The System.ServiceModel..::.InstanceContext manages the association between the channel and the user-defined service objects. Use the InstanceContextMode enumeration with the ServiceBehaviorAttribute..::.InstanceContextMode property to specify the lifetime of the InstanceContext object. Windows Communication Foundation (WCF) can create a new InstanceContext object for every call, every session or specify that the InstanceContext object is bound to a single service object. For a working example, see the Instancing Sample.

The Single value specifies that a single InstanceContext object should be used for the lifetime of the service.

Note:
If the InstanceContextMode value is set to Single the result is that your service can only process one message at a time unless you also set the ConcurrencyMode value to Multiple.

The following code illustrates how to set the InstanceContextMode for a service class:

' Service class which implements the service contract.
Public Class CalculatorService
Implements ICalculator

    Public Function Add(n1 As Double, n2 As Double) As Double Implements ICalculator.Add
        Return n1 + n2
    End Function

    Public Function Subtract(n1 As Double, n2 As Double) As Double Implements ICalculator.Subtract
        Return n1 - n2
    End Function

    Public Function Multiply(n1 As Double, n2 As Double) As Double Implements ICalculator.Multiply
        Return n1 * n2
    End Function

    Public Function Divide(n1 As Double, n2 As Double) As Double Implements ICalculator.Divide
        Return n1 / n2
    End Function

End Class

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003

.NET Framework

Reference