Question

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 ?

Answer 1

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.