Question

I would pass a control of my form to another class where I will be creating events for the control, its parent control etc. I also need to detach those events at some point. But I need to ensure I wont be duplicating the events, if the event is already attached.

So I always attach events like this, for eg:

internal static void X(Control c, MouseEventHandler mouseDownEvent)
{
    c.TopLevelControl.MouseDown -= mouseDownEvent;
    c.TopLevelControl.MouseDown += mouseDownEvent;
}

Now I would need similar event attaching for other handlers too. For eg,

internal static void X(Control c, EventHandler event)
{
    c.Enter -= event;
    c.Enter += event;
}

Now I dont want to spray around this -= and += all around, instead would like to have one simple utility function so that I can call it everywhere.

Something like:

internal static void AttachEvent(this Control c, 
                                 Func<Control, MouseEventHandler> e, 
                                 MouseEventHandler m)
{
    e(c) -= m;
    e(c) += m;
}

So that I can call:

AttachEvent(c, control => control.MouseDown, mouseDownEvent);

But this wouldn’t compile, I get two errors:

The event 'System.Windows.Forms.Control.MouseDown' can only appear on the left hand side of += or -=

and

The left-hand side of an assignment must be a variable, property or indexer

I would like to have the AttachEvent take any event as input argument, but if that’s too complicated, I can live with the MouseEvents alone.

Answer 1

I think this is not possible. Event in C# – like a property – consists of two methods: add and remove. So if you expand event in two methods in your mind

e.add_MouseOver(Delegate)
e.remove_MouseOver(Delegate) 

you will see it is not possible to pass reference to event and do something with it in other method.

Also += is part of C# syntax and only works with referencing event at left side of expression.