使用匿名方法订阅事件

Question

I am having problems understanding how the below works (questions below code):

private delegate void MyDelegate(int i);
private event MyDelegate myEvent;

public void EventTests() {
    //Option One
    myEvent += One;

    //Option Two
    myEvent += delegate{ Two(true); };

    //Option Three
    myEvent += () => { Two(true); };
}

private void One(int i) { }

private void Two(bool j) { }

Questions:

1. I can understand why Option One works, as the event myEvent expects a delegate with an int parameter to be attached to it. Why does Option Two work though? it is attaching an anonymous method with incorrect signature to the event, no?

2. So if Option Two works, why does option Three not work? It seems that the signature needs to be (int i) => { Two(true); }; (int i) => { Two(true); }; as opposed to () as written above. But Option Two worked without the right signature, so why does this anonymous method cause an error?

Thanks a lot.

Answer 1

Option two works because the compiler automatically figures out what the function signature is when the parameter list is omitted. Once you add the () to the delegate keyword, you've defined a specific parameter list of none and the compiler throws a fit.

Option three is a lambda expression with an incorrect parameter list defined.

Microsoft's C# Programming Guide states:

There is one case in which an anonymous method provides functionality not found in lambda expressions. Anonymous methods enable you to omit the parameter list. This means that an anonymous method can be converted to delegates with a variety of signatures. This is not possible with lambda expressions.