在VB.NET中，C＃的`default`等价于什么？ [英] What is the equivalent of C#'s `default` in VB.NET?

问题描述

我通常在C＃中使用C＃，我正在考虑一些VB.NET代码中的性能问题 - 我想能够比较某种类型的默认值（类似于C＃的 default 关键字）。

I'm normally at home in C#, and I'm looking at a performance issue in some VB.NET code -- I want to be able to compare something to the default value for a type (kind of like C#'s default keyword).

public class GenericThing<T1, T2>
{
    public T1 Foo( T2 id )
    {
        if( id != default(T2) ) // There doesn't appear to be an equivalent in VB.NET for this(?)
        {
            // ...
        }
    }
}

我被引导认为 Nothing 在语义上是相同的，但是如果我这样做的话：

I was led to believe that Nothing was semantically the same, yet if I do:

Public Class GenericThing(Of T1, T2)
    Public Function Foo( id As T2 ) As T1
        If id IsNot Nothing Then
            ' ...
        End If
    End Function
End Class

然后，当 T2 是整数，并且 id 是 0 ，条件仍然通过，并且的主体if 被评估。但是，如果我这样做：
$ b

Then when T2 is Integer, and the value of id is 0, the condition still passes, and the body of the if is evaluated. However if I do:

    Public Function Bar( id As Integer ) As T1
        If id <> Nothing Then
            ' ...
        End If
    End Function

然后条件不符合，并且不评估正文...

Then the condition is not met, and the body is not evaluated...

推荐答案

这不是一个完整的解决方案，因为你原来的C＃代码不能编译。您可以通过局部变量使用Nothing：

This is not a complete solution, as your original C# code doesn't compile. You can use Nothing via a local variable:

Public Class GenericThing(Of T)
    Public Sub Foo(id As T)
        Dim defaultValue As T = Nothing
        If id <> defaultValue Then
            Console.WriteLine("Not default")
        Else
            Console.WriteLine("Default")
        End If
    End Function
End Class

不能编译，就像C＃版本不编译一样 - 你可以比较不受约束的类型参数的值。

That doesn't compile, in the same way that the C# version doesn't compile - you can't compare values of unconstrained type parameters like that.

可以使用 EqualityComparer（Of T） - 但那么你甚至不需要局部变量：

You can use EqualityComparer(Of T) though - and then you don't even need the local variable:

If Not EqualityComparer(Of T).Default.Equals(id, Nothing) Then

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