Singleton
Factory method
Publish/subscribe
Delegates are used to code the publish/subscribe pattern. The subscriber creates a delegate and registers it with the publisher. The publisher notifies the subscribers by invoking through the delegate as needed.

The types of the objects involved in the registration and callback
The name of the callback method
The parameter list and return type of the callback method
The delegate describes the interaction between the two parties by specifying the type of information that flows between them; that is, the delegate specifies the parameters and return type of the callback.

None - only an event can have registered subscribers
One
As many as needed
A delegate can handle as many subscribers as needed.

Zero
null
Empty string
Delegates are references types, so they have a default value of null.

The delegate is automatically made public
Client code is limited to accessing the delegate using only the += and -= operators
The delegate is limited to handling only one subscriber
The event keyword prevents client code from having direct access to a delegate. In particular, the client code cannot directly assign to the delegate nor invoke the delegate. The client is limited to registering using the += operator or de-registering using the -= operator.