.NET Framework 中的类型转换Type Conversion in the .NET Framework

每个值都有与之关联的类型,此类型定义分配给该值的空间大小、它可以具有的可能值的范围以及它可以提供的成员等属性。Every value has an associated type, which defines attributes such as the amount of space allocated to the value, the range of possible values it can have, and the members that it makes available. 许多值可以表示为多种类型。Many values can be expressed as more than one type. 例如,值 4 可以表示为整数或浮点值。For example, the value 4 can be expressed as an integer or a floating-point value. 类型转换可以创建一个等同于旧类型值的新类型值,但却不必保留原始对象的恒等值(或精确值)。Type conversion creates a value in a new type that is equivalent to the value of an old type, but does not necessarily preserve the identity (or exact value) of the original object.

.NET Framework 自动支持以下转换:The .NET Framework automatically supports the following conversions:

  • 从派生类转换为基类。Conversion from a derived class to a base class. 例如,这意味着可将任何类或结构的实例转换为 Object 实例。This means, for example, that an instance of any class or structure can be converted to an Object instance. 此转换不需要强制转换或转换运算符。This conversion does not require a casting or conversion operator.

  • 从基类转换回原始的派生类。Conversion from a base class back to the original derived class. 在 C# 中,此转换需要强制转换运算符。In C#, this conversion requires a casting operator. 在 Visual Basic 中,如果 Option Strict 处于开启状态,则它需要 CType 运算符。In Visual Basic, it requires the CType operator if Option Strict is on.

  • 从实现接口的类型转换为表示该接口的接口对象。Conversion from a type that implements an interface to an interface object that represents that interface. 此转换不需要强制转换或转换运算符。This conversion does not require a casting or conversion operator.

  • 从接口对象转换回实现该接口的原始类型。Conversion from an interface object back to the original type that implements that interface. 在 C# 中,此转换需要强制转换运算符。In C#, this conversion requires a casting operator. 在 Visual Basic 中,如果 Option Strict 处于开启状态,则它需要 CType 运算符。In Visual Basic, it requires the CType operator if Option Strict is on.

除这些自动转换外,.NET Framework 还提供支持自定义类型转换的多种功能。In addition to these automatic conversions, the .NET Framework provides several features that support custom type conversion. 其中包括:These include the following:

使用隐式运算符的隐式转换Implicit Conversion with the Implicit Operator

扩大转换涉及从现有类型的值创建一个新值,该现有类型比目标类型具有限制性更强的范围或限制性更强的成员列表。Widening conversions involve the creation of a new value from the value of an existing type that has either a more restrictive range or a more restricted member list than the target type. 扩大转换不会导致数据丢失(但可能导致精度损失)。Widening conversions cannot result in data loss (although they may result in a loss of precision). 由于不会丢失数据,因此编译器可以隐式或透明地处理转换,无需使用显式转换方法或强制转换运算符。Because data cannot be lost, compilers can handle the conversion implicitly or transparently, without requiring the use of an explicit conversion method or a casting operator.

备注

虽然执行隐式转换的代码可以调用转换方法或使用强制转换运算符,但支持隐式转换的编译器不需要调用转换方法或使用强制转换运算符。Although code that performs an implicit conversion can call a conversion method or use a casting operator, their use is not required by compilers that support implicit conversions.

例如,Decimal 类型支持从 ByteCharInt16Int32Int64SByteUInt16UInt32UInt64 值进行的隐式转换。For example, the Decimal type supports implicit conversions from Byte, Char, Int16, Int32, Int64, SByte, UInt16, UInt32, and UInt64 values. 下面的示例通过为 Decimal 变量赋值演示了其中的一些隐式转换。The following example illustrates some of these implicit conversions in assigning values to a Decimal variable.

byte byteValue = 16;
short shortValue = -1024;
int intValue = -1034000;
long longValue = 1152921504606846976;
ulong ulongValue = UInt64.MaxValue;

decimal decimalValue;

decimalValue = byteValue;
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.", 
                  byteValue.GetType().Name, decimalValue); 
                         
decimalValue = shortValue;
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.", 
                  shortValue.GetType().Name, decimalValue); 

decimalValue = intValue;
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.", 
                  intValue.GetType().Name, decimalValue); 

decimalValue = longValue;
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.", 
                  longValue.GetType().Name, decimalValue); 
                    
decimalValue = ulongValue;
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.", 
                  longValue.GetType().Name, decimalValue); 
// The example displays the following output:
//    After assigning a Byte value, the Decimal value is 16.
//    After assigning a Int16 value, the Decimal value is -1024.
//    After assigning a Int32 value, the Decimal value is -1034000.
//    After assigning a Int64 value, the Decimal value is 1152921504606846976.
//    After assigning a Int64 value, the Decimal value is 18446744073709551615.
Dim byteValue As Byte = 16
Dim shortValue As Short = -1024
Dim intValue As Integer = -1034000
Dim longValue As Long = CLng(1024^6)
Dim ulongValue As ULong = ULong.MaxValue

Dim decimalValue As Decimal

decimalValue = byteValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  byteValue.GetType().Name, decimalValue) 
                         
decimalValue = shortValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  shortValue.GetType().Name, decimalValue) 

decimalValue = intValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  intValue.GetType().Name, decimalValue) 

decimalValue = longValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  longValue.GetType().Name, decimalValue) 
                  
decimalValue = ulongValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  longValue.GetType().Name, decimalValue) 
' The example displays the following output:
'    After assigning a Byte value, the Decimal value is 16.
'    After assigning a Int16 value, the Decimal value is -1024.
'    After assigning a Int32 value, the Decimal value is -1034000.
'    After assigning a Int64 value, the Decimal value is 1152921504606846976.
'    After assigning a Int64 value, the Decimal value is 18446744073709551615.

如果特定语言编译器支持自定义运算符,则您还可以在自己的自定义类型中定义隐式转换。If a particular language compiler supports custom operators, you can also define implicit conversions in your own custom types. 下面的示例提供了一个名为 ByteWithSign 的有符号字节数据类型的分部实现,该分部实现使用符号数值表示法。The following example provides a partial implementation of a signed byte data type named ByteWithSign that uses sign-and-magnitude representation. 它支持 ByteSByte 值到 ByteWithSign 值的隐式转换。It supports implicit conversion of Byte and SByte values to ByteWithSign values.

public struct ByteWithSign
{
   private SByte signValue; 
   private Byte value;
   
   public static implicit operator ByteWithSign(SByte value) 
   {
      ByteWithSign newValue;
      newValue.signValue = (SByte) Math.Sign(value);
      newValue.value = (byte) Math.Abs(value);
      return newValue;
   }  
   
   public static implicit operator ByteWithSign(Byte value)
   {
      ByteWithSign  newValue;
      newValue.signValue = 1;
      newValue.value = value;
      return newValue;
   }
   
   public override string ToString()
   { 
      return (signValue * value).ToString();
   }
}
Public Structure ByteWithSign
   Private signValue As SByte 
   Private value As Byte
   
   Public Overloads Shared Widening Operator CType(value As SByte) As ByteWithSign
      Dim newValue As ByteWithSign
      newValue.signValue = CSByte(Math.Sign(value))
      newValue.value = CByte(Math.Abs(value))
      Return newValue
   End Operator  
   
   Public Overloads Shared Widening Operator CType(value As Byte) As ByteWithSign
      Dim NewValue As ByteWithSign
      newValue.signValue = 1
      newValue.value = value
      Return newValue
   End Operator
   
   Public Overrides Function ToString() As String 
      Return (signValue * value).ToString()
   End Function
End Structure

然后,客户端代码可以声明一个 ByteWithSign 变量,并为该变量赋予 ByteSByte 值,而无需执行任何显式转换或使用任何强制转换运算符,如下面的示例所示。Client code can then declare a ByteWithSign variable and assign it Byte and SByte values without performing any explicit conversions or using any casting operators, as the following example shows.

