Contracts in WCF

Contracts

• A contract defines certain aspects of the service such as format & structure of the message etc.
• A contract is an agreement between client & service
• Contract describes the data, punitions their return types, parameters, etc to be communicated with the service

Types of contracts in WCF

WCF supports 5 types of contracts

• Data contracts
• Operation contracts
• Message contracts
• Fault contracts
• Service contracts

Data contract

The data type that a service should be use in metadata to enable others  to interoperate with the service/ described as data contract.

• In general, these data contracts are created with in the data contract class by preceding with [data contract] attribute.
• The descriptions of the data types are known as the data contract.
• The types used / described with in the data contract can be used in any part of the messages as parameters / return types tec.
• If the service is using only simple types it is not required to create data contract separately.

Operation contract

An operation contract defines the operations, their parameters & the return types.

• All operations should be preceded with operation contract attribute that are to be used outside the WCF service
• In WCF services the operations can be modeled as taking a single message & returning a single message.
• Message contract:-
• A message contract defines / describes the format of a message.
• Message contract declares whether message elements should go in headers / with in the body etc
• Message contract also describes that what level of security should be applied to which elements of the message.

Fault contract

A fault contract can be associated with service operation to denote errors that can be return

• An operation can have zero% more fault contracts associated with it.
• These errors are soap faults that are modeled as exceptions in the programming model

Service contract

A service contract can be said as the combination of other contracts mentioned above.

• The service contract bundles together multi related operations into a single functional unit.
• A service contract contains internally data contracts, operation contracts, message contracts & fault contracts.
• The service contract can define the settings to be followed at service level
• In most of the cases, the service contract is defined by creating an interface & preceding it with service contract attribute.
• The actual service contract code is resulted by implementing that interface.
• The service contract outlines wheel functionality that end point exposes to the client

Endpoint

An end point is a construct, at which messages are sent/received.

• An endpoint contains a location i.e., an address., a specification of the communication mechanism i.e., binding & definition for set of messages that can be sent / received i.e., contract.
• So, endpoint can be set as combination of ABC’s
• All communication with in the WCF services will occur through the endpoints of the service.
• End points will provide the clients complete access to the functionality that WCF service offers
• A WCF service is exposed to it’s clients as a collection of endpoints.
• A WCF service should contain minimum one endpoint.

Structure of a service with endpoints

    An endpoint is defined in a web.confing file by using the 3 attributes address, binding & contract like <endpoint address = “Value” binding = “Value” contract = “Value”> </enclpoint>    

Message & Message Patterns

A message is a self contained unit of data which contains several parts like body, header, etc. Message are sent between endpoints of dient & service. WCF supports following message patterns.

1. Simple
2. Duplex
3. Request – reply

Simplex This is one way communication from client to service. In this mechanism / pattern client serds request / message to the service, service consumes that request but doesn’t respond back to the client. To work / implement this message pattern set “ Is one way” property to “true” along with [operation contract] attribute like, [operation contract (is one way = true)] The methods which are following simplex message pattern should not return a value i.e., should not have return type (or) should not contain reference & out parameters. Otherwise, they through an conception “System invalid operation” exception.      

Duplex

This is a two way / bidirectional communicate service and service respond to the client by conserve that message. Client & service will exchange the messages cuing channel A duplex service contract is a message exchange pull in which both client & service endpoints can semi messages to each other independently. To implement this message pattern set “I someway property of [operation contract] attribute to “true” create the call back interface that defines the of operations for invoking on the client         Request – Reply This is the default messaging path followed in WCF. In this type of pattern client makes a call to service tions & waits for a response from the service Client request & service response will be through event channels.