问题描述

我有一个类，用户可以将 Action 传递给(或不传递给).

I have a class that the user can pass an Action into (or not).

public class FooClass<T> : BaseClass<T>
{
    public FooClass()
        : this((o) => ()) //This doesn't work...
    {
    }

    public FooClass(Action<T> myAction)
        : base(myAction)
    {
    }
}

<小时>基本上，我不能将 null 传递给 Action 的基类.但是，与此同时，我不想强​​迫我的用户传入 Action.相反，我希望能够即时创建无所事事"动作.

推荐答案

你想说

this(t => { })

这样想.你需要 t =>匿名表达式主体.在这种情况下，anonymous-expression-body 是 expression 或 block.你不能有一个空的表达式，所以你不能用 expression 来表示一个空的方法体.因此，你必须使用一个 block 在这种情况下你可以说 { } 来表示一个 block 有一个空的 statement-列表，因此是空的主体.

Think of it like this. You need t => anonymous-expression-body. In this case, anonymous-expression-body is an expression or a block. You can't have an empty expression so you can't indicate an empty method body with an expression. Therefore, you have to use a block in which case you can say { } to indicate the a block has an empty statement-list and is therefore the empty body.

详见语法规范，附录B.

For details, see the grammar specification, appendix B.

这是另一种思考方式，您可以用这种方式自己发现这一点.Action<T> 是一个接受 T 并返回 void 的方法.您可以通过非匿名方法或匿名方法定义 Action<T>.您正试图弄清楚如何使用匿名方法(或者更确切地说，一种非常特殊的匿名方法，即 lambda 表达式)来完成它.如果你想通过非匿名方法做到这一点，你会说

And here's another way to think of it, which is a way that you could use to discover this for yourself. An Action<T> is a method that takes in a T and returns void. You can define an Action<T> via an non-anonymous method or via an anonymous method. You are trying to figure out how to do it using an anonymous method (or rather, a very special anonymous method, namely a lambda expression). If you wanted to do this via a non-anonymous method you would say

private void MyAction<T>(T t) { }

然后你可以说

this(MyAction)

它使用方法组的概念.但是现在您想将其转换为 lambda 表达式.所以，让我们把那个方法体变成一个 lambda 表达式.因此，我们丢弃 private void MyAction(T t) 并用 t => 替换它并逐字复制方法主体 { }.

this(t => { })