当作为普通委托参数提供时，为什么必须强制转换 lambda 表达式

问题描述

取方法System.Windows.Forms.Control.Invoke(Delegate方法)

Take the method System.Windows.Forms.Control.Invoke(Delegate method)

为什么会出现编译时错误:

Why does this give a compile time error:

string str = "woop";
Invoke(() => this.Text = str);
// Error: Cannot convert lambda expression to type 'System.Delegate'
// because it is not a delegate type

但这工作正常:

string str = "woop";
Invoke((Action)(() => this.Text = str));

什么时候方法需要一个普通的委托?

When the method expects a plain Delegate?

推荐答案

lambda 表达式可以转换为委托类型或表达式树 - 但它必须知道哪个委托类型.仅仅知道签名是不够的.例如，假设我有:

A lambda expression can either be converted to a delegate type or an expression tree - but it has to know which delegate type. Just knowing the signature isn't enough. For instance, suppose I have:

public delegate void Action1();
public delegate void Action2();

...

Delegate x = () => Console.WriteLine("hi");

您希望 x 引用的对象的具体类型是什么?是的，编译器可以生成具有适当签名的新委托类型，但这很少有用，而且您最终检查错误的机会更少.

What would you expect the concrete type of the object referred to by x to be? Yes, the compiler could generate a new delegate type with an appropriate signature, but that's rarely useful and you end up with less opportunity for error checking.

如果你想让 Control.Invoke 使用 Action 更容易调用，最简单的方法是向 Control 添加扩展方法:

If you want to make it easy to call Control.Invoke with an Action the easiest thing to do is add an extension method to Control:

public static void Invoke(this Control control, Action action)
{
    control.Invoke((Delegate) action);
}

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