Windows窗体中的通用事件处理程序的解决方法

Question

Quite some time ago, I noticed that the Windows Forms editor of Visual Studio does not support events which contain generic type parameters. For example, an event like

public event EventHandler<ListEventArgs<int>> MyStrangeEvent { add { ... } remove { ... } }

where

public class ListEventArgs<T> : EventArgs { List<T> args; }

does not even show up in the event list in the property manager of Visual Studio. Now, this is a somewhat artificial example that could easily be modified to work in Visual Studio by rewriting the classes and their events. However, I am currently working on a project where I cannot change some classes for compatibility reasons. The only thing I can do is to change the events of my user control. The events of this control currently look like this:

public event EventHandler<Plane<GDISurface>.DrawingErrorEventArgs> DrawingError { add { _Plane.DrawingError += value; } remove { _Plane.DrawingError -= value; } }

Note that the underlying Plane class (represented by the _Plane instance which is a protected field) cannot be changed. Its DrawingError event and its EventArgs type are declared in the Plane class like this:

public class Plane<T> where T : ISurface
{
    ...
    public event EventHandler<DrawingErrorEventArgs> DrawingError = null;
    ...
    public class DrawingErrorEventArgs : EventArgs { ... /* Uses T */ ... }
}

Of course, the Windows Forms editor of Visual Studio does not show any of the events of my user control. I have been looking for a number of workarounds to get them shown again, but have not been able to find a workaround that actually works. Here are some things that I tried:

1. Created a MyPlane class which inherits from Plane and used that instead: public event EventHandler<MyPlane.DrawingErrorEventArgs> DrawingError ... . For reasons unknown to me, the events still don't show up in the editor. Perhaps this is due to the parameters of the event, some of which still are generic. Find a minimal working example below.
2. Created a helper class which defines implicit conversion operators between EventHandler<Plane<GDISurface>.DrawingErrorEventArgs> and EventHandler<GDIPlane.DrawingErrorEventArgs> where GDIPlane is just a dummy class which inherits from Plane<GDISurface> . This does work to some extent, but duplicates event calls since the conversion creates new event handlers which are passed down to _Plane which cannot be removed/unregistered properly.
3. Tried to inherit from EventHandler<Plane<GDISurface>.DrawingErrorEventArgs> , which obviously does not work since EventHandler<T> is sealed.

Are there any other ways to make my events visible again in the Windows Forms editor?

Best regards Andreas

EDIT: Minimal working example for 1:

public interface ISurface { }

public class GDISurface : ISurface { }

public class Plane<T> where T : ISurface
{
    public event EventHandler<DrawingErrorEventArgs> DrawingError = null;
    public class DrawingErrorEventArgs : EventArgs { T stuff; }
}

public class TestControl : UserControl
{
    public class GDIPlane : Plane<GDISurface>  { }
    GDIPlane _Plane = null;
    public event EventHandler<GDIPlane.DrawingErrorEventArgs> DrawingError { add { _Plane.DrawingError += value; } remove { _Plane.DrawingError -= value; } }
}

DrawingError does not show up in the list of events in the property manager when clicking on a TestControl instance.

EDIT2: This is the original problem (without any workarounds) where the DrawingError event does of TestControl does not show up either:

public interface ISurface { }

public class GDISurface : ISurface { }

public class Plane<T> where T : ISurface
{
    public event EventHandler<DrawingErrorEventArgs> DrawingError = null;
    public class DrawingErrorEventArgs : EventArgs { T stuff; }
}

public class TestControl : UserControl
{
    Plane<GDISurface> _Plane = null;
    public event EventHandler<Plane<GDISurface>.DrawingErrorEventArgs> DrawingError { add { _Plane.DrawingError += value; } remove { _Plane.DrawingError -= value; } }
}

Answer 1

This is behavior specific to Visual Studio, and the cause is rooted in the fact that EventHandler<> does not specify covariance on its 'TEventArgs' (it would impose seemingly silly restrictions) and the tools do not perform enough introspection of your code to suss out an appropriate type (even though you've left a trail of type data in constructing the control.) Thus, it seems as though VS does not support generic event properties. You may consider filing a feature request on Microsoft Connect , I wouldn't suggest filing it as a bug as they may label it "by design" and close it.

As a general rule, if you need generic type parameters on your events and you need design time support for them (which are different implementation concerns), you're looking at wrapping them in a presentation-specific facade (eg "extra layer of code to facilitate design-time needs".)