SByte sbyteValue = -120;
ByteWithSign value = sbyteValue;
Console.WriteLine(value);
value = Byte.MaxValue;
Console.WriteLine(value);
// The example displays the following output:
//       -120
//       255
Dim sbyteValue As SByte = -120
Dim value As ByteWithSign = sbyteValue
Console.WriteLine(value.ToString()) 
value = Byte.MaxValue
Console.WriteLine(value.ToString()) 
' The example displays the following output:
'       -120
'       255

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使用显式运算符的显式转换Explicit Conversion with the Explicit Operator

收缩转换涉及从现有类型的值创建一个新值,该现有类型比目标类型具有更大的范围和更大的成员列表。Narrowing conversions involve the creation of a new value from the value of an existing type that has either a greater range or a larger member list than the target type. 由于收缩转换可以导致数据丢失,因此编译器通常需要通过调用转换方法或使用强制转换运算符来进行显式转换。Because a narrowing conversion can result in a loss of data, compilers often require that the conversion be made explicit through a call to a conversion method or a casting operator. 也就是说,必须在开发人员代码中显式处理收缩转换。That is, the conversion must be handled explicitly in developer code.

备注

收缩转换之所以需要使用转换方法或强制转换运算符,主要是为提醒开发人员可能会丢失数据或引发 OverflowException,以便可以在代码中对其进行处理。The major purpose of requiring a conversion method or casting operator for narrowing conversions is to make the developer aware of the possibility of data loss or an OverflowException so that it can be handled in code. 但是,有些编译器可以放宽此要求。However, some compilers can relax this requirement. 例如,在 Visual Basic 中,如果 Option Strict 关闭(其默认设置),则 Visual Basic 编译器会尝试隐式执行收缩转换。For example, in Visual Basic, if Option Strict is off (its default setting), the Visual Basic compiler tries to perform narrowing conversions implicitly.

例如,UInt32Int64UInt64 数据类型均具有超过 Int32 数据类型的范围,如下表所示。For example, the UInt32, Int64, and UInt64 data types have ranges that exceed that the Int32 data type, as the following table shows.

类型Type 与 Int32 范围的比较Comparison with range of Int32
Int64 Int64.MaxValue 大于 Int32.MaxValueInt64.MinValue 小于 Int32.MinValue(即比后者具有更大的负范围)。Int64.MaxValue is greater than Int32.MaxValue, and Int64.MinValue is less than (has a greater negative range than) Int32.MinValue.
UInt32 UInt32.MaxValue 大于 Int32.MaxValueUInt32.MaxValue is greater than Int32.MaxValue.
UInt64 UInt64.MaxValue 大于 Int32.MaxValueUInt64.MaxValue is greater than Int32.MaxValue.

为了处理这种收缩转换,.NET Framework 允许类型定义 Explicit 运算符。To handle such narrowing conversions, the .NET Framework allows types to define an Explicit operator. 然后,各种语言编译器可以使用自己的语法实现此运算符,也可以调用 Convert 类的成员来执行此转换。Individual language compilers can then implement this operator using their own syntax, or a member of the Convert class can be called to perform the conversion. (有关 Convert 类的详细信息,请参阅本主题后面的 Convert 类。)下面的示例演示如何使用语言功能来处理这些可能超出范围的整数值到 Int32 值的显式转换。(For more information about the Convert class, see The Convert Class later in this topic.) The following example illustrates the use of language features to handle the explicit conversion of these potentially out-of-range integer values to Int32 values.

long number1 = int.MaxValue + 20L;
uint number2 = int.MaxValue - 1000;
ulong number3 = int.MaxValue;

int intNumber;

try {
   intNumber = checked((int) number1);
   Console.WriteLine("After assigning a {0} value, the Integer value is {1}.", 
                     number1.GetType().Name, intNumber); 
}
catch (OverflowException) {
   if (number1 > int.MaxValue)
      Console.WriteLine("Conversion failed: {0} exceeds {1}.", 
                        number1, int.MaxValue);
   else
      Console.WriteLine("Conversion failed: {0} is less than {1}.", 
                        number1, int.MinValue);
}

try {
   intNumber = checked((int) number2);
   Console.WriteLine("After assigning a {0} value, the Integer value is {1}.", 
                     number2.GetType().Name, intNumber); 
}
catch (OverflowException) {
   Console.WriteLine("Conversion failed: {0} exceeds {1}.", 
                     number2, int.MaxValue);
}

try {
   intNumber = checked((int) number3);
   Console.WriteLine("After assigning a {0} value, the Integer value is {1}.", 
                     number3.GetType().Name, intNumber); 
}
catch (OverflowException) {
   Console.WriteLine("Conversion failed: {0} exceeds {1}.", 
                     number1, int.MaxValue);
}

// The example displays the following output:
//    Conversion failed: 2147483667 exceeds 2147483647.
//    After assigning a UInt32 value, the Integer value is 2147482647.
//    After assigning a UInt64 value, the Integer value is 2147483647.
Dim number1 As Long = Integer.MaxValue + 20L
Dim number2 As UInteger = Integer.MaxValue - 1000
Dim number3 As ULong = Integer.MaxValue

Dim intNumber As Integer

Try
   intNumber = CInt(number1)
   Console.WriteLine("After assigning a {0} value, the Integer value is {1}.", 
                       number1.GetType().Name, intNumber)
Catch e As OverflowException
   If number1 > Integer.MaxValue Then
      Console.WriteLine("Conversion failed: {0} exceeds {1}.", 
                                        number1, Integer.MaxValue)
   Else
      Console.WriteLine("Conversion failed: {0} is less than {1}.\n", 
                                        number1, Integer.MinValue)
   End If
End Try

Try
   intNumber = CInt(number2)
   Console.WriteLine("After assigning a {0} value, the Integer value is {1}.", 
                       number2.GetType().Name, intNumber)
Catch e As OverflowException
   Console.WriteLine("Conversion failed: {0} exceeds {1}.", 
                                     number2, Integer.MaxValue)
End Try

Try
   intNumber = CInt(number3)
   Console.WriteLine("After assigning a {0} value, the Integer value is {1}.", 
                       number3.GetType().Name, intNumber)
Catch e As OverflowException
   Console.WriteLine("Conversion failed: {0} exceeds {1}.",
                                     number1, Integer.MaxValue)
End Try
' The example displays the following output:
'    Conversion failed: 2147483667 exceeds 2147483647.
'    After assigning a UInt32 value, the Integer value is 2147482647.
'    After assigning a UInt64 value, the Integer value is 2147483647.

