Got this question yesterday; can the ASP.NET session object be used to store state in a web service? The answer is yes, but the web service method must be marked with EnableSession = true.

Below is a small code snippet that shows how it's done:

[WebMethod(EnableSession = true)]
public string HelloWorld()
{   
   // Check if item in cache, else add it.
   if (HttpContext.Current.Session["stateItem"] == null)
   {
      HttpContext.Current.Session["stateItem"] = "some-item-to-store";
   }

   return (string)HttpContext.Current.Session["stateItem"];
}

Simple, but yet easy to forget...