无法将lambda表达式转换为委托类型，因为委托不是类型

Question

I'm having a problem that I can't seem to figure out, although its kind of a standard question here on StackOverflow.

base.Invoke(() => prgWork.Value = 100);

What am I doing wrong? When I compile it I get the error message:

Cannot convert lambda expression to type delegate because delegate is not a type

Answer 1

尝试这个：

base.Invoke((Action)(() => { prgWork.Value = 100; }));

Answer 2

Invoke takes a Delegate , rather than a typed Delegate to make the method as generic as possible. The problem is that type Delegate is an abstract type, and in order for the compiler to infer the number and Types of arguments or the return type of your lambda, you must provide a "concrete" Delegate type. A common solution to this is to wrap your lambda in an Action delegate, as Action derives from Delegate . In fact, you can pass any type that inherits from Delegate , directly or indirectly, including custom delegates you define in your project.

Try something like this:

base.Invoke(new Action(() => { prgWork.Value = 100; }));

If you find yourself using Invoke often, I would suggest creating an extension method such as this:

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

Or with C#6 syntax

public static void Invoke(this Control control, Action action) => control?.Invoke(action);

And now you will be able to use it like this:

someControl.Invoke(() => { prgWork.Value = 100; });

Here the compiler will create a new Action object for you, cleaning up your code. This also sheds some light on why your attempt did not work. You were basically asking the compiler to create an instance of the abstract class Delegate .