是否有一个VB.NET等价于C#的'??' 运营商？

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)))

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.

    ''' 

    ''' Returns the first non-null T based on a collection of the root object and the args.
    ''' 
    ''' 
    ''' 
    ''' 
    ''' 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
    '''
    ''' 
     _
    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/ ||)

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.