Subclassing HTMLTextWriter results in NPE

[JCDenton]

I found a nice sample (http://weblogs.asp.net/scottgu/archive/2007/02/26/tip-trick-url-rewriting-with-asp-net.aspx) on how to set the action attribute of a form HTML element. However when an ajax request is made the code throws a NPE:

 

public class FormControlAdapter : ControlAdapter
{
   protected override void Render(HtmlTextWriter writer)
   { 
       base.Render(new FormWriter(writer)); //Throws NPE when a ajax request is made
   }
}

public class FormWriter : HtmlTextWriter
{
   public FormWriter(HtmlTextWriter writer)
       : base(writer)
   {
       base.InnerWriter = writer;
   }

   public FormWriter(TextWriter writer)
       : base(writer)
   {
       base.InnerWriter = writer;
   }

   public override void WriteAttribute(string name, string value, bool fEncode)
   {
       if (name == "action")
       {
           if (HttpContext.Current.Items["ActionWritten"] == null)
           {
               value = HttpContext.Current.Request.RawUrl;
               HttpContext.Current.Items["ActionWritten"] = true;
           }
       }

       base.WriteAttribute(name, value, fEncode);
   }

}



System.NullReferenceException was unhandled by user code
 Message="Object reference not set to an instance of an object."
 Source="System.Web.Extensions"
 StackTrace:
      at System.Web.UI.PageRequestManager.RenderFormCallback(HtmlTextWriter writer, Control containerControl)
      at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
      at System.Web.UI.Control.RenderChildren(HtmlTextWriter writer)
      at System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer)
      at System.Web.UI.HtmlControls.HtmlForm.Render(HtmlTextWriter output)
      at System.Web.UI.Adapters.ControlAdapter.Render(HtmlTextWriter writer)

      at FormControlAdapter.Render(HtmlTextWriter writer) in line 23

      at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
      at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter)
      at System.Web.UI.HtmlControls.HtmlForm.RenderControl(HtmlTextWriter writer)
      at System.Web.UI.HtmlFormWrapper.System.Web.UI.IHtmlForm.RenderControl(HtmlTextWriter writer)
      at System.Web.UI.PageRequestManager.RenderPageCallback(HtmlTextWriter writer, Control pageControl)
      at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
      at System.Web.UI.Control.RenderChildren(HtmlTextWriter writer)
      at System.Web.UI.Page.Render(HtmlTextWriter writer)
      at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
      at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter)
      at System.Web.UI.Control.RenderControl(HtmlTextWriter writer)
      at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
 InnerException:

 

I would appreciate any solution on how to set the action attribute for a form or solving this peculiar problem.. Thanks

Edited May 22, 2008 by JCDenton