In WCF, all services expose contracts. The contract is a platform-neutral and standard way of describing what the service does. WCF defines four types of contracts,
- Service contract: Describe which operations the client can perform on the service.
- Data contract: Define which data types are passed to and from the service. WCF defines implicit contracts for built-in types such as int and string, but we can easily define explicit opt-in data contracts for custom types.
- Fault contract: Define which errors are raised by the service, and how the service handles and propagates errors to its clients.
- Message contract: Allow the service to interact directly with messages. Message contracts can be typed or untyped, and are useful in interoperability cases and when there is an existing message format we have to comply with.