显式转换在不同的语言中可能会产生不同的结果,并且这些结果可能因对应的 Convert 方法所返回的值而异。Explicit conversions can produce different results in different languages, and these results can differ from the value returned by the corresponding Convert method. 例如,如果将 Double 值 12.63251 转换为 Int32,则 Visual Basic CInt 方法和 .NET Framework Convert.ToInt32(Double) 方法会对 Double 进行舍入以返回值 13,而 C# (int) 运算符会截断 Double 以返回值 12。For example, if the Double value 12.63251 is converted to an Int32, both the Visual Basic CInt method and the .NET Framework Convert.ToInt32(Double) method round the Double to return a value of 13, but the C# (int) operator truncates the Double to return a value of 12. 类似地,C# (int) 运算符不支持从布尔值到整数的转换,而 Visual Basic CInt 方法会将值 true 转换为 -1。Similarly, the C# (int) operator does not support Boolean-to-integer conversion, but the Visual Basic CInt method converts a value of true to -1. 另一方面,Convert.ToInt32(Boolean) 方法将值 true 转换为 1。On the other hand, the Convert.ToInt32(Boolean) method converts a value of true to 1.

大多数编译器允许以有检查或无检查的方式执行显式转换。Most compilers allow explicit conversions to be performed in a checked or unchecked manner. 当执行有检查转换时,如果被转换类型的值超出了目标类型的范围,则会引发 OverflowExceptionWhen a checked conversion is performed, an OverflowException is thrown when the value of the type to be converted is outside the range of the target type. 在相同条件下执行无检查转换时,转换可能不会引发异常,但无法确定确切的行为,并且可能产生不正确的值。When an unchecked conversion is performed under the same conditions, the conversion might not throw an exception, but the exact behavior becomes undefined and an incorrect value might result.

备注

在 C# 中,可将 checked 关键字与强制转换运算符一起使用来执行有检查转换,也可通过指定 /checked+ 编译器选项来执行有检查转换。In C#, checked conversions can be performed by using the checked keyword together with a casting operator, or by specifying the /checked+ compiler option. 反过来,可将 unchecked 关键字与强制转换运算符一起使用来执行无检查转换,或者通过指定 /checked- 编译器选项来执行无检查转换。Conversely, unchecked conversions can be performed by using the unchecked keyword together with the casting operator, or by specifying the /checked- compiler option. 默认情况下,显式转换将为无检查转换。By default, explicit conversions are unchecked. 在 Visual Basic 中,通过清除项目的“高级编译器设置”对话框中的“不做整数溢出检查”复选框或指定 /removeintchecks- 编译器选项,可以执行有检查转换。In Visual Basic, checked conversions can be performed by clearing the Remove integer overflow checks check box in the project's Advanced Compiler Settings dialog box, or by specifying the /removeintchecks- compiler option. 反之,通过选中项目的“高级编译器设置”对话框中的“不做整数溢出检查”复选框,或者指定 /removeintchecks+ 编译器选项,可以执行无检查转换。Conversely, unchecked conversions can be performed by selecting the Remove integer overflow checks check box in the project's Advanced Compiler Settings dialog box or by specifying the /removeintchecks+ compiler option. 默认情况下,显式转换将为有检查转换。By default, explicit conversions are checked.

下面的 C# 示例使用 checkedunchecked 关键字阐释了将 Byte 范围外的值转换为 Byte 时的行为差异。The following C# example uses the checked and unchecked keywords to illustrate the difference in behavior when a value outside the range of a Byte is converted to a Byte. 有检查转换会引发异常,但无检查转换会向 Byte.MaxValue 变量赋予 ByteThe checked conversion throws an exception, but the unchecked conversion assigns Byte.MaxValue to the Byte variable.

   int largeValue = Int32.MaxValue;
   byte newValue;
   
   try {
      newValue = unchecked((byte) largeValue);
      Console.WriteLine("Converted the {0} value {1} to the {2} value {3}.", 
                        largeValue.GetType().Name, largeValue,
                        newValue.GetType().Name, newValue);
   }
   catch (OverflowException) {
      Console.WriteLine("{0} is outside the range of the Byte data type.", 
                        largeValue);
   }

   try {
      newValue = checked((byte) largeValue);
      Console.WriteLine("Converted the {0} value {1} to the {2} value {3}.", 
                        largeValue.GetType().Name, largeValue,
                        newValue.GetType().Name, newValue);
   }
   catch (OverflowException) {
      Console.WriteLine("{0} is outside the range of the Byte data type.", 
                        largeValue);
   }
   // The example displays the following output:
   //    Converted the Int32 value 2147483647 to the Byte value 255.
   //    2147483647 is outside the range of the Byte data type.

如果特定语言编译器支持自定义重载运算符,您还可以在自己的自定义类型中定义显式转换。If a particular language compiler supports custom overloaded operators, you can also define explicit conversions in your own custom types. 下面的示例提供了一个名为 ByteWithSign 的有符号字节数据类型的分部实现,该分部实现使用符号数值表示法。The following example provides a partial implementation of a signed byte data type named ByteWithSign that uses sign-and-magnitude representation. 它支持 Int32UInt32 值到 ByteWithSign 值的显式转换。It supports explicit conversion of Int32 and UInt32 values to ByteWithSign values.

public struct ByteWithSign
{
   private SByte signValue; 
   private Byte value;

   private const byte MaxValue = byte.MaxValue;
   private const int MinValue = -1 * byte.MaxValue;
   
   public static explicit operator ByteWithSign(int value) 
   {
      // Check for overflow.
      if (value > ByteWithSign.MaxValue || value < ByteWithSign.MinValue)
         throw new OverflowException(String.Format("'{0}' is out of range of the ByteWithSign data type.", 
                                                   value));
      
      ByteWithSign newValue;
      newValue.signValue = (SByte) Math.Sign(value);
      newValue.value = (byte) Math.Abs(value);
      return newValue;
   }  
   
   public static explicit operator ByteWithSign(uint value)
   {
      if (value > ByteWithSign.MaxValue) 
         throw new OverflowException(String.Format("'{0}' is out of range of the ByteWithSign data type.", 
                                                   value));

      ByteWithSign newValue;
      newValue.signValue = 1;
      newValue.value = (byte) value;
      return newValue;
   }
   
   public override string ToString()
   { 
      return (signValue * value).ToString();
   }
}
Public Structure ByteWithSign
   Private signValue As SByte 
   Private value As Byte
   
   Private Const MaxValue As Byte = Byte.MaxValue
   Private Const MinValue As Integer = -1 * Byte.MaxValue

   Public Overloads Shared Narrowing Operator CType(value As Integer) As ByteWithSign
      ' Check for overflow.
      If value > ByteWithSign.MaxValue Or value < ByteWithSign.MinValue Then
         Throw New OverflowException(String.Format("'{0}' is out of range of the ByteWithSign data type.", value))
      End If
      
      Dim newValue As ByteWithSign
      
      newValue.signValue = CSByte(Math.Sign(value))
      newValue.value = CByte(Math.Abs(value))
      Return newValue
   End Operator
   
   Public Overloads Shared Narrowing Operator CType(value As UInteger) As ByteWithSign
      If value > ByteWithSign.MaxValue Then 
         Throw New OverflowException(String.Format("'{0}' is out of range of the ByteWithSign data type.", value))
      End If
       
      Dim NewValue As ByteWithSign

      newValue.signValue = 1
      newValue.value = CByte(value)
      Return newValue
   End Operator
   
   Public Overrides Function ToString() As String 
      Return (signValue * value).ToString()
   End Function
End Structure

然后,客户端代码可以声明一个 ByteWithSign 变量,并为该变量赋予 Int32UInt32 值(如果赋值中包括一个强制转换运算符或转换方法),如下面的示例所示。Client code can then declare a ByteWithSign variable and assign it Int32 and UInt32 values if the assignments include a casting operator or a conversion method, as the following example shows.

