Recently at a job interview during a technical test I had to answer a question:

What is the difference between a delegate and a multicast delegate?

Unfortunately I did not have the answer, I have used delegates plenty but I have never obviously used them enough to know this difference, and the dissapointing thing is that the answer was glaringly obvious.

A multicast delegate is multiple instances of the same delegate chained together using the + operator, Events in .NET make use of this to attach multiple event handlers to an event.

An example: 

public delegate void SaySomething(string something);

 class Program
 {
     static void Main(string[] args)
     {
         // At first, something is just a delegate
         SaySomething something = new SaySomething((s) => Console.WriteLine(s));

         // By adding together more instances of the SaySomething delegate.
         // we get a multicast delegate
         something += new SaySomething((s) => Console.WriteLine(s.ToUpper()));
         something += new SaySomething((s) => Console.WriteLine(s.ToLower()));

         something("Hello");
     }
 } 

The output would give:

Hello
HELLO
hello

It pains me to think I have now been at this for 6 years and there are still aspects to C# that I have not yet come across, yet I do not think of myself as a bad developer, I am sure there are plenty of things that I am aware of that the interviewer has no idea about, and it makes me wonder if the whole point is to see areas of where I could improve my knowledge, or the purpose is to scrutinise my knowledge in a way that an incorrect answer would make me out to be some kind of fraud.

I guess at least now if this question comes up again I will be able to answer it.