Contracts are useful to build the WCF service application. The Contracts defines what protocol (binding) the service uses, how communication will be done, what message exchange format to use and much more.
The following are the contracts which are available in WCF.

• Service contracts
• Data contracts
• Message contracts
• Fault Contract 
• Operation contract
• Policies and Binding

   [ServiceContract]
    public interface IService1
    {

        // TODO: Add your service operations here
    }

Operation contract

   public interface IService1
    {
        [OperationContract]
        string GetData(int value);

        [OperationContract]
        CompositeType GetDataUsingDataContract(CompositeType composite);
  
    }

    [DataContract]
    public class Student
    {
        private string _Name;

        private string _City;


        [DataMember]
        public string Name
        {
            get { return _Name; }
            set { _Name = value; }
        }
}

[ServiceContract]
    public interface IGetDetailsService
    {
        [OperationContract]
        [FaultContract(typeof(Student))]
        Student GetDetails(string Name);
    }

    [DataContract]
    public class Student
    {
        private string _Name;

        private string _City;

        [DataMember]
        public string Name
        {
            get { return _Name; }
            set { _Name = value; }
        }

        [DataMember]
        public string City
        {
            get { return _City; }
            set { _City = value; }
         }
}

Default SOAP message format is provided by the WCF run time for communication between the client and service. If it is not meeting your requirements, then we can create our own message format. This can be achieved by using Message Contract attribute.
It can be defined as

Policies and Binding in WCF

Specify conditions required to communicate with a service, e.g., security requirement to communicate with a service, protocol and encoding used for binding.

Service Run-time in WCF

It contains the behaviors that occur during the run-time of service. The following behaviors are managed by the service run-time layer.

• Throttling Behavior
• Error Behavior 
• Metadata Behavior
• Instance Behavior 
• Transaction Behavior
• Dispatch Behavior 
• Concurrency Behavior
• Parameter filtering
• Message inspection

Throttling Behavior: It controls how many messages are processed.
Error Behavior: Specifies what occurs when internal error occurs on the service.
Metadata Behavior: Tells how and whether metadata is available to the outside world.
Instance Behavior: Specifies how many instances of the service have to be created while running.
Transaction Behavior: Enables the rollback of transacted operations if a failure occurs.
Dispatch Behavior: Controls how a message is processed by the WCF Infrastructure.
Concurrency Behavior:  It's used to control how many threads can access a given instance of service.
Parameter filtering: It filters the message headers and executes preset actions based on the filters of the message headers. 
Message inspection: It's used to inspect a specific part or all parts of the message through service.

Messaging in WCF Service

The messaging layer is composed of channels. A channel is a component that processes a message in some way, for example, by authenticating a message. A set of channels is also known as a channel stack. Channels are the core abstraction for sending message to and receiving message from an Endpoint. Generally, we can categories channels as follows:

• Transport Channels
• Protocol Channels  

Handles sending and receiving messages from the network. Protocols like HTTP, TCP, name pipes and MSMQ. Services can be hosted or executed, so that it will be available to everyone accessing the client. The WCF service can be hosted by the following mechanism.Internet information Service provides number of advantages if a Service uses Http as protocol. It does not require Host code to activate the service, it automatically activates service code.WAS is the new process activation mechanism that ships with IIS 7.0. In addition to HTTP based communication, WCF can also use WAS to provide message-based activation over other protocols, such as TCP and named pipes.

Self-Hosting

The WCF service can be self-hosted as a console application, Win Forms or WPF application with a graphical UI.
WCF can also be hosted as a Windows Service, so that it is under the control of the Service Control Manager (SCM).

• Support for different protocols
• Security over transport as well as Message level
• Easy to implement MSMQ and REST Service.
• Load balancing & support scaling.
• It can control concurrency.
• It can be hosted on IIS, WAS, Self hosting, Windows services.
• Multiple Message Patterns.
• Unhanded Exceptions does not return to the client as SOAP faults. WCF supports better exception handling by using Fault Contract.

Note

Please refer my following articles for complete understanding

• Creating WCF Service 
• End Points in WCF 
• Consuming WCF Service in Web Application 

Summary

I hope this article is useful for all students and beginners. If you have any suggestion related to this article, then please contact me.