C＃的'??'是否有VB.NET等效项 操作员？

Question

C＃??运算符是否有等效的VB.NET ？

Answer 1

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))

Answer 2

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，取自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 )

如果操作员用两个参数调用

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)))