Nullable<T>.GetValueOrDefault Nullable<T>.GetValueOrDefault Nullable<T>.GetValueOrDefault Nullable<T>.GetValueOrDefault Method

定义

检索当前 Nullable<T> 对象的值或默认值。Retrieves the value of the current Nullable<T> object, or a default value.

重载

GetValueOrDefault() GetValueOrDefault() GetValueOrDefault() GetValueOrDefault()

检索当前 Nullable<T> 对象的值,或基础类型的默认值。Retrieves the value of the current Nullable<T> object, or the default value of the underlying type.

GetValueOrDefault(T) GetValueOrDefault(T) GetValueOrDefault(T) GetValueOrDefault(T)

检索当前 Nullable<T> 对象的值或指定的默认值。Retrieves the value of the current Nullable<T> object, or the specified default value.

GetValueOrDefault() GetValueOrDefault() GetValueOrDefault() GetValueOrDefault()

检索当前 Nullable<T> 对象的值,或基础类型的默认值。Retrieves the value of the current Nullable<T> object, or the default value of the underlying type.

public:
 T GetValueOrDefault();
public T GetValueOrDefault ();
member this.GetValueOrDefault : unit -> 'T
Public Function GetValueOrDefault () As T

返回

T T T T

如果 HasValue 属性为 true,则为 Value 属性的值;否则为基础类型的默认值。The value of the Value property if the HasValue property is true; otherwise, the default value of the underlying type.

示例

下面的代码示例检索Nullable<T>对象的值(如果定义了该值); 否则,它将检索默认值或特定的默认值。The following code example retrieves the value of a Nullable<T> object if that value is defined; otherwise, it retrieves the default value or a specific default value.

// This code example demonstrates the 
// Nullable<T>.GetValueOrDefault methods.

using System;

class Sample 
{
    public static void Main() 
    {
    float? mySingle = 12.34f;
    float? yourSingle = -1.0f;

   Console.WriteLine("*** Display a value or the default value ***\n");
// Display the values of mySingle and yourSingle.

    Display("A1", mySingle, yourSingle);

// Assign the value of mySingle to yourSingle, then display the values 
// of mySingle and yourSingle. The yourSingle variable is assigned the 
// value 12.34 because mySingle has a value.

    yourSingle = mySingle.GetValueOrDefault();
    Display("A2", mySingle, yourSingle);

// Assign null (Nothing in Visual Basic) to mySingle, which means no value is
// defined for mySingle. Then assign the value of mySingle to yourSingle and
// display the values of both variables. The default value of all binary zeroes 
// is assigned to yourSingle because mySingle has no value.

    mySingle = null;
    yourSingle = mySingle.GetValueOrDefault();
    Display("A3", mySingle, yourSingle);

// Reassign the original values of mySingle and yourSingle.
    mySingle = 12.34f;
    yourSingle = -1.0f;

    Console.Write("\n*** Display a value or the ");
    Console.WriteLine("specified default value ***\n");

// Display the values of mySingle and yourSingle.
    Display("B1", mySingle, yourSingle);

// Assign the value of mySingle to yourSingle, then display the values 
// of mySingle and yourSingle. The yourSingle variable is assigned the 
// value 12.34 because mySingle has a value.

    yourSingle = mySingle.GetValueOrDefault(-222.22f);
    Display("B2", mySingle, yourSingle);

// Assign null (Nothing in Visual Basic) to mySingle, which means no value is
// defined for mySingle. Then assign the value of mySingle to yourSingle and
// display the values of both variables. The specified default value of -333.33
// is assigned to yourSingle because mySingle has no value.

    mySingle = null;
    yourSingle = mySingle.GetValueOrDefault(-333.33f);
    Display("B3", mySingle, yourSingle);

    }

// Display the values of two nullable of System.Single structures.
// The Console.WriteLine method automatically calls the ToString methods of 
// each input argument to display its values. If no value is defined for a
// nullable type, the ToString method for that argument returns the empty
// string ("").
    public static void Display(string title, float? dspMySingle, float? dspYourSingle)
    {
    Console.WriteLine("{0}) mySingle = [{1}], yourSingle = [{2}]", 
                      title, dspMySingle, dspYourSingle);    
    }
}

