Why events does not support binding inherited types?

In these delegates:

EventHandler

public delegate void EventHandler(object sender, EventArgs e);

FormClosingEventHandler

public delegate void FormClosingEventHandler(object sender, FormClosingEventArgs e);

FormClosingEventArgs is inherited from EventArgs . Why can't I bind an FormClosing event with an event handler from EventHandler delegate?

I know that the event handler's signiture must match its delegate, but why it doesn't suppot matching inherited types ?

Well, it's interesting...

You can bind an eventhandler using a method group conversion with compatible types:

public void GenericHandlerMethod(object sender, EventArgs e) {}

...
// Valid
foo.FormClosingEvent += GenericHandlerMethod;

This will actually create an instance of FormClosingEventHandler , not EventHandler .

However you can't subscribe directly with an existing delegate of type EventHandler:

EventHandler genericHandler = GenericHandlerMethod;
// Invalid
foo.FormClosingEvent += genericHandler;

... but you can create a new delegate based on an existing one, if the types are compatible:

EventHandler generic = GenericHandlerMethod;
FormClosingEventHandler closingHandler = new FormClosingEventHandler(generic);
// Valid
foo.FormClosingEvent += closingHandler;

Basically you need to remember that all the syntactic sugar is effectively calling a method like this:

foo.AddFormClosingHandler(handler);

where the method has a signature of:

public void AddFormClosingHandler(FormClosingHandler handler)

Now remember that although they have compatible signatures, there's no reference conversion available from EventHandler to FormClosingHandler . It's not like one inherits from the other.

It gets even more confusing with generic covariance/contravariance, but we'll leave it there for now... hopefully that's given you something to chew on and options for working round the restrictions.

A delegate which points directly to an object method holds three pieces of information: