Question

Assuming we declare a class DerivedEventArgs:

public class DerivedEventArgs : EventArgs { ... }

then EventHandler delegate is able to accept methods with the following signature:

public static void Some_Method(object o, DerivedEventArgs e) { ... }

But if we try to subscribe a method with the above signature to the event implementing EventHandler delegate:

public event EventHandler MyEvent;

, then we get an error. Why is that?

thanx

Answer 1

Suppose the code raising the event specified a value which wasn’t a DerivedEventArgs – what would you expect it to do? Basically you’d lose type safety.

EDIT Note that you can do it the other way round – you can subscribe to an event with a more specific parameter type using a method with a less specific parameter type – because the event is still guaranteeing that it will call the handler with something compatible. Here’s an example:

using System;

class Test
{
    public class DerivedEventArgs : EventArgs { }

    public EventHandler<DerivedEventArgs> SpecialistEvent;

    static void Main()
    {
        Test t = new Test();
        t.SpecialistEvent += GeneralHandler;
    }

    static void GeneralHandler(object sender, EventArgs e)
    {
    }
}