Session Management Using WCF - Silverlight

Hi,

I want to handle Session Management using WCF in Silverlight. Here is the sample code:

=============================================================================

Imports System.ServiceModel

<ServiceContract(Namespace:="")>
Public Interface IService1

    <OperationContract()>
    Sub SetSessionVariable(ByVal SessionKey As String, ByVal SessionValue As String)

    <OperationContract()>
    Function GetSessionVariable(ByVal SessionKey As String) As String

End Interface

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Imports System.ServiceModel.Activation

<AspNetCompatibilityRequirements(RequirementsMode:=AspNetCompatibilityRequirementsMode.Allowed)> _
Public Class Service1
    Implements IService1

    Public Sub SetSessionVariable(ByVal SessionKey As String, ByVal SessionValue As String) Implements IService1.SetSessionVariable
        System.Web.HttpContext.Current.Session(SessionKey) = SessionValue
        System.Web.HttpContext.Current.Session.Timeout = 20
    End Sub

    Public Function GetSessionVariable(ByVal SessionKey As String) As String Implements IService1.GetSessionVariable
        Return System.Web.HttpContext.Current.Session(SessionKey)
    End Function

End Class

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Private Sub MainPage_Loaded(sender As Object, e As System.Windows.RoutedEventArgs) Handles Me.Loaded
        Dim svc As New ServiceReference1.Service1Client()
        svc.SetSessionVariableAsync("Key", "Hello World !")
End Sub

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I am getting "Object reference not set to an instance of an object." at:

System.Web.HttpContext.Current.Session(SessionKey) = SessionValue

Can you please change my code and make it working ? My WCF Service is in .WEB project. I am accessing that by using proxy in Silverlight project.

Thanks