The behavior and state that a class provides, which is matched with what a client of that class can expect to hold. A contract is expressed partly by the signatures for all public fields, methods, properties, and events of that class. This is augmented by a description (usually in simple descriptive text) of what each field or property represents, together with what each method does. In the .NET Framework add-in programming model, a non-versioning interface that defines the methods and specifies the data types for transferring data over the communication pipeline between the host and the add-in. The contract is in the middle of the communication pipeline between the add-in and the host. The contract assembly is loaded into the application domains of both the add-in and the host.