Confusion over `Action` delegate and lambda expres

问题:

private void StringAction(string aString) // method to be called
{
    return;
}

private void TestDelegateStatement1() // doesn't work
{
    var stringAction = new System.Action(StringAction("a string"));
    // Error: "Method expected"
}

private void TestDelegateStatement2() // doesn't work
{
    var stringAction = new System.Action(param => StringAction("a string"));
    // Error: "System.Argument doesn't take 1 arguments"

    stringAction();
}

private void TestDelegateStatement3() // this is ok
{
    var stringAction = new System.Action(StringActionCaller);

    stringAction();
}

private void StringActionCaller()
{
    StringAction("a string");
}

I don't understand why TestDelegateStatement3 works but TestDelegateStatement1 fails. In both cases, Action is supplied with a method that takes zero parameters. They may call a method that takes a single parameter (aString), but that should be irrelevant. They don't take a parameter. Is this just not possible to do with lamda expressions, or am I doing something wrong?

回答1:

As you said, Action doesn't take any parameters. If you do this:

var stringAction = new System.Action(StringAction("a string"));

You actually execute the method here, so that is not a method parameter.

if you do this:

var stringAction = new System.Action(param => StringAction("a string"));

you tell it that your method takes a parameter called param, which Action does not.

So the correct way to do this would be:

var stringAction = new System.Action( () => StringAction("a string"));

or more compact:

Action stringAction = () => StringAction("a string");

the empty brackets are used to indicate the lambda doesn't take any parameters.

回答2:

Action delegate is defined as delegate to method, that has no parameters and returns void. In sample 1, you are making 2 mistakes:
1. You are trying to give method, that takes parameter
2. You are invoking the method, and not giving it as a parameter (it should be new Action(methodName)), although, it wouldn't work because of 1.

In sample 2, you are making the same mistake again, your lambda is taking a parameter, you should write it like this:
new Action(() => StringAction("a string"));

If you want to create a delegate, that will take a parameter, you should do it like this:
new Action<string>(myStringParam => StringAction(myStringParam));

So, in your case, complete code would look like this:

private void StringAction(string aString) // method to be called
{
    return;
}

private void TestDelegateStatement1() // now it works
{
    var stringAction = new Action<string>(StringAction);
    //You can call it now:
    stringAction("my string");
}

private void TestDelegateStatement2() // now it works
{
    var stringAction = () => StringAction("a string");
    //Or the same, with a param:
    var stringActionParam = (param) => StringAction(param);

    //You can now call both:
    stringAction();
    stringActionParam("my string");
}

private void TestDelegateStatement3() // this is ok
{
    var stringAction = new System.Action(StringActionCaller);

    stringAction();
}

private void StringActionCaller()
{
    StringAction("a string");
}

回答3:

I'm not an expert on this, but have you tried this?

public void TestDelegateStatement4
{
    var stringAction = () => StringAction("a string");
}

回答4:

In C# 2.0, the Action delegate is a void delegate that doesn't accept parameters. In later versions, there's the generic Action<T> delegate, where T specifies the parameter type.

This should work:

var stringAction = new Action<string>(param => StringAction(param));

or even better:

var stringAction = new Action<string>(StringAction); // using method group conversion

then, you can call

stringAction("Hello world");