Calling C# events from outside the owning class?

Question

Is it possible under any set of circumstances to be able to accomplish this?

My current circumstances are this:

public class CustomForm : Form
{
    public class CustomGUIElement
    {
    ...
        public event MouseEventHandler Click;
        // etc, and so forth.
    ...
    }

    private List<CustomGUIElement> _elements;

    ...

    public void CustomForm_Click(object sender, MouseEventArgs e)
    {
        // we might want to call one of the _elements[n].Click in here
        // but we can't because we aren't in the same class.
    }
}

My first thought was to have a function similar to:

internal enum GUIElementHandlers { Click, ... }
internal void CustomGUIElement::CallHandler(GUIElementHandler h, object[] args) {
    switch (h) {
        case Click:
            this.Click(this, (EventArgs)args[0]);
            break;
        ... // etc and so forth
    }
}

It's a horribly ugly kludge, but it should work... There must be a more elegant solution though? The .NET library does this all the time with message handlers and calling events in Control's. Does anyone else have any other/better ideas?

Answer 1

You really should wrap the code you want to be able to execute from the outside in a method. That method can then do whatever your event would do - and that event would also instead call that method.

Answer 2

You just need to add a public method for invoking the event. Microsoft already does this for some events such as PerformClick for controls that expose a Click event.

public class CustomGUIElement    
{
    public void PerformClick()
    {
        OnClick(EventArgs.Empty);
    }

    protected virtual void OnClick(EventArgs e)
    {
        if (Click != null)
            Click(this, e);
    }
}

You would then do the following inside your example event handler...

public void CustomForm_Click(object sender, MouseEventArgs e)        
{
    _elements[0].PerformClick();
}

Answer 3

The event keyword in c# modifies the declaration of the delegate. It prevents direct assignment to the delegate (you can only use += and -= on an event), and it prevents invocation of the delegate from outside the class.

So you could alter your code to look like this:

public class CustomGUIElement
{
...
    public MouseEventHandler Click;
    // etc, and so forth.
...
}

Then you can invoke the event from outside the class like this.

myCustomGUIElement.Click(sender,args);

The drawback is that code using the class can overwrite any registered handlers very easily with code like this:

myCustomGUIElement.Click = null;

which is not allowed if the Click delegate is declared as an event.