Confusion over `Action` delegate and lambda expressions

Question

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?

Answer 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.