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.
Extracted from Microsoft Technet articles.
