Question

I’m working on an form custom control.the control is a MonthCalendar like Visual studio(C#) MonthCalendar control and I want to define an event for my control.
How can define a new event for this form custom control?

Answer 1

If your event should not provide any additional info (Foo is name of your event):

public event EventHandler Foo;

And raise it this way:

protected virtual void OnFoo()
{
    if (Foo != null)
        Foo(this, EventArgs.Empty);
}

If you need to pass some additional info to event handlers, then create custom arguments class by inheriting from EvenArgs class

public class FooEventArgs : EventArgs
{
    public string Message { get; private set; }

    public FooEventArgs(string message)
    {
        Message = message;
    }
}

Declare event this way:

public event EventHandler<FooEventArgs> Foo;

And raise it this way:

protected virtual void OnFoo(string message)
{
   if (Foo != null)
       Foo(this, new FooEventArgs(message));
}

Its good practice to create protected methods for raising events by descendants of class where event is declared. Also good practice to use event naming convention:

• add suffix -ing to event name for events which raised before
  something happened (often you could cancel such events) (e.g. Validating)
• add suffix -ed to event name for events which raised after something happened (e.g. Clicked)

As Thorsten stated, good practice to create virtual methods for raising events. Its not only allows to raise events from descendants, but also disable event raising, or adding some behavior prior/after event raising.