Overview

• The interface implementation relationship is a “can do” relationship: The class can do what the interface requires.
• The interface defines the contract between the classes that implement the interface and the classes that use the interface.

Interface Implementation

• Declaring a class to implement an interface is similar to deriving from a base class.
  • The only difference is that classes can implement multiple interfaces.

public class Contact : PdaItem, IListable, IComparable

• Interfaces can never be instantiated; and therefore, interfaces cannot have constructors or finalizers.
• Interface instances are available only from types that implement them.
• Interfaces cannot include static members.
• It is not possible to use the abstract modifier on interface members explicitly.
• Extension methods work with interfaces in addition to classes.

Implicit Member Implementation

• With implicit member implementation, you access the interface methods and properties as if they were part of the class.

public class Test : ITest
{
    public string Id => "Implicit";
}

• Implicit member implementation doesn’t require a cast because the member is not hidden from direct invocation on the implementing class.
• Implicit member implementations must be public.
• virtual is optional depending on whether derived classes may override the implementation.
  • Eliminating virtual will cause the member to behave as though it is sealed.
• override is not allowed because the interface declaration of the member does not include implementation, so override is not meaningful.

Explicit Member Implementation

• Explicitly implemented methods are available only by calling through the interface itself.
  • This is typically achieved by casting an object to the interface.