WebOperationContext Object reference not set to an instance of an object

Question

When I try to use WebOperationContext.Current in a WCF project, the Current is null. Below is the example. Could anyone please shed a light on it?

WebForm - default.aspx:

    ServiceClient sc = new ServiceClient();

    Response.Write(sc.DoWork(1) + "<br />");

    WebOperationContext c = WebOperationContext.Current;  --Current is null

//WCF Interface

[ServiceContract]
public interface IService
{
    [OperationContract]
    [WebGet]
    int DoWork(int num);
}

//WCF Implementation

public class Service : IService
{
    public int DoWork(int num)
    {           
        return num;
    }
}

System Settings: ASP.NET 3.5

Thank you in advance.

---

The description of my question is changed to below:

When I try to use WebOperationContext.Current in a WCF project, the Current is null. Below is the example. Could anyone please shed a light on it?

What I need is a transparent way (or a way that changes as little to the existing code) to make the existing code to do a specified work based on a token. This is whay HttpModule is used here, show below:

//HttpModule: Insert a token on which works in the pipneline can be based on. As mentioned, HttpModule is used to make minimum changes to existing code.

public class ImageIntercept : IHttpModule
{
    public void Init(HttpApplication context)
    {
        context.BeginRequest += new EventHandler(context_BeginRequest);
    }

    void context_BeginRequest(object sender, EventArgs e)
    {

        WebOperationContext.Current.IncomingRequest.Headers.Add("image", "true");  //a token on which works will be based

    }

    public void Dispose()
    { }

}

//WCF Interface

[ServiceContract]
public interface IService
{
    [OperationContract]
    [WebGet]
    int DoWork(int num);
}

//WCF Implementation

public class Service : IService
{
    public int DoWork(int num)
    {
        string isImage = WebOperationContext.Current.IncomingRequest.Headers["image"];
        if(isImage == "true")
        {
            //this is what I need to do something here
        }

        return num;
    }
}

System Settings: ASP.NET 3.5

Thank you in advance.