/*
This code example produces the following results:

A1) mySingle = [12.34], yourSingle = [-1]
A2) mySingle = [12.34], yourSingle = [12.34]
A3) mySingle = [], yourSingle = [0]

*** Display a value or the specified default value ***

B1) mySingle = [12.34], yourSingle = [-1]
B2) mySingle = [12.34], yourSingle = [12.34]
B3) mySingle = [], yourSingle = [-333.33]

*/
' This code example demonstrates the 
' Nullable(Of T).GetValueOrDefault methods.

Class Sample
    Public Shared Sub Main() 
        Dim mySingle As Nullable(Of System.Single) = 12.34F 
        Dim yourSingle As Nullable(Of System.Single) = - 1.0F 
        
        Console.WriteLine("*** Display a value or the default value ***" & vbCrLf)
    ' Display the values of mySingle and yourSingle.
        Display("A1", mySingle, yourSingle)
        
    ' Assign the value of mySingle to yourSingle, then display the values 
    ' of mySingle and yourSingle. The yourSingle variable is assigned the 
    ' value 12.34 because mySingle has a value.
        yourSingle = mySingle.GetValueOrDefault()
        Display("A2", mySingle, yourSingle)
        
    ' Assign null (Nothing in Visual Basic) to mySingle, which means no value is
    ' defined for mySingle. Then assign the value of mySingle to yourSingle and
    ' display the values of both variables. The default value of all binary zeroes 
    ' is assigned to yourSingle because mySingle has no value.
        mySingle = Nothing
        yourSingle = mySingle.GetValueOrDefault()
        Display("A3", mySingle, yourSingle)
        
    ' Reassign the original values of mySingle and yourSingle.
        mySingle = 12.34F
        yourSingle = - 1.0F
        
        Console.Write(vbCrLf & "*** Display a value or the ")
        Console.WriteLine("specified default value ***" & vbCrLf)
        
    ' Display the values of mySingle and yourSingle.
        Display("B1", mySingle, yourSingle)
        
    ' Assign the value of mySingle to yourSingle, then display the values 
    ' of mySingle and yourSingle. The yourSingle variable is assigned the 
    ' value 12.34 because mySingle has a value.
        yourSingle = mySingle.GetValueOrDefault(- 222.22F)
        Display("B2", mySingle, yourSingle)
        
    ' Assign null (Nothing in Visual Basic) to mySingle, which means no value is
    ' defined for mySingle. Then assign the value of mySingle to yourSingle and
    ' display the values of both variables. The specified default value of -333.33
    ' is assigned to yourSingle because mySingle has no value.
        mySingle = Nothing
        yourSingle = mySingle.GetValueOrDefault(- 333.33F)
        Display("B3", mySingle, yourSingle)
    
    End Sub
     
    
    ' Display the values of two nullable of System.Single structures.
    ' The Console.WriteLine method automatically calls the ToString methods of 
    ' each input argument to display its values. If no value is defined for a
    ' nullable type, the ToString method for that argument returns the empty
    ' string ("").

    Public Shared Sub Display(ByVal title As String, _
                              ByVal dspMySingle As Nullable(Of System.Single), _
                              ByVal dspYourSingle As Nullable(Of System.Single))
        If (True) Then
            Console.WriteLine("{0}) mySingle = [{1}], yourSingle = [{2}]", _
                               title, dspMySingle, dspYourSingle)
        End If
    End Sub
End Class

'
'This code example produces the following results:
'
'A1) mySingle = [12.34], yourSingle = [-1]
'A2) mySingle = [12.34], yourSingle = [12.34]
'A3) mySingle = [], yourSingle = [0]
'
'*** Display a value or the specified default value ***
'
'B1) mySingle = [12.34], yourSingle = [-1]
'B2) mySingle = [12.34], yourSingle = [12.34]
'B3) mySingle = [], yourSingle = [-333.33]
'

注解

即使GetValueOrDefault HasValue 属性为false (与属性不同,后者引发异常),该方法也会返回一个值。ValueThe GetValueOrDefault method returns a value even if the HasValue property is false (unlike the Value property, which throws an exception). 如果属性为false,则该方法将返回基础类型的默认值。 HasValueIf the HasValue property is false, the method returns the default value of the underlying type.