ByteWithSign value;

try {
   int intValue = -120;
   value = (ByteWithSign) intValue;
   Console.WriteLine(value);
}
catch (OverflowException e) {
   Console.WriteLine(e.Message);
}

try {
   uint uintValue = 1024;
   value = (ByteWithSign) uintValue;
   Console.WriteLine(value);
}
catch (OverflowException e) {
    Console.WriteLine(e.Message);
}
// The example displays the following output:
//       -120
//       '1024' is out of range of the ByteWithSign data type.
Dim value As ByteWithSign
 
Try  
   Dim intValue As Integer = -120
   value = CType(intValue, ByteWithSign)
   Console.WriteLine(value) 
Catch e As OverflowException
   Console.WriteLine(e.Message)
End Try

Try
   Dim uintValue As UInteger = 1024
   value = CType(uintValue, ByteWithSign)
   Console.WriteLine(value) 
Catch e As OverflowException
   Console.WriteLine(e.Message)
End Try
' The example displays the following output:
'       -120
'       '1024' is out of range of the ByteWithSign data type.

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IConvertible 接口The IConvertible Interface

为了支持任意类型到公共语言运行时基类型的转换,.NET Framework 提供了 IConvertible 接口。To support the conversion of any type to a common language runtime base type, the .NET Framework provides the IConvertible interface. 需要使用实现类型以提供以下方法:The implementing type is required to provide the following:

  • 一个返回实现类型的 TypeCode 的方法。A method that returns the TypeCode of the implementing type.

  • 用于将实现类型转换为公共语言运行时的每一种基类型(BooleanByteDateTimeDecimalDouble 等)的各种方法。Methods to convert the implementing type to each common language runtime base type (Boolean, Byte, DateTime, Decimal, Double, and so on).

  • 一个用于将实现类型的实例转换为另一个指定类型的通用转换方法。A generalized conversion method to convert an instance of the implementing type to another specified type. 不支持的转换应引发 InvalidCastExceptionConversions that are not supported should throw an InvalidCastException.

公共语言运行时的每一种基类型(即 BooleanByteCharDateTimeDecimalDoubleInt16Int32Int64SByteSingleStringUInt16UInt32UInt64)以及 DBNullEnum 类型都可以实现 IConvertible 接口。Each common language runtime base type (that is, the Boolean, Byte, Char, DateTime, Decimal, Double, Int16, Int32, Int64, SByte, Single, String, UInt16, UInt32, and UInt64), as well as the DBNull and Enum types, implement the IConvertible interface. 但是,这些是显式接口实现;因此只能通过 IConvertible 接口变量来调用转换方法,如下面的示例所示。However, these are explicit interface implementations; the conversion method can be called only through an IConvertible interface variable, as the following example shows. 此示例将一个 Int32 值转换为其等效的 Char 值。This example converts an Int32 value to its equivalent Char value.

int codePoint = 1067;
IConvertible iConv = codePoint;
char ch = iConv.ToChar(null);
Console.WriteLine("Converted {0} to {1}.", codePoint, ch);
Dim codePoint As Integer = 1067
Dim iConv As IConvertible = codePoint
Dim ch As Char = iConv.ToChar(Nothing)
Console.WriteLine("Converted {0} to {1}.", codePoint, ch)

对转换方法的接口(而不是实现类型)调用转换方法的要求使显式接口实现成为一种代价相对较大的操作。The requirement to call the conversion method on its interface rather than on the implementing type makes explicit interface implementations relatively expensive. 因此,在公共语言运行时基类型之间进行转换时,建议您调用 Convert 类的适当成员。Instead, we recommend that you call the appropriate member of the Convert class to convert between common language runtime base types. 有关详细信息,请参阅下一部分 Convert 类For more information, see the next section, The Convert Class.

备注

除了 .NET Framework 提供的 IConvertible 接口和 Convert 类,各种语言还可能会提供其他方法来执行转换。In addition to the IConvertible interface and the Convert class provided by the .NET Framework, individual languages may also provide ways to perform conversions. 例如,C# 使用强制转换运算符;Visual Basic 使用编译器实现的转换函数,例如 CTypeCIntDirectCastFor example, C# uses casting operators; Visual Basic uses compiler-implemented conversion functions such as CType, CInt, and DirectCast.

大多数情况下,IConvertible 接口设计为支持 .NET Framework 中基类型之间的转换。For the most part, the IConvertible interface is designed to support conversion between the base types in the .NET Framework. 但是,通过自定义类型也可以实现该接口,以便支持该类型到其他自定义类型的转换。However, the interface can also be implemented by a custom type to support conversion of that type to other custom types. 有关详细信息,请参阅本主题后面的使用 ChangeType 方法的自定义转换部分。For more information, see the section Custom Conversions with the ChangeType Method later in this topic.

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Convert 类The Convert Class

虽然可以调用每个基类型的 IConvertible 接口实现来执行类型转换,但从一种基类型转换为另一种基类型时,建议您调用 System.Convert 类的方法,这种方式与语言无关。Although each base type's IConvertible interface implementation can be called to perform a type conversion, calling the methods of the System.Convert class is the recommended language-neutral way to convert from one base type to another. 此外,Convert.ChangeType(Object, Type, IFormatProvider) 方法还可用于从一个指定的自定义类型转换为另一种类型。In addition, the Convert.ChangeType(Object, Type, IFormatProvider) method can be used to convert from a specified custom type to another type.

基类型之间的转换Conversions Between Base Types

Convert 类提供了一种与语言无关的方式来执行基类型之间的转换,并且该类可用于面向公共语言运行时的所有语言。The Convert class provides a language-neutral way to perform conversions between base types and is available to all languages that target the common language runtime. 它为扩大转换和收缩转换提供了一组完整的方法,并且会对不支持的转换(例如 InvalidCastException 值到整数值的转换)引发 DateTimeIt provides a complete set of methods for both widening and narrowing conversions, and throws an InvalidCastException for conversions that are not supported (such as the conversion of a DateTime value to an integer value). 收缩转换是在已检查的上下文中执行的,如果转换失败,将引发 OverflowExceptionNarrowing conversions are performed in a checked context, and an OverflowException is thrown if the conversion fails.

重要

由于 Convert 类包含用于转换为每个基类型和从每个基类型进行转换的方法,因此不再需要调用每个基类型的 IConvertible 显式接口实现。Because the Convert class includes methods to convert to and from each base type, it eliminates the need to call each base type's IConvertible explicit interface implementation.

