer that indicates whether the value of this instance is less than, equal to, or greater than the value of the specified double-precision floating-point number. 比較這個執行個體與特定物件，並且傳回一個整數，指出這個執行個體的值是小於、等於或大於特定物件的值。Compares this instance to a specified object and returns an integer that indicates whether the value of this instance is less than, equal to, or greater than the value of the specified object. 傳回數值，指示這個執行個體和指定的 Double 物件是否表示相同的值。Returns a value indicating whether this instance and a specified Double object represent the same value. 傳回值，指出這個執行個體 (Instance) 是否和指定的物件相等。Returns a value indicating whether this instance is equal to a specified object. 傳回這個執行個體的雜湊碼。Returns the hash code for this instance. 傳回實值類型 TypeCode 的 Double。Returns the TypeCode for value type Double. 判斷指定的值是否為有限 (零、偏低或一般)。Determines whether the specified value is finite (zero, subnormal, or normal). 傳回值，指出指定的數字是否計算結果為負或正的無限大。Returns a value indicating whether the specified number evaluates to negative or positive infinity. 傳回值，這個值表示指定的值是否不是數字 (NaN)。Returns a value that indicates whether the specified value is not a number (NaN). 判斷指定的值是否為負數。Determines whether the specified value is negative. 傳回值，指出指定的數字是否計算結果為負的無限大。Returns a value indicating whether the specified number evaluates to negative infinity. 判斷指定的值是否為正常。Determines whether the specified value is normal. 傳回值，指出指定數字是否計算結果為正的無限大。Returns a value indicating whether the specified number evaluates to positive infinity. 判斷指定的值是否為偏低。Determines whether the specified value is subnormal. 將包含數字字串表示 (使用指定樣式和特定文化特性格式) 的字元範圍轉換為其對等的雙精確度浮點數。Converts a character span that contains the string representation of a number in a specified style and culture-specific format to its double-precision floating-point number equivalent. 將數字的字串表示轉換為其相等的雙精確度浮點數。Converts the string representation of a number to its double-precision floating-point number equivalent. 將數字的字串表示 (使用指定的特定文化特性格式) 轉換為其相等的雙精確度浮點數。Converts the string representation of a number in a specified culture-specific format to its double-precision floating-point number equivalent. 將數字的字串表示 (使用指定樣式) 轉換為其相等的雙精確度浮點數。Converts the string representation of a number in a specified style to its double-precision floating-point number equivalent. 使用指定樣式和特定文化特性格式，將數字的字串表示轉換為其相等的雙精確度浮點數。Converts the string representation of a number in a specified style and culture-specific format to its double-precision floating-point number equivalent. 將這個執行個體的數值轉換為它的相等字串表示。Converts the numeric value of this instance to its equivalent string representation. 使用指定的特定文化特性格式資訊，將這個執行個體的數值轉換成它的相等字串表示。Converts the numeric value of this instance to its equivalent string representation using the specified culture-specific format information. 使用指定格式，將這個執行個體的數值轉換成它的相等字串表示。Converts the numeric value of this instance to its equivalent string representation, using the specified format. 使用指定的格式和特定文化特性格式資訊，將這個執行個體的數值轉換成它的相等字串表示。Converts the numeric value of this instance to its equivalent string representation using the specified format and culture-specific format information. 嘗試將目前雙精確度浮點數執行個體的值格式化為所提供的字元範圍。Tries to format the value of the current double instance into the provided span of characters. 將數字的範圍表示 (使用指定樣式和特定文化特性格式) 轉換為其對等的雙精確度浮點數。Converts the span representation of a number in a specified style and culture-specific format to its double-precision floating-point number equivalent. 傳回值會指出轉換成功或失敗。A return value indicates whether the conversion succeeded or failed. 將包含數字字串表示 (使用指定樣式和特定文化特性格式) 的字元範圍轉換為其對等的雙精確度浮點數。Converts a character span containing the string representation of a number in a specified style and culture-specific format to its double-precision floating-point number equivalent. 傳回值會指出轉換成功或失敗。A return value indicates whether the conversion succeeded or failed. 將數字的字串表示轉換為其相等的雙精確度浮點數。Converts the string representation of a number to its double-precision floating-point number equivalent. 傳回值會指出轉換成功或失敗。A return value indicates whether the conversion succeeded or failed. 使用指定樣式和特定文化特性格式，將數字的字串表示轉換為其相等的雙精確度浮點數。Converts the string representation of a number in a specified style and culture-specific format to its double-precision floating-point number equivalent. 傳回值會指出轉換成功或失敗。A return value indicates whether the conversion succeeded or failed.

## 運算子

 傳回值，這個值表示兩個指定的 Double 值是否相等。Returns a value that indicates whether two specified Double values are equal. 傳回值，這個值表示指定的 Double 值是否大於另一個指定的 Double 值。Returns a value that indicates whether a specified Double value is greater than another specified Double value. 傳回值，這個值表示指定的 Double 值是否大於或等於另一個指定的 Double 值。Returns a value that indicates whether a specified Double value is greater than or equal to another specified Double value. 傳回值，這個值表示兩個指定的 Double 值是否不相等。Returns a value that indicates whether two specified Double values are not equal. 傳回值，這個值表示指定的 Double 值是否小於另一個指定的 Double 值。Returns a value that indicates whether a specified Double value is less than another specified Double value. 傳回值，這個值表示指定的 Double 值是否小於或等於另一個指定的 Double 值。Returns a value that indicates whether a specified Double value is less than or equal to another specified Double value.

## 明確介面實作

 如需這個成員的說明，請參閱 ToBoolean(IFormatProvider)。For a description of this member, see ToBoolean(IFormatProvider). 如需這個成員的說明，請參閱 ToByte(IFormatProvider)。For a description of this member, see ToByte(IFormatProvider). 不支援這個轉換。This conversion is not supported. 嘗試使用這個方法會擲回 InvalidCastException。Attempting to use this method throws an InvalidCastException. 不支援這個轉換。This conversion is not supported. 嘗試使用這個方法會擲回 InvalidCastExceptionAttempting to use this method throws an InvalidCastException 如需這個成員的說明，請參閱 ToDecimal(IFormatProvider)。For a description of this member, see ToDecimal(IFormatProvider). 如需這個成員的說明，請參閱 ToDouble(IFormatProvider)。For a description of this member, see ToDouble(IFormatProvider). 如需這個成員的說明，請參閱 ToInt16(IFormatProvider)。For a description of this member, see ToInt16(IFormatProvider). 如需這個成員的說明，請參閱 ToInt32(IFormatProvider)。For a description of this member, see ToInt32(IFormatProvider). 如需這個成員的說明，請參閱 ToInt64(IFormatProvider)。For a description of this member, see ToInt64(IFormatProvider). 如需這個成員的說明，請參閱 ToSByte(IFormatProvider)。For a description of this member, see ToSByte(IFormatProvider). 如需這個成員的說明，請參閱 ToSingle(IFormatProvider)。For a description of this member, see ToSingle(IFormatProvider). 如需這個成員的說明，請參閱 ToType(Type, IFormatProvider)。For a description of this member, see ToType(Type, IFormatProvider). 如需這個成員的說明，請參閱 ToUInt16(IFormatProvider)。For a description of this member, see ToUInt16(IFormatProvider). 如需這個成員的說明，請參閱 ToUInt32(IFormatProvider)。For a description of this member, see ToUInt32(IFormatProvider). 如需這個成員的說明，請參閱 ToUInt64(IFormatProvider)。For a description of this member, see ToUInt64(IFormatProvider).