另请参阅

GetValueOrDefault(T) GetValueOrDefault(T) GetValueOrDefault(T) GetValueOrDefault(T)

检索当前 Nullable<T> 对象的值或指定的默认值。Retrieves the value of the current Nullable<T> object, or the specified default value.

public:
 T GetValueOrDefault(T defaultValue);
public T GetValueOrDefault (T defaultValue);
member this.GetValueOrDefault : 'T -> 'T
Public Function GetValueOrDefault (defaultValue As T) As T

参数

defaultValue
T T T T

如果 HasValue 属性为 false,则为一个返回值。A value to return if the HasValue property is false.

返回

T T T T

如果 Value 属性为 true,则为 HasValue 属性的值;否则为 defaultValue 参数。The value of the Value property if the HasValue property is true; otherwise, the defaultValue parameter.

示例

下面的代码示例检索Nullable<T>对象的值(如果定义了该值); 否则,它将检索默认值或特定的默认值。The following code example retrieves the value of a Nullable<T> object if that value is defined; otherwise, it retrieves the default value or a specific default value.

// This code example demonstrates the 
// Nullable<T>.GetValueOrDefault methods.

using System;

class Sample 
{
    public static void Main() 
    {
    float? mySingle = 12.34f;
    float? yourSingle = -1.0f;

   Console.WriteLine("*** Display a value or the default value ***\n");
// Display the values of mySingle and yourSingle.

    Display("A1", mySingle, yourSingle);

// Assign the value of mySingle to yourSingle, then display the values 
// of mySingle and yourSingle. The yourSingle variable is assigned the 
// value 12.34 because mySingle has a value.

    yourSingle = mySingle.GetValueOrDefault();
    Display("A2", mySingle, yourSingle);

// Assign null (Nothing in Visual Basic) to mySingle, which means no value is
// defined for mySingle. Then assign the value of mySingle to yourSingle and
// display the values of both variables. The default value of all binary zeroes 
// is assigned to yourSingle because mySingle has no value.

    mySingle = null;
    yourSingle = mySingle.GetValueOrDefault();
    Display("A3", mySingle, yourSingle);

// Reassign the original values of mySingle and yourSingle.
    mySingle = 12.34f;
    yourSingle = -1.0f;

    Console.Write("\n*** Display a value or the ");
    Console.WriteLine("specified default value ***\n");

// Display the values of mySingle and yourSingle.
    Display("B1", mySingle, yourSingle);

// Assign the value of mySingle to yourSingle, then display the values 
// of mySingle and yourSingle. The yourSingle variable is assigned the 
// value 12.34 because mySingle has a value.

    yourSingle = mySingle.GetValueOrDefault(-222.22f);
    Display("B2", mySingle, yourSingle);

// Assign null (Nothing in Visual Basic) to mySingle, which means no value is
// defined for mySingle. Then assign the value of mySingle to yourSingle and
// display the values of both variables. The specified default value of -333.33
// is assigned to yourSingle because mySingle has no value.

    mySingle = null;
    yourSingle = mySingle.GetValueOrDefault(-333.33f);
    Display("B3", mySingle, yourSingle);

    }

// Display the values of two nullable of System.Single structures.
// The Console.WriteLine method automatically calls the ToString methods of 
// each input argument to display its values. If no value is defined for a
// nullable type, the ToString method for that argument returns the empty
// string ("").
    public static void Display(string title, float? dspMySingle, float? dspYourSingle)
    {
    Console.WriteLine("{0}) mySingle = [{1}], yourSingle = [{2}]", 
                      title, dspMySingle, dspYourSingle);    
    }
}

/*
This code example produces the following results:

A1) mySingle = [12.34], yourSingle = [-1]
A2) mySingle = [12.34], yourSingle = [12.34]
A3) mySingle = [], yourSingle = [0]

*** Display a value or the specified default value ***

B1) mySingle = [12.34], yourSingle = [-1]
B2) mySingle = [12.34], yourSingle = [12.34]
B3) mySingle = [], yourSingle = [-333.33]

*/
' This code example demonstrates the 
' Nullable(Of T).GetValueOrDefault methods.

Class Sample
    Public Shared Sub Main() 
        Dim mySingle As Nullable(Of System.Single) = 12.34F 
        Dim yourSingle As Nullable(Of System.Single) = - 1.0F 
        
        Console.WriteLine("*** Display a value or the default value ***" & vbCrLf)
    ' Display the values of mySingle and yourSingle.
        Display("A1", mySingle, yourSingle)
        
    ' Assign the value of mySingle to yourSingle, then display the values 
    ' of mySingle and yourSingle. The yourSingle variable is assigned the 
    ' value 12.34 because mySingle has a value.
        yourSingle = mySingle.GetValueOrDefault()
        Display("A2", mySingle, yourSingle)
        
    ' Assign null (Nothing in Visual Basic) to mySingle, which means no value is
    ' defined for mySingle. Then assign the value of mySingle to yourSingle and
    ' display the values of both variables. The default value of all binary zeroes 
    ' is assigned to yourSingle because mySingle has no value.
        mySingle = Nothing
        yourSingle = mySingle.GetValueOrDefault()
        Display("A3", mySingle, yourSingle)
        
    ' Reassign the original values of mySingle and yourSingle.
        mySingle = 12.34F
        yourSingle = - 1.0F
        
        Console.Write(vbCrLf & "*** Display a value or the ")
        Console.WriteLine("specified default value ***" & vbCrLf)
        
    ' Display the values of mySingle and yourSingle.
        Display("B1", mySingle, yourSingle)
        
    ' Assign the value of mySingle to yourSingle, then display the values 
    ' of mySingle and yourSingle. The yourSingle variable is assigned the 
    ' value 12.34 because mySingle has a value.
        yourSingle = mySingle.GetValueOrDefault(- 222.22F)
        Display("B2", mySingle, yourSingle)
        
    ' Assign null (Nothing in Visual Basic) to mySingle, which means no value is
    ' defined for mySingle. Then assign the value of mySingle to yourSingle and
    ' display the values of both variables. The specified default value of -333.33
    ' is assigned to yourSingle because mySingle has no value.
        mySingle = Nothing
        yourSingle = mySingle.GetValueOrDefault(- 333.33F)
        Display("B3", mySingle, yourSingle)
    
    End Sub
     
    
    ' Display the values of two nullable of System.Single structures.
    ' The Console.WriteLine method automatically calls the ToString methods of 
    ' each input argument to display its values. If no value is defined for a
    ' nullable type, the ToString method for that argument returns the empty
    ' string ("").

    Public Shared Sub Display(ByVal title As String, _
                              ByVal dspMySingle As Nullable(Of System.Single), _
                              ByVal dspYourSingle As Nullable(Of System.Single))
        If (True) Then
            Console.WriteLine("{0}) mySingle = [{1}], yourSingle = [{2}]", _
                               title, dspMySingle, dspYourSingle)
        End If
    End Sub
End Class

'
'This code example produces the following results:
'
'A1) mySingle = [12.34], yourSingle = [-1]
'A2) mySingle = [12.34], yourSingle = [12.34]
'A3) mySingle = [], yourSingle = [0]
'
'*** Display a value or the specified default value ***
'
'B1) mySingle = [12.34], yourSingle = [-1]
'B2) mySingle = [12.34], yourSingle = [12.34]
'B3) mySingle = [], yourSingle = [-333.33]
'

注解

即使GetValueOrDefault HasValue 属性为false (与属性不同,后者引发异常),该方法也会返回一个值。ValueThe GetValueOrDefault method returns a value even if the HasValue property is false (unlike the Value property, which throws an exception).

另请参阅

适用于