下面的示例演示如何使用 System.Convert 类来执行 .NET Framework 基类型之间的多种扩大转换和收缩转换。The following example illustrates the use of the System.Convert class to perform several widening and narrowing conversions between .NET Framework base types.

// Convert an Int32 value to a Decimal (a widening conversion).
int integralValue = 12534;
decimal decimalValue = Convert.ToDecimal(integralValue);
Console.WriteLine("Converted the {0} value {1} to " +  
                                  "the {2} value {3:N2}.", 
                                  integralValue.GetType().Name, 
                                  integralValue, 
                                  decimalValue.GetType().Name, 
                                  decimalValue);
// Convert a Byte value to an Int32 value (a widening conversion).
byte byteValue = Byte.MaxValue;
int integralValue2 = Convert.ToInt32(byteValue);                                  
Console.WriteLine("Converted the {0} value {1} to " +  
                                  "the {2} value {3:G}.", 
                                  byteValue.GetType().Name, 
                                  byteValue, 
                                  integralValue2.GetType().Name, 
                                  integralValue2);

// Convert a Double value to an Int32 value (a narrowing conversion).
double doubleValue = 16.32513e12;
try {
   long longValue = Convert.ToInt64(doubleValue);
   Console.WriteLine("Converted the {0} value {1:E} to " +  
                                     "the {2} value {3:N0}.", 
                                     doubleValue.GetType().Name, 
                                     doubleValue, 
                                     longValue.GetType().Name, 
                                     longValue);
}
catch (OverflowException) {
   Console.WriteLine("Unable to convert the {0:E} value {1}.", 
                                     doubleValue.GetType().Name, doubleValue);
}
      
// Convert a signed byte to a byte (a narrowing conversion).     
sbyte sbyteValue = -16;
try {
   byte byteValue2 = Convert.ToByte(sbyteValue);
   Console.WriteLine("Converted the {0} value {1} to " +  
                                     "the {2} value {3:G}.", 
                                     sbyteValue.GetType().Name, 
                                     sbyteValue, 
                                     byteValue2.GetType().Name, 
                                     byteValue2);
}
catch (OverflowException) {
   Console.WriteLine("Unable to convert the {0} value {1}.", 
                                     sbyteValue.GetType().Name, sbyteValue);
}                                         
// The example displays the following output:
//       Converted the Int32 value 12534 to the Decimal value 12,534.00.
//       Converted the Byte value 255 to the Int32 value 255.
//       Converted the Double value 1.632513E+013 to the Int64 value 16,325,130,000,000.
//       Unable to convert the SByte value -16.                                                                  
' Convert an Int32 value to a Decimal (a widening conversion).
Dim integralValue As Integer = 12534
Dim decimalValue As Decimal = Convert.ToDecimal(integralValue)
Console.WriteLine("Converted the {0} value {1} to the {2} value {3:N2}.", 
                  integralValue.GetType().Name,
                  integralValue,
                  decimalValue.GetType().Name,
                  decimalValue)

' Convert a Byte value to an Int32 value (a widening conversion).
Dim byteValue As Byte = Byte.MaxValue
Dim integralValue2 As Integer = Convert.ToInt32(byteValue)                                  
Console.WriteLine("Converted the {0} value {1} to " + 
                                  "the {2} value {3:G}.",
                                  byteValue.GetType().Name,
                                  byteValue,
                                  integralValue2.GetType().Name,
                                  integralValue2)

' Convert a Double value to an Int32 value (a narrowing conversion).
Dim doubleValue As Double = 16.32513e12
Try
   Dim longValue As Long = Convert.ToInt64(doubleValue)
   Console.WriteLine("Converted the {0} value {1:E} to " + 
                                     "the {2} value {3:N0}.",
                                     doubleValue.GetType().Name,
                                     doubleValue,
                                     longValue.GetType().Name,
                                     longValue)
Catch e As OverflowException
   Console.WriteLine("Unable to convert the {0:E} value {1}.",
                                     doubleValue.GetType().Name, doubleValue)
End Try                                                             
      
' Convert a signed byte to a byte (a narrowing conversion).     
Dim sbyteValue As SByte = -16
Try
   Dim byteValue2 As Byte = Convert.ToByte(sbyteValue)
   Console.WriteLine("Converted the {0} value {1} to " + 
                                     "the {2} value {3:G}.",
                                     sbyteValue.GetType().Name,
                                     sbyteValue,
                                     byteValue2.GetType().Name,
                                     byteValue2)
Catch e As OverflowException
   Console.WriteLine("Unable to convert the {0} value {1}.",
                                     sbyteValue.GetType().Name, sbyteValue)
End Try 
' The example displays the following output:
'       Converted the Int32 value 12534 to the Decimal value 12,534.00.
'       Converted the Byte value 255 to the Int32 value 255.
'       Converted the Double value 1.632513E+013 to the Int64 value 16,325,130,000,000.
'       Unable to convert the SByte value -16.                                                                  

在某些情况下,尤其是当转换为浮点值和从浮点值进行转换时,转换可能会丢失精度,即使不引发 OverflowException 时也是如此。In some cases, particularly when converting to and from floating-point values, a conversion may involve a loss of precision, even though it does not throw an OverflowException. 下面的示例演示了这种精度丢失。The following example illustrates this loss of precision. 在第一种情况下,Decimal 值在转换为 Double 后精度降低(有效位减少)。In the first case, a Decimal value has less precision (fewer significant digits) when it is converted to a Double. 在第二种情况下,Double 值从 42.72 四舍五入为 43 以完成转换。In the second case, a Double value is rounded from 42.72 to 43 in order to complete the conversion.

double doubleValue; 

// Convert a Double to a Decimal.
decimal decimalValue = 13956810.96702888123451471211m;
doubleValue = Convert.ToDouble(decimalValue);
Console.WriteLine("{0} converted to {1}.", decimalValue, doubleValue);

doubleValue = 42.72;
try {
   int integerValue = Convert.ToInt32(doubleValue);
   Console.WriteLine("{0} converted to {1}.", 
                                     doubleValue, integerValue);
}
catch (OverflowException) {      
   Console.WriteLine("Unable to convert {0} to an integer.", 
                                     doubleValue);
}   
// The example displays the following output:
//       13956810.96702888123451471211 converted to 13956810.9670289.
//       42.72 converted to 43.
Dim doubleValue As Double 

' Convert a Double to a Decimal.
Dim decimalValue As Decimal = 13956810.96702888123451471211d
doubleValue = Convert.ToDouble(decimalValue)
Console.WriteLine("{0} converted to {1}.", decimalValue, doubleValue)

doubleValue = 42.72
Try
   Dim integerValue As Integer = Convert.ToInt32(doubleValue)
   Console.WriteLine("{0} converted to {1}.",
                                     doubleValue, integerValue)
Catch e As OverflowException
   Console.WriteLine("Unable to convert {0} to an integer.",
                                     doubleValue)
End Try   
' The example displays the following output:
'       13956810.96702888123451471211 converted to 13956810.9670289.
'       42.72 converted to 43.

有关列出 Convert 类支持的扩大转换和收缩转换的表,请参阅类型转换表For a table that lists both the widening and narrowing conversions supported by the Convert class, see Type Conversion Tables.

