C＃??运算符是否有等效的VB.NET ？

将If()运算符与两个参数一起使用（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))

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

接受的答案没有任何解释，仅是一个链接。
因此，我想留下一个解释该If操作员如何工作的答案（来自MSDN）：

如果运算符（Visual Basic）

使用短路评估有条件地返回两个值之一。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 )

如果运算符调用了两个参数

If的第一个参数可以省略。这样就可以仅使用两个参数来调用运算符。以下列表仅在使用两个参数调用If运算符时适用。

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，将返回第一个参数的值。以下示例说明了此评估的工作方式。

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))

有关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。这和SQL一样工作COALESCE，可能对您要测试的内容有些过分，但它可以工作。

    ''' <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)只能处理两个可为空的选择。在这里，可以Coalesce根据需要设置多个参数。将返回第一个非空值，此后将不评估其余参数（短路，如AndAlso/ &&和OrElse/ ||）

这些解决方案中的大多数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??。

内置If运算符将不评估后续参数，除非较早的参数评估为空。

以下语句是等效的：

C＃

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

VB

dim value = if(expression1,if(expression2,if(expression3,expression4)))

这将在所有可行的情况下??起作用。使用任何其他解决方案都必须格外谨慎，因为它们很容易引入运行时错误。

在此处查看有关If Operator（Visual Basic）的Microsoft文档：https : //docs.microsoft.com/zh-cn/dotnet/visual-basic/language-reference/operators/if-operator

If( [argument1,] argument2, argument3 )

这是一些例子（VB.Net）

' This statement prints TruePart, because the first argument is true.
Console.WriteLine(If(True, "TruePart", "FalsePart"))

' This statement prints FalsePart, because the first argument is false.
Console.WriteLine(If(False, "TruePart", "FalsePart"))

Dim number = 3
' With number set to 3, this statement prints Positive.
Console.WriteLine(If(number >= 0, "Positive", "Negative"))

number = -1
' With number set to -1, this statement prints Negative.
Console.WriteLine(If(number >= 0, "Positive", "Negative"))