Four Different ways to use knowntype attribute

Data Contract -: 

Data Contract attribute is used for the type of data that will be sent/received between a service and a client. Some time data is not know to client an example is derived type, in that case there will be some problem in De-Serialization. De-Serialization engine could not recognize the derived data contract and will threw error.

• Global Level
• Using Code at base type
• Using Web.config
• Service Contract Level
• Operation Contract Level

Global using Code

[KnownType(typeof(Santro))]
[KnownType(typeof(Maruti))]
[DataContract]
public class Car
{
}
[DataContract]
public class Santro : Car
{

[OperationContract]

void GetModal( );
}
[DataContract]
public class Maruti : Car
{

[OperationContract]

void GetModal( );
}
using on base type it is applicable for all functions.

Global Level using config

<system.runtime.serialization>

     <dataContractSerializer>

                  <declaredTypes>

                             MyService.Car, 

                                                MyService, Version=, 

                                                Culture=Neutral, PublicKeyToken=null”>

                                         <knownType type=”MyService.Santro, 

                                                MyService, Version=<version here>, 

                                                Culture=Neutral, PublicKeyToken=null” />

                                         <knownType type=”MyService.Maruti, 

                                                MyService, Version=<version here>, 

                                                Culture=Neutral, PublicKeyToken=null” />

                            </add>

                  </declaredTypes>

     </dataContractSerializer>

</system.runtime.serialization>

Service Contract 

[ServiceKnownType(typeof(Car))]

[ServiceKnownType(typeof(Truck))]
[ServiceContract]
public Interface ICarService
{
[OperationContract]
Vehicle AddNewCar(Car car);

[OperationContract]
bool UpdateCar(Car car);
}

will  be applicable for all functioned declared within this interface but not for other interfaces.

Operation Contract Level

[ServiceContract]

public Interface ICarService

{

      [ServiceKnownType(typeof(Santro))]

      [ServiceKnownType(typeof(Maruti))]

      [OperationContract]

      Vehicle AddNewCar(Car car);

      [OperationContract]

      bool UpdateCar(Car car);  

}

will be applicable only for contract on which ServiceKnownType is defined.