是否有一个VB.NET等价于C#的'??' 运营商？

Question

C#的??运营商是否有VB.NET等价物？

Answer 1

使用If()带有两个参数的运算符(Microsoft文档):

' Variable first is a nullable type.
Dim first? As Integer = 3
Dim second As Integer = 6

' Variable first <> Nothing, so its value, 3, is returned.
Console.WriteLine(If(first, second))

second = Nothing
' Variable first <> Nothing, so the value of first is returned again. 
Console.WriteLine(If(first, second))

first = Nothing second = 6
' Variable first = Nothing, so 6 is returned.
Console.WriteLine(If(first, second))

Answer 2

该IF()运营商应该为你做的伎俩:

value = If(nullable, defaultValueIfNull)

http://visualstudiomagazine.com/listings/list.aspx?id=252

Answer 3

接受的答案没有任何解释,只是一个链接.
因此,我想我会留下一个答案,解释If运营商如何从MSDN中获取工作:

如果运算符(Visual Basic)

使用短路评估有条件地返回两个值中的一个.该如果操作员可以用三个参数或两个参数来调用.

If( [argument1,] argument2, argument3 )

如果操作员使用两个参数调用

If的第一个参数可以省略.这使得只使用两个参数就可以调用运算符.仅当使用两个参数调用If运算符时,以下列表才适用.

部分

Term         Definition
----         ----------

argument2    Required. Object. Must be a reference or nullable type. 
             Evaluated and returned when it evaluates to anything 
             other than Nothing.

argument3    Required. Object.
             Evaluated and returned if argument2 evaluates to Nothing.

当布尔省略参数,所述第一参数必须是参考或空类型.如果第一个参数的计算结果为 Nothing,则返回第二个参数的值.在所有其他情况下,返回第一个参数的值.以下示例说明了此评估的工作原理.

VB

' Variable first is a nullable type. 
Dim first? As Integer = 3
Dim second As Integer = 6

' Variable first <> Nothing, so its value, 3, is returned.
Console.WriteLine(If(first, second))

second = Nothing 
' Variable first <> Nothing, so the value of first is returned again.
Console.WriteLine(If(first, second))

first = Nothing
second = 6
' Variable first = Nothing, so 6 is returned.
Console.WriteLine(If(first, second))

如何处理两个以上值(嵌套ifs)的示例:

Dim first? As Integer = Nothing
Dim second? As Integer = Nothing
Dim third? As Integer = 6
' The LAST parameter doesn't have to be nullable.
'Alternative: Dim third As Integer = 6

' Writes "6", because the first two values are "Nothing".
Console.WriteLine(If(first, If(second, third)))

Answer 4

您可以使用扩展方法.这个就像SQL一样工作COALESCE,对于你想要测试的东西来说可能有些过分,但是它有效.

    ''' <summary>
    ''' Returns the first non-null T based on a collection of the root object and the args.
    ''' </summary>
    ''' <param name="obj"></param>
    ''' <param name="args"></param>
    ''' <returns></returns>
    ''' <remarks>Usage
    ''' Dim val as String = "MyVal"
    ''' Dim result as String = val.Coalesce(String.Empty)
    ''' *** returns "MyVal"
    '''
    ''' val = Nothing
    ''' result = val.Coalesce(String.Empty, "MyVal", "YourVal")
    ''' *** returns String.Empty
    '''
    ''' </remarks>
    <System.Runtime.CompilerServices.Extension()> _
    Public Function Coalesce(Of T)(ByVal obj As T, ByVal ParamArray args() As T) As T

        If obj IsNot Nothing Then
            Return obj
        End If

        Dim arg As T
        For Each arg In args
            If arg IsNot Nothing Then
                Return arg
            End If
        Next

        Return Nothing

    End Function

内置If(nullable, secondChoice)只能处理两个可空的选择.在这里,可以Coalesce根据需要获得尽可能多的参数.将返回第一个非空的,并且之后不评估其余参数(短路,如AndAlso/ &&和OrElse/ ||)

Answer 5

大多数这些解决方案的一个重要限制是它们不会短路.因此它们实际上并不等同于??

除非先前的参数计算为空,否则内置的"if"运算符不会评估后续参数.

以下陈述是等效的.

C#

var value = expression1 ?? expression2 ?? expression3 ?? expression4;

VB

dim value = if(exression1,if(expression2,if(expression3,expression4)))

这将适用于"??"的所有情况 作品.任何其他解决方案都必须非常谨慎使用,因为它们很容易引入运行时错误.