使用 ChangeType 方法的自定义转换Custom Conversions with the ChangeType Method

除了支持到每个基类型的转换外,Convert 类还可用于将一个自定义类型转换为一个或多个预定义类型。In addition to supporting conversions to each of the base types, the Convert class can be used to convert a custom type to one or more predefined types. 此转换是通过 Convert.ChangeType(Object, Type, IFormatProvider) 方法执行的,而此方法包装了对 IConvertible.ToType 参数的 value 方法的调用。This conversion is performed by the Convert.ChangeType(Object, Type, IFormatProvider) method, which in turn wraps a call to the IConvertible.ToType method of the value parameter. 这意味着 value 参数所表示的对象必须提供 IConvertible 接口的实现。This means that the object represented by the value parameter must provide an implementation of the IConvertible interface.

备注

由于 Convert.ChangeType(Object, Type)Convert.ChangeType(Object, Type, IFormatProvider) 方法使用 Type 对象来指定 value 将转换为的目标类型,因此它们可用于执行对象(其类型在编译时是未知的)的动态转换。Because the Convert.ChangeType(Object, Type) and Convert.ChangeType(Object, Type, IFormatProvider) methods use a Type object to specify the target type to which value is converted, they can be used to perform a dynamic conversion to an object whose type is not known at compile time. 但请注意,IConvertiblevalue 实现必须仍支持此转换。However, note that the IConvertible implementation of value must still support this conversion.

下面的示例演示 IConvertible 接口的一个可能实现,该实现允许将 TemperatureCelsius 对象转换为 TemperatureFahrenheit 对象,反之亦然。The following example illustrates a possible implementation of the IConvertible interface that allows a TemperatureCelsius object to be converted to a TemperatureFahrenheit object and vice versa. 此示例定义一个基类 Temperature,该基类实现 IConvertible 接口并重写 Object.ToString 方法。The example defines a base class, Temperature, that implements the IConvertible interface and overrides the Object.ToString method. 派生的 TemperatureCelsiusTemperatureFahrenheit 类分别重写该基类的 ToTypeToString 方法。The derived TemperatureCelsius and TemperatureFahrenheit classes each override the ToType and the ToString methods of the base class.

using System;

public abstract class Temperature : IConvertible
{
   protected decimal temp;
   
   public Temperature(decimal temperature)
   {
      this.temp = temperature;
   }

   public decimal Value
   { 
      get { return this.temp; } 
      set { this.temp = value; }        
   }

   public override string ToString()
   {
      return temp.ToString(null as IFormatProvider) + "º";
   }

   // IConvertible implementations.
   public TypeCode GetTypeCode() { 
      return TypeCode.Object;
   }   

   public bool ToBoolean(IFormatProvider provider) {
      throw new InvalidCastException(String.Format("Temperature-to-Boolean conversion is not supported."));
   }
   
   public byte ToByte(IFormatProvider provider) {
      if (temp < Byte.MinValue || temp > Byte.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Byte data type.", temp));
      else
         return (byte) temp;
   }

   public char ToChar(IFormatProvider provider) {
      throw new InvalidCastException("Temperature-to-Char conversion is not supported.");
   }
   
   public DateTime ToDateTime(IFormatProvider provider) {
      throw new InvalidCastException("Temperature-to-DateTime conversion is not supported.");
   }
   
   public decimal ToDecimal(IFormatProvider provider) {
      return temp;
   }
   
   public double ToDouble(IFormatProvider provider) {
      return (double) temp;
   }
   
   public short ToInt16(IFormatProvider provider) {
      if (temp < Int16.MinValue || temp > Int16.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Int16 data type.", temp));
      else
         return (short) Math.Round(temp);
   }

   public int ToInt32(IFormatProvider provider) {
      if (temp < Int32.MinValue || temp > Int32.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Int32 data type.", temp));
      else
         return (int) Math.Round(temp);
   }
   
   public long ToInt64(IFormatProvider provider) {
      if (temp < Int64.MinValue || temp > Int64.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Int64 data type.", temp));
      else
         return (long) Math.Round(temp);
   }
   
   public sbyte ToSByte(IFormatProvider provider) {
      if (temp < SByte.MinValue || temp > SByte.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the SByte data type.", temp));
      else
         return (sbyte) temp;
   }

   public float ToSingle(IFormatProvider provider) {
      return (float) temp;
   }
   
   public virtual string ToString(IFormatProvider provider) {
      return temp.ToString(provider) + "°";
   }
   
   // If conversionType is implemented by another IConvertible method, call it.
   public virtual object ToType(Type conversionType, IFormatProvider provider) {
      switch (Type.GetTypeCode(conversionType))
      {
         case TypeCode.Boolean:
            return this.ToBoolean(provider);
         case TypeCode.Byte:
            return this.ToByte(provider);
         case TypeCode.Char:
            return this.ToChar(provider);
         case TypeCode.DateTime:
            return this.ToDateTime(provider);
         case TypeCode.Decimal:
            return this.ToDecimal(provider);
         case TypeCode.Double:
            return this.ToDouble(provider);
         case TypeCode.Empty:
            throw new NullReferenceException("The target type is null.");
         case TypeCode.Int16:
            return this.ToInt16(provider);
         case TypeCode.Int32:
            return this.ToInt32(provider);
         case TypeCode.Int64:
            return this.ToInt64(provider);
         case TypeCode.Object:
            // Leave conversion of non-base types to derived classes.
            throw new InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", 
                                           conversionType.Name));
         case TypeCode.SByte:
            return this.ToSByte(provider);
         case TypeCode.Single:
            return this.ToSingle(provider);
         case TypeCode.String:
            IConvertible iconv = this;
            return iconv.ToString(provider);
         case TypeCode.UInt16:
            return this.ToUInt16(provider);
         case TypeCode.UInt32:
            return this.ToUInt32(provider);
         case TypeCode.UInt64:
            return this.ToUInt64(provider);
         default:
            throw new InvalidCastException("Conversion not supported.");
      }
   }
   
   public ushort ToUInt16(IFormatProvider provider) {
      if (temp < UInt16.MinValue || temp > UInt16.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the UInt16 data type.", temp));
      else
         return (ushort) Math.Round(temp);
   }
   
   public uint ToUInt32(IFormatProvider provider) {
      if (temp < UInt32.MinValue || temp > UInt32.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the UInt32 data type.", temp));
      else
         return (uint) Math.Round(temp);
   }
   
   public ulong ToUInt64(IFormatProvider provider) {
      if (temp < UInt64.MinValue || temp > UInt64.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the UInt64 data type.", temp));
      else
         return (ulong) Math.Round(temp);
   }
}

public class TemperatureCelsius : Temperature, IConvertible
{
   public TemperatureCelsius(decimal value) : base(value)
   {
   }
   
   // Override ToString methods.
   public override string ToString()
   {
      return this.ToString(null);
   }

   public override string ToString(IFormatProvider provider)
   {
      return temp.ToString(provider) + "°C"; 
   }
   
