Eyal’s Posts

ASP.NET Web Services and STA COM Objects

Posted on March 4th, 2006.

Eran’s post (where he mentions a hotfix for ASP.NET with regard to STA COM objects) reminded me of a problem we had with web services and STA COM objects (We worked for the same company..).
If you have no idea what STA COM objects are, You didn’t miss anything and I really envy you. You can jump to the end of the article where there’s a line about you.
ASP.NET web services do not support the aspcompat attribute which is available for asp.net web pages. This means calls to STA COM objects are made from an MTA thread and always incur a thread switch to the STA thread.
There are a few solutions and guidelines but they don’t compare to the convenience provided by the aspcompat attribute and they generally suffer from performance problems.
Since web services support a “stateful” mode (see EnableSession in the WebMethod attribute) which was nicely aligned with the way our COM objects where designed, we needed a way to make sure that whenever a web method is called from the same session, it will run on the same thread and that this thread will be a STA thread. This is the same behavior aspcompat pages have and we needed a way to duplicate that.
The solution I came up with was actually very simple – But it took quite a while to find.
To understand the solution, you need to realize that an asmx (web service) page is handled just like any other asp.net page. ASP.NET reads the httpHandlers section in the web.config file and finds the handler for asmx pages. What I did was to “hijack” asmx handling and redirect it to my own System.Web.UI.Page derived object. Since this object is derived from Page, it has the infrastructure for the aspcompat attribute. When that page was loaded, I simply loaded the real asmx page and handed the request to it. Here’s some source code:

..
using System.Web.Services.Protocols;
using System.Web.SessionState;
..
public class STAWebServiceHandler: System.Web.UI.Page, IHttpAsyncHandler,IRequiresSessionState
{
  //This is the real handler factory for asmx pages
  private static WebServiceHandlerFactory defaultFactory= new WebServiceHandlerFactory();

  override protected void OnInit(EventArgs e)
  {
    //Get the real handler and ask it to process the request
    IHttpHandler Handler=defaultFactory.GetHandler(Context,Context.
      Request.HttpMethod,Context.Request.RawUrl,Context.Request.PhysicalPath);
   
    if (Handler!=null)
      Handler.ProcessRequest(Context); }

    //This is where the magic happens. Our BeginProcessRequest
    //and EndProcessRequest calls the inherited
    //AspCompatBeginProcessRequest and AspCompatEndProcessRequest
    //to process this page in an “aspcompat” way.
    public IAsyncResult BeginProcessRequest(HttpContext context,
             AsyncCallback cb, object extraData)
    {
        return AspCompatBeginProcessRequest(context,cb,extraData);
    }
   
    public void EndProcessRequest(IAsyncResult result)
    {
      AspCompatEndProcessRequest(result);
     } 
  }
}

You also need to make sure your web.config has an entry in the httpHandlers section that points to this class.
I hope that no one will actually need to use this code. I’m really looking forward to the days where STA COM objects will be to programmers what COBOL is to me..:) (No offense, COBOL guys..)

Technorati: STA, COM, ASP.NET, Web Services

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