Question

I recently attended a interview in C# where i was asked a question about Events and delegates in C#

The person asked me when a event say button gets clicked, which gets called first event or delegate?

Does the delegate calls the event or the event calls delegate?

Can we have a event without a delegate in c#?

Answer 1

An event is simply a code construct implemented in .NET as a multi-cast delegate.

When an event is “raised” (which can only be done by code in the same class as the event; event raising must happen within “protected” scope), the delegates are invoked one at a time, in a synchronous fashion but not necessarily in any deterministic order. The event IS the delegate, so when the event is raised for a button being clicked, the delegates are invoked by the runtime, which has received the Windows message that the user clicked on the GUI area for the button.

The statements “the event is raised” and “the delegates are invoked” are equivalent statements; it’s like asking “which comes first, the chicken or the gallus domesticus?”.

Now, events often cascade, especially when we’re talking about UI. There is a MouseUp event, invoked when the mouse button is released, which can fire one or more other events such as MouseClick, MouseDoubleClick, DragDrop, etc. You may not attach a handler to the MouseUp event, but there is built-in logic behind the scenes of MouseUp to raise the MouseClick event which you DO handle. So, in this sense, you could say that the MouseUp event “comes first” and calls the MouseClick handler delegate.