What are contracts in WCF?
Let us explain what contracts are in WCF.
A WCF Contract is a collection of Operations that specifies what the Endpoint communicates to the outside world. Each operation is a simple message exchange, for example one-way or request/reply message exchange.
The ContractDescription class is used to describe WCF Contracts and their operations. Within a ContractDescription, each Contract operation has a corresponding OperationDescription that describes aspects of the operation such as whether the operation is one-way or request/reply. Each OperationDescription also describes the messages that make up the operation using a collection of MessageDescriptions.
A ContractDescription is usually created from an interface or class that defines the Contract using the WCF programming model. This type is annotated with ServiceContractAttribute and its methods that correspond to Endpoint Operations are annotated with OperationContractAttribute. You can also build a ContractDescription by hand without starting with a CLR type annotated with attributes.
A duplex Contract defines two logical sets of operations: A set that the Service exposes for the Client to call and a set that the Client exposes for the Service to call. The programming model for defining a duplex Contract is to split each set in a separate type (each type must be a class or an interface) and annotate the contract that represents the service's operations with ServiceContractAttribute, referencing the contract that defines the client (or callback) operations. In addition, ContractDescription contains a reference to each of the types thereby grouping them into one duplex Contract.
Similar to Bindings, each Contract has a Name and Namespace that uniquely identify it in the Service's metadata.
Each Contract also has a collection of ContractBehaviors that are modules that modify or extend the contract's behavior
Here is a Code example of how to Define a Contract in WCF
Here is a Code example of how to Implement a Contract in WCF
Different types of WCF Contracts:
Service Contract Example
Data Contract Example