Personally, I would reduce the generic typing you have in play now, it seems a bit excessive and if you don't understand covariance/contravariance in generic types it might put you in a tight spot at some point, such as now.

However, to work around your problem:

Consider using a custom event args class which could transport data in a non-generic property, and also use a non-generic EventHandler event/property. Understanding the 'type' of the event is then shifted away from generic type parameters and made the responsibility of your non-generic event args instead. If the 'class' of the event args is insufficient, you can add a property to convey the event type (or data type) so that receiving code can properly interpret it (assuming, of course, that it does not already know by some other means.):

public class DataEventArgs : EventArgs
{
    //public string EventTypeOrPurpose { get; set; }
    public object Data { get; set; }
}

This is most often only used to ferry data through an event chain, and it is usually implemented as follows:

public class DataEventArgs<T> : EventArgs
{
    public T Data { get; set; }
}

Unfortunately, this also has a covariance problem, to resolve it you would actually want something more like this:

public interface IDataArgs<out T>
{
    T Data { get; }
}

public class DataEventArgs<T> : EventArgs, IDataArgs<T>
{
    public DataEventArgs<T>(T data) 
    {
        _data = data;
    }
    private T _data;
    public T Data { get { return _data; } }
}

Even so, these generic versions still don't work around Visual Studio's limitations , this is merely more proper alternative forms of what you already have shown us.

UPDATE: As requested, here is what a "purpose built facade" might look like in the most basic sense. Note that the usercontrol functions as a facade layer in this case as the eventhandler it exposes delegates to the underlying object model. There is no direct access to underlying object model from the user control (from consumer/designer perspective.)

Please note the reference tracking for event handlers is not necessary unless you dispose of these user controls throughout the lifetime of the app (it is only done to ensure proper delegate removal based on the delegate provided, which is wrapped in a closure/delegate, as you see below.)

Also worth noting I did not test-run this code beyond verifying that the designer shows DrawingError in the property grid when dropped onto a form.

namespace SampleCase3
{
    public interface ISurface { }

    public class GDISurface : ISurface { }

    public class Plane<T> where T : ISurface
    {
        public event EventHandler<DrawingErrorEventArgs> DrawingError;
        public class DrawingErrorEventArgs : EventArgs { T stuff; }
    }

    public class TestControl : UserControl
    {
        private Plane<GDISurface> _Plane = new Plane<GDISurface>(); // requires initialization for my own testing

        public TestControl()
        {
        }

        // i am adding this map *only* so that the removal of an event handler can be done properly
        private Dictionary<EventHandler, EventHandler<Plane<GDISurface>.DrawingErrorEventArgs>> _cleanupMap = new Dictionary<EventHandler, EventHandler<Plane<GDISurface>.DrawingErrorEventArgs>>();

        public event EventHandler DrawingError
        {
            add
            {
                var nonGenericHandler = value;
                var genericHandler = (EventHandler<Plane<GDISurface>.DrawingErrorEventArgs>)delegate(object sender, Plane<GDISurface>.DrawingErrorEventArgs e)
                {
                    nonGenericHandler(sender, e);
                };
                _Plane.DrawingError += genericHandler;
                _cleanupMap[nonGenericHandler] = genericHandler;
            }
            remove
            {
                var nonGenericHandler = value;
                var genericHandler = default(EventHandler<Plane<GDISurface>.DrawingErrorEventArgs>);
                if (_cleanupMap.TryGetValue(nonGenericHandler, out genericHandler))
                {
                    _Plane.DrawingError -= genericHandler;
                    _cleanupMap.Remove(nonGenericHandler);
                }
            }
        }
    }
}

To complement the above, here is what a non-generic event handler would now look like:

private void testControl1_DrawingError(object sender, EventArgs e)
{
    var genericDrawingErrorEventArgs = e as Plane<GDISurface>.DrawingErrorEventArgs;
    if (genericDrawingErrorEventArgs != null)
    {
        // TODO:
    }
}

Note that the consumer here has to have knowledge of the type for e to perform conversion. The use of the as operator will bypass ancestry checks under the assumption that the conversion should succeed.

Something like this is as close as you're going to get. Yes it is ugly by most of our standards, but if you absolutely 'need' design-time support on top of these components and you cannot change Plane<T> (which would be more appropriate) then this, or something close to this, is the only viable workaround.

HTH