[ServiceContract(Namespace="http://Microsoft.ServiceModel.Samples")]
public interface ICalculatorSession
{
    [OperationContract]
    void Clear();
    [OperationContract]
    void AddTo(double n);
    [OperationContract]
    void SubtractFrom(double n);
    [OperationContract]
    void MultiplyBy(double n);
    [OperationContract]
    void DivideBy(double n);
    [OperationContract]
    double Equals();
}

[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Required)]
public class CalculatorService : ICalculatorSession
{
    double result
    {   // Store result in AspNet session.
        get
        {
            if (HttpContext.Current.Session["Result"] != null)
                return (double)HttpContext.Current.Session["Result"];
            return 0.0D;
        }
        set
        {
            HttpContext.Current.Session["Result"] = value;
        }
    }

    public void Clear()
    {
    }

    public void AddTo(double n)
    {
        result += n;
    }

    public void SubtractFrom(double n)
    {
        result -= n;
    }

    public void MultiplyBy(double n)
    {
        result *= n;
    }

    public void DivideBy(double n)
    {
        result /= n;
    }

    public double Equals()
    {
        return result;
    }
}