Sometimes it is handy to have the HttpSession available from your custom IHttpHandlers. By default the session state is unavailable from IHttpHandlers, and attempting to access it will result in a NullReferenceException. To make session state available simply implement the IRequireSessionState interface.

using System.Web;
using System.Web.SessionState;

public class SomeHandler : IHttpHandler, IRequiresSessionState {

     public void ProcessRequest(HttpContext context){
          //do processing that requires session state.
	  //use the context.Session to acquire the session state.
     }

     public bool IsReusable {
          get { return false; }
     }
}

The IRequiresSessionState interface is simply a marker interface, thus it has no members to implement. Once implemented your IHttpHandler can acquire the session state using the Session property of the HttpContext parameter on the ProcessRequest method.