参数类型“ void”不能分配给参数类型“ System.Action” [英] Argument type 'void' is not assignable to parameter type 'System.Action'

问题描述

这是我的测试代码：

class PassingInActionStatement
{
    static void Main(string[] args)
    {
        var dsufac = new DoSomethingUsefulForAChange();

        dsufac.Do(WriteToConsole);
        dsufac.Do2(s => WriteToConsoleWithSomethingExtra("Test"));
        dsufac.Do(WriteToConsoleWithSomethingExtra("Test")); // Does not compile
    }

    internal static void WriteToConsole()
    {
        Console.WriteLine("Done");
    }

    internal static void WriteToConsoleWithSomethingExtra(String input)
    {
        Console.WriteLine(input);
    }
}

internal class DoSomethingUsefulForAChange
{
    internal void Do(Action action)
    {
        action();
    }

    internal void Do2(Action<String> action)
    {
        action("");
    }
}

前2个通话有效，但我想知道为什么第三个人没有。我不喜欢Do2中的代码，因为奇怪的是我在那里键入了 action（）类型，以便使其正常工作。

The first 2 calls work but I am wondering why the 3rd one does not. I do not fancy the code inside Do2 as it seems strange that I have type type action("") in there in order to get it to work.

有人可以解释我不明白的两件事吗？

Could someone please explain the 2 things I do not understand please?

1. 为什么我不能写第三行调用Do


2. 为什么我必须编写action（）才能使其在Do2中工作

推荐答案

dsufac.Do(WriteToConsoleWithSomethingExtra("Test"));

实际上首先调用该函数（ WriteToConsoleWithSomethingExtra（ Test）），然后尝试将结果传递到 Do 中。由于没有结果（ void ），所以不可能。

actually calls the function first (WriteToConsoleWithSomethingExtra("Test")) and then attempts to pass the result into Do. Since there is no result (void), it's not possible.

您真正想要的是：

dsufac.Do(() => WriteToConsoleWithSomethingExtra("Test"));

内部声明一个不带任何内容的函数（（）=> 位），在执行时调用 WriteToConsoleWithSomethingExtra（ Test）。 然后您的 dsufac.Do 呼叫将收到预期的操作。

The inner part declares a function that takes nothing (the () => bit), which calls WriteToConsoleWithSomethingExtra("Test") when executed. Then your dsufac.Do call will receive an action, like it expects.

至于 Do2 -您已经声明它采取了 Action< String> ，这意味着 action 是一个采用一个参数的函数。您必须将其传递一个字符串。该字符串可能为空，例如您的 action（）示例，或者它可能是从外部传递的，例如：

As for Do2 - you've declared it as taking Action<String>, which means that action is a function that takes one argument. You have to pass it a string. That string might be empty, like in your action("") example, or it might be passed in externally, as in something like this:

dsufac.Do3(WriteToConsole, "Test");

...

internal void Do3(Action<String> action, String str)
{
    action(str);
}

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