5.1. Interfaces Define Types

An interface declaration defines a reference type. Within variables of this type, you can store a reference to an object on the garbage collected heap that implements the contract of the interface type. Each variable in the CLR is stored in a storage location, whether it be on the heap or on the stack. Each of these storage locations has a type associated with it. Therefore, an interface type can describe the type associated with a specific storage location. When a variable—say, a reference to an object—is stored in that location, it must be the same type as the location, or it must be convertible to the type attached to the location. If it can be converted automatically to the type of the location, then it is implicitly convertible to the storage location type. If it requires a specific cast syntax to perform the conversion, then it is explicitly convertible to the storage location type.

Many examples use a fictitious GUI framework as their basis for demonstration purposes, so I'll do the same here. Take a look at the following code snippet: