When to Use Delegates Instead of Interfaces (C# Programming Guide)
Both delegates and interfaces enable a class designer to separate type declarations and implementation. A given interface can be inherited and implemented by any class or struct. A delegate can be created for a method on any class, as long as the method fits the method signature for the delegate. An interface reference or a delegate can be used by an object that has no knowledge of the class that implements the interface or delegate method. Given these similarities, when should a class designer use a delegate and when should it use an interface?
Use a delegate in the following circumstances:
An eventing design pattern is used.
It is desirable to encapsulate a static method.
The caller has no need to access other properties, methods, or interfaces on the object implementing the method.
Easy composition is desired.
A class may need more than one implementation of the method.
Use an interface in the following circumstances:
There is a group of related methods that may be called.
A class only needs one implementation of the method.
The class using the interface will want to cast that interface to other interface or class types.
The method being implemented is linked to the type or identity of the class: for example, comparison methods.
One good example of using a single-method interface instead of a delegate is IComparable or the generic version, IComparable<T>. IComparable declares the CompareTo method, which returns an integer that specifies a less than, equal to, or greater than relationship between two objects of the same type. IComparable can be used as the basis of a sort algorithm. Although using a delegate comparison method as the basis of a sort algorithm would be valid, it is not ideal. Because the ability to compare belongs to the class and the comparison algorithm does not change at run time, a single-method interface is ideal.