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