Support for generic sender parameter in EventHandler

[simplexidev]

When using event handlers EventHandler and EventHandler<TEventArgs>, it is not possible to pass a generic sender to the delegate without writing your own delegate type. There should be a event handler that has a second generic parameter to specify the type of sender.

Rationale/Usage

This is a nice-to-have that would reduce the number of custom event handers (that take just a sender and args) by exposing a new delegate, such as EventHandler<TSender, TEventArgs>.

For example, say I had a class with an event that was specific to it and it's derivatives:

public delegate void LoadingEventHandler(FooObject sender, EventArgs e);

public class FooObject
{
    public FooObject()
    {
        Loading.Invoke(this, EventArgs.Empty);
    }
    public event LoadingEventHandler Loading;
}

With a EventHandler<TSender, TEventArgs> delegate, the above could turn into:

public class FooObject
{
    public FooObject()
    {
        Loading.Invoke(this, EventArgs.Empty);
    }
    public event EventHandler<FooObject, EventArgs> Loading;
}

Proposed API

namespace System
{
    /// <summary>
    /// Represents the method that will handle an event when the event provides data.
    /// </summary>
    /// <typeparam name="TSender">The type of sender object.</typeparam>
    /// <typeparam name="TEventArgs">The type of the event data generated by the event.</typeparam>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">An object that contains the event data.</param>
    public delegate void EventHandler<TSender, TEventArgs>(TSender sender, TEventArgs e)
        where TSender : object;
}

Updates

• Updated this issue to follow the API review process/other guidelines.
• Updated the Proposed API with type parameters.
• Updated the Proposed API: Removed type parameter constraints to align with the existing EventHandler implementations, as TEventArgs does not have to be of type EventArgs. The type constraint for TSender remains as object.

[utkumaden]

This would cause a huge change in the framework because you can't do optional generics (to my knowledge). Could break a lot of things.

[simplexidev]

@utkumaden I'm not sure it would break anything, as you would have to specify TSender as well as TEventArgs. I don't think this addition would have any effect on existing types using EventHandler or EventHandler<TEventArgs>.

[utkumaden]

@tom-corwin, now that you put it that way it makes more sense.

[joshfree]

@jeffschwMSFT @KKhurin

[adrientetar]

winrt has TypedEventHandler<TSender, TArgs> for this… it's not present in .NET. Maybe it could be added which would also make things more consistent?

[maxkatz6]

Any updates on the issue?

[simplexidev]

Whether we bring overTypedEventHandler or have a new delegate for EventHandler, I would be happy with it. Are there any further updates on this issue?