Vb.Net动作委托有问题吗？ [英] Vb.Net action delegate problem?

问题描述

是vb.net新手。这个问题可能是新手，以前曾回答过，但我找不到。我正在尝试使用lambda功能，并在这里碰到了。

Am vb.net newbie. This question might be very novice and answered before, but i couldn't find. I was trying the lambda features and got struck here.

 Private Function HigerOrderTest(highFunction as Func(Of Int16,Int16)) As Action(of String)
  Dim sam = highFunction(3)    
  Dim DoIt as Action(of String)
  DoIt = sub(s) console.WriteLine(s)            
    return DoIt  
 End Function

我在预期的表达式上获得了 DoIt = sub（s）控制台。WriteLine（s）。当我将其更改为DoIt = function（s）console.WriteLine（s）时，我得到 Expression不会产生值。错误。是什么问题？

I got "Expression expected." at line DoIt = sub(s) console.WriteLine(s). And when i changed this to DoIt = function(s) console.WriteLine(s) i got Expression does not produce a value. error. Whats the problem?

推荐答案

如果使用的是Visual Studio 2008（VB.NET 9），则VB中存在限制。 NET要求lambda表达式返回一个值，因此您不能使用 Sub 。这在VB.NET 10中已更改，因此在那种环境下，您的代码应该可以按预期工作。

If you are using Visual Studio 2008 (VB.NET 9), there is a limitation in VB.NET that requires that lambda expressions returns a value, so you cannot use Sub. This has changed in VB.NET 10, so in that environment your code should work as expected.

问题是，一方面需要将lambda表达式转换为功能，而不是 Sub ，而另一方面， Console.WriteLine 没有返回值。解决方案是将其包装到一个调用 Console.WriteLine 且返回值的函数中：

The problem is that on one hand you need to make your lambda expression into a Function, not a Sub, while on the other hand Console.WriteLine does not have a return value. The solution is to wrap this into a function that calls Console.WriteLine and returns a value:

Private Function ConsoleWriteLine(ByVal text As String) As String
    Console.WriteLine(text)
    Return text
End Function

然后您可以在lambda表达式中使用该函数：

Then you can use that function in your lambda expression:

Dim DoIt As Action(Of String)
DoIt = Function(s) ConsoleWriteLine(s)

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