The dictionary defines an interface as “a point where two systems, subjects, organizations, etc., meet and interact.” In C#, the interface concept can be applied to the interaction between classes where one class implements an interface. An interface can be thought of as a class specification in that it declares properties and methods, but you can’t instantiate an instance of an interface. An interface is declared with the keyword interface and, by convention, has an “I” prefixed to the name. A class implements an interface by appending the interface name to the class definition, similar to class inheritance. However, a class can implement multiple interfaces. In turn, interfaces can inherit properties and methods from multiple parent interfaces. A class must declare all the properties of all the interfaces it implements, including those inherited indirectly. The car class implements properties of IIceVehicle and ITransport. Interfaces can have methods with code called default interface methods, so a class doesn’t have to implement those methods but does have to implement declaration-only methods. Interface properties and methods implemented by a class are public.

The most common interface use case is mimicking multiple inheritance. Interface types can be used as method parameters or in property declarations. You can assign any object to a variable or parameter declared as an interface type, just as long as, at the very least, the object has the properties and methods of that interface, with the exception of default interface methods.

You could think of interfaces, like costumes, where you aren’t allowed entry to a theme party unless you wear the correct themed costume. Once at the party, you can only behave in a way consistent with the costume. If you go to a Star Wars themed party as R2D2, you must behave exactly as R2D2, i.e., no eating, drinking, or talking. Suppose you have a suitcase with multiple costumes, analogous to implementing multiple interfaces. In that case, you can leave the Star Wars party, put on a gorilla suit, and go to a Planet of the Apes party, where you must eat vegetarian food, drink and beat your chest.

My name is Hallam Webber, and I hope you’ve found this course on C# interfaces instructive and enjoyable. Please give this course a rating, and if you have any questions or comments, please let us know.