namespace DOTA.