   // If conversionType is a implemented by another IConvertible method, call it.
   public override object ToType(Type conversionType, IFormatProvider provider) {
      // For non-objects, call base method.
      if (Type.GetTypeCode(conversionType) != TypeCode.Object) {
         return base.ToType(conversionType, provider);
      }   
      else
      {   
         if (conversionType.Equals(typeof(TemperatureCelsius)))
            return this;
         else if (conversionType.Equals(typeof(TemperatureFahrenheit)))
            return new TemperatureFahrenheit((decimal) this.temp * 9 / 5 + 32);
         else
            throw new InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", 
                                           conversionType.Name));
      }
   }
}

public class TemperatureFahrenheit : Temperature, IConvertible 
{
   public TemperatureFahrenheit(decimal value) : base(value)
   {
   }
   
   // Override ToString methods.
   public override string ToString()
   {
      return this.ToString(null);
   }

   public override string ToString(IFormatProvider provider)
   {
      return temp.ToString(provider) + "°F"; 
   }

   public override object ToType(Type conversionType, IFormatProvider provider)
   { 
      // For non-objects, call base methood.
      if (Type.GetTypeCode(conversionType) != TypeCode.Object) {
         return base.ToType(conversionType, provider);
      }   
      else
      {   
         // Handle conversion between derived classes.
         if (conversionType.Equals(typeof(TemperatureFahrenheit))) 
            return this;
         else if (conversionType.Equals(typeof(TemperatureCelsius)))
            return new TemperatureCelsius((decimal) (this.temp - 32) * 5 / 9);
         // Unspecified object type: throw an InvalidCastException.
         else
            throw new InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", 
                                           conversionType.Name));
      }                                 
   }
}   
Public MustInherit Class Temperature
   Implements IConvertible
   
   Protected temp As Decimal
   
   Public Sub New(temperature As Decimal)
      Me.temp = temperature
   End Sub

   Public Property Value As Decimal
      Get 
         Return Me.temp
      End Get
      Set
         Me.temp = Value
      End Set   
   End Property

   Public Overrides Function ToString() As String
      Return temp.ToString() & "º"
   End Function

   ' IConvertible implementations.
   Public Function GetTypeCode() As TypeCode Implements IConvertible.GetTypeCode
      Return TypeCode.Object
   End Function   

   Public Function ToBoolean(provider As IFormatProvider) As Boolean Implements IConvertible.ToBoolean
      Throw New InvalidCastException(String.Format("Temperature-to-Boolean conversion is not supported."))
   End Function
   
   Public Function ToByte(provider As IFormatProvider) As Byte Implements IConvertible.ToByte
      If temp < Byte.MinValue Or temp > Byte.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Byte data type.", temp))
      Else
         Return CByte(temp)
      End If    
   End Function

   Public Function ToChar(provider As IFormatProvider) As Char Implements IConvertible.ToChar
      Throw New InvalidCastException("Temperature-to-Char conversion is not supported.")
   End Function
   
   Public Function ToDateTime(provider As IFormatProvider) As DateTime Implements IConvertible.ToDateTime
      Throw New InvalidCastException("Temperature-to-DateTime conversion is not supported.")
   End Function
   
   Public Function ToDecimal(provider As IFormatProvider) As Decimal Implements IConvertible.ToDecimal
      Return temp
   End Function
   
   Public Function ToDouble(provider As IFormatProvider) As Double Implements IConvertible.ToDouble
      Return CDbl(temp)
   End Function
   
   Public Function ToInt16(provider As IFormatProvider) As Int16 Implements IConvertible.ToInt16
      If temp < Int16.MinValue Or temp > Int16.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int16 data type.", temp))
      End If
      Return CShort(Math.Round(temp))
   End Function

   Public Function ToInt32(provider As IFormatProvider) As Int32 Implements IConvertible.ToInt32
      If temp < Int32.MinValue Or temp > Int32.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int32 data type.", temp))
      End If
      Return CInt(Math.Round(temp))
   End Function
   
   Public Function ToInt64(provider As IFormatProvider) As Int64 Implements IConvertible.ToInt64
      If temp < Int64.MinValue Or temp > Int64.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int64 data type.", temp))
      End If
      Return CLng(Math.Round(temp))
   End Function
   
   Public Function ToSByte(provider As IFormatProvider) As SByte Implements IConvertible.ToSByte
      If temp < SByte.MinValue Or temp > SByte.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the SByte data type.", temp))
      Else
         Return CSByte(temp)
      End If    
   End Function

   Public Function ToSingle(provider As IFormatProvider) As Single Implements IConvertible.ToSingle
      Return CSng(temp)
   End Function
   
   Public Overridable Overloads Function ToString(provider As IFormatProvider) As String Implements IConvertible.ToString
      Return temp.ToString(provider) & " °C"
   End Function
   
   ' If conversionType is a implemented by another IConvertible method, call it.
   Public Overridable Function ToType(conversionType As Type, provider As IFormatProvider) As Object Implements IConvertible.ToType
      Select Case Type.GetTypeCode(conversionType)
         Case TypeCode.Boolean
            Return Me.ToBoolean(provider)
         Case TypeCode.Byte
            Return Me.ToByte(provider)
         Case TypeCode.Char
            Return Me.ToChar(provider)   
         Case TypeCode.DateTime
            Return Me.ToDateTime(provider)
         Case TypeCode.Decimal
            Return Me.ToDecimal(provider)
         Case TypeCode.Double
            Return Me.ToDouble(provider)
         Case TypeCode.Empty
            Throw New NullReferenceException("The target type is null.")
         Case TypeCode.Int16
            Return Me.ToInt16(provider)
         Case TypeCode.Int32
            Return Me.ToInt32(provider)
         Case TypeCode.Int64
            Return Me.ToInt64(provider)
         Case TypeCode.Object
            ' Leave conversion of non-base types to derived classes.
            Throw New InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", _
                                           conversionType.Name))
         Case TypeCode.SByte
            Return Me.ToSByte(provider)
         Case TypeCode.Single
            Return Me.ToSingle(provider)
         Case TypeCode.String
            Return Me.ToString(provider)
         Case TypeCode.UInt16
            Return Me.ToUInt16(provider)
         Case TypeCode.UInt32
            Return Me.ToUInt32(provider)
         Case TypeCode.UInt64
            Return Me.ToUInt64(provider)
         Case Else
            Throw New InvalidCastException("Conversion not supported.")   
      End Select
   End Function
   
   Public Function ToUInt16(provider As IFormatProvider) As UInt16 Implements IConvertible.ToUInt16
      If temp < UInt16.MinValue Or temp > UInt16.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt16 data type.", temp))
      End If
      Return CUShort(Math.Round(temp))
   End Function
   
   Public Function ToUInt32(provider As IFormatProvider) As UInt32 Implements IConvertible.ToUInt32
      If temp < UInt32.MinValue Or temp > UInt32.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt32 data type.", temp))
      End If
      Return CUInt(Math.Round(temp))
   End Function
   
   Public Function ToUInt64(provider As IFormatProvider) As UInt64 Implements IConvertible.ToUInt64
      If temp < UInt64.MinValue Or temp > UInt64.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt64 data type.", temp))
      End If
      Return CULng(Math.Round(temp))
   End Function
End Class

Public Class TemperatureCelsius : Inherits Temperature : Implements IConvertible
   Public Sub New(value As Decimal)
      MyBase.New(value)
   End Sub

   ' Override ToString methods.
   Public Overrides Function ToString() As String
      Return Me.ToString(Nothing)
   End Function

