如何转换EventHandler <T> 到EventHandler <EventArgs> ？

Question

I'm creating a centralized event aggregator that keeps a registry that accepts EventHandler delegates. The aggregator class itself is not generic, but the register method is:

public void RegisterHandler<T>(EventHandler<T> handler) where T : EventArgs
{
    Debug.Assert(handler != null);
    if (handler != null)
    {
        if (this._eventRegistry != null)
        {
            this._eventRegistry.AddValue(typeof(T), handler);
        }
        else { /* Do nothing. */ }
    }
    else 
    { 
        throw new ArgumentNullException(ReflectionUtilities.GetVariableName(() => handler)); 
    }
}

The aim is to allow registry of any type that derives from EventArgs. But the registry (a type of Dictionary) accepts EventHandler of type EventArgs and not EventHandler of type T where T is EventArgs.

The line

this._eventRegistry.AddValue(typeof(T), handler);

will not compile because it says it has some invalid arguments (specifically 'handler'). If I change that argument to

this._eventRegistry.AddValue(typeof(T), handler as EventHandler<EventArgs>);

then the code compiles but the argument 'handler' has a null value.

Similarly, if I do this:

EventHandler<EventArgs> eventHandler = handler as EventHandler<EventArgs>;

then 'eventHandler' is null.

If I know that T is constrained to be type or subtype of EventArgs, how can I cast

EventHandler<T>

to

EventHandler<EventArgs>

?

Answer 1

You can't cast an EventHandler<T> to an EventHandler<EventArgs> . A somewhat ugly but simple workaround, is to change this line of your code:

this._eventRegistry.AddValue(typeof(T), handler);

to

this._eventRegistry.AddValue(typeof(T), (sender, evt) => handler(sender, (T)evt));