Contracts often occur in business affairs to restrict the operations between the operators that are working with each other. For distributed communication services, Contracts also play a very important role in making sure that the service consumers can co-operate with the service providers correctly. Looking around, we can see the term SOA (Service-Oriented Architecture) being widely used. Technically speaking, SOAP (Simple Object Access Protocol) can be explained as a set of components that can be invoked, and whose interface descriptions can be published and discovered. From an SOA perspective, with Contracts properly defined, service consumers can get an idea of how to work with the target service without knowing how the service is actually implemented.

As a unified communication programming platform, WCF provides complete support for Contract-related design in various parts of WCF service development. These include ServiceContract, OperationContract, DataContract, MessageContract, FaultContract, and so on. ServiceContract and OperationContract are used to represent a Service and its operations' definition (such as the operation collection and operation signatures). DataContract is used to represent an agreement of the data that will be exchanged between the service client and server. If the service designer wants to take full control over the data envelope transferred between client and server, they can use MessageContract to control the underlying service messages. WCF also provides FaultContract for the service designer to declaratively associate custom Exception types to certain service operations, and the corresponding fault content will be returned when an error occurs.

This chapter provides seven recipes on how to work with various contracts in WCF service development. These include defining a one-way service operation that helps you get familiar with standard ServiceContract and OperationContract declaration. Next, we will cover how to use FaultContractAttribute to associate a custom SOAP fault data type with certain service operations that need a customized error format. With the third, fourth, and fifth recipes, we will focus on DataContract designing topics, such as DataContract versioning, using XMLSerializer for the DataContract types serialization, and the contract-first approach for DataContract generation. The last two recipes describe how to use MessageContract to perform low-level manipulation on the service operations message formatting, such as returning arbitrary XML data as message content and adding a custom SOAPHeader through MessageContract class members.