   Public Overrides Function ToString(provider As IFormatProvider ) As String
      Return temp.ToString(provider) + "°C" 
   End Function
   
  ' If conversionType is a implemented by another IConvertible method, call it.
   Public Overrides Function ToType(conversionType As Type, provider As IFormatProvider) As Object
      ' For non-objects, call base method.
      If Type.GetTypeCode(conversionType) <> TypeCode.Object Then
         Return MyBase.ToType(conversionType, provider)
      Else   
         If conversionType.Equals(GetType(TemperatureCelsius)) Then
            Return Me
         ElseIf conversionType.Equals(GetType(TemperatureFahrenheit))
            Return New TemperatureFahrenheit(CDec(Me.temp * 9 / 5 + 32))
         ' Unspecified object type: throw an InvalidCastException.
         Else
            Throw New InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", _ 
                                           conversionType.Name))
         End If
      End If                                  
   End Function
End Class

Public Class TemperatureFahrenheit : Inherits Temperature : Implements IConvertible
   Public Sub New(value As Decimal)
      MyBase.New(value)
   End Sub
   
   ' Override ToString methods.
   Public Overrides Function ToString() As String
      Return Me.ToString(Nothing)
   End Function

   Public Overrides Function ToString(provider As IFormatProvider ) As String
      Return temp.ToString(provider) + "°F" 
   End Function
   
   Public Overrides Function ToType(conversionType As Type, provider As IFormatProvider) As Object
      ' For non-objects, call base methood.
      If Type.GetTypeCode(conversionType) <> TypeCode.Object Then
         Return MyBase.ToType(conversionType, provider)
      Else   
         ' Handle conversion between derived classes.
         If conversionType.Equals(GetType(TemperatureFahrenheit)) Then 
            Return Me
         ElseIf conversionType.Equals(GetType(TemperatureCelsius))
            Return New TemperatureCelsius(CDec((MyBase.temp - 32) * 5 / 9))
         ' Unspecified object type: throw an InvalidCastException.
         Else
            Throw New InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", _
                                           conversionType.Name))
         End If
      End If                                  
   End Function
End Class   

下面的示例演示对这些 IConvertible 实现的多个调用,以实现 TemperatureCelsius 对象和 TemperatureFahrenheit 对象之间的相互转换。The following example illustrates several calls to these IConvertible implementations to convert TemperatureCelsius objects to TemperatureFahrenheit objects and vice versa.

TemperatureCelsius tempC1 = new TemperatureCelsius(0);
TemperatureFahrenheit tempF1 = (TemperatureFahrenheit) Convert.ChangeType(tempC1, typeof(TemperatureFahrenheit), null);
Console.WriteLine("{0} equals {1}.", tempC1, tempF1);
TemperatureCelsius tempC2 = (TemperatureCelsius) Convert.ChangeType(tempC1, typeof(TemperatureCelsius), null);
Console.WriteLine("{0} equals {1}.", tempC1, tempC2);
TemperatureFahrenheit tempF2 = new TemperatureFahrenheit(212);
TemperatureCelsius tempC3 = (TemperatureCelsius) Convert.ChangeType(tempF2, typeof(TemperatureCelsius), null);
Console.WriteLine("{0} equals {1}.", tempF2, tempC3);
TemperatureFahrenheit tempF3 = (TemperatureFahrenheit) Convert.ChangeType(tempF2, typeof(TemperatureFahrenheit), null);
Console.WriteLine("{0} equals {1}.", tempF2, tempF3);
// The example displays the following output:
//       0°C equals 32°F.
//       0°C equals 0°C.
//       212°F equals 100°C.
//       212°F equals 212°F.
Dim tempC1 As New TemperatureCelsius(0)
Dim tempF1 As TemperatureFahrenheit = CType(Convert.ChangeType(tempC1, GetType(TemperatureFahrenheit), Nothing), TemperatureFahrenheit)
Console.WriteLine("{0} equals {1}.", tempC1, tempF1) 
Dim tempC2 As TemperatureCelsius = CType(Convert.ChangeType(tempC1, GetType(TemperatureCelsius), Nothing), TemperatureCelsius)
Console.WriteLine("{0} equals {1}.", tempC1, tempC2) 
Dim tempF2 As New TemperatureFahrenheit(212)
Dim tempC3 As TEmperatureCelsius = CType(Convert.ChangeType(tempF2, GEtType(TemperatureCelsius), Nothing), TemperatureCelsius)
Console.WriteLine("{0} equals {1}.", tempF2, tempC3) 
Dim tempF3 As TemperatureFahrenheit = CType(Convert.ChangeType(tempF2, GetType(TemperatureFahrenheit), Nothing), TemperatureFahrenheit)
Console.WriteLine("{0} equals {1}.", tempF2, tempF3)
' The example displays the following output:
'       0°C equals 32°F.
'       0°C equals 0°C.
'       212°F equals 100°C.
'       212°F equals 212°F.      

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TypeConverter 类The TypeConverter Class

.NET Framework 还允许您通过下面的方法为自定义类型定义类型转换器:扩展 System.ComponentModel.TypeConverter 类,然后通过 System.ComponentModel.TypeConverterAttribute 特性将类型转换器与该类型关联。The .NET Framework also allows you to define a type converter for a custom type by extending the System.ComponentModel.TypeConverter class and associating the type converter with the type through a System.ComponentModel.TypeConverterAttribute attribute. 下表列出了此方法与为自定义类型实现 IConvertible 接口之间的差异。The following table highlights the differences between this approach and implementing the IConvertible interface for a custom type.

备注

只能为已定义了类型转换器的自定义类型提供设计时支持。Design-time support can be provided for a custom type only if it has a type converter defined for it.

使用 TypeConverter 转换Conversion using TypeConverter 使用 IConvertible 转换Conversion using IConvertible
通过从 TypeConverter 派生单独的类来为自定义类型实现。Is implemented for a custom type by deriving a separate class from TypeConverter. 此派生类通过应用 TypeConverterAttribute 特性与自定义类型关联。This derived class is associated with the custom type by applying a TypeConverterAttribute attribute. 由自定义类型实现,以执行转换。Is implemented by a custom type to perform conversion. 该类型的用户必须对该类型调用 IConvertible 转换方法。A user of the type invokes an IConvertible conversion method on the type.
在设计时和运行时都可以使用。Can be used both at design time and at run time. 只能在运行时使用。Can be used only at run time.
使用反射;因此,比 IConvertible 所启用的转换慢。Uses reflection; therefore, is slower than conversion enabled by IConvertible. 不使用反射。Does not use reflection.
允许自定义类型和其他数据类型间的双向类型转换。Allows two-way type conversions from the custom type to other data types, and from other data types to the custom type. 例如,为 TypeConverter 定义的 MyType 允许从 MyType 转换为 String 以及从 String 转换为 MyTypeFor example, a TypeConverter defined for MyType allows conversions from MyType to String, and from String to MyType. 允许从自定义类型转换为其他数据类型,但不允许从其他数据类型转换为自定义类型。Allows conversion from a custom type to other data types, but not from other data types to the custom type.

有关使用类型转换器执行转换的更多信息,请参见 System.ComponentModel.TypeConverterFor more information about using type converters to perform conversions, see System.ComponentModel.TypeConverter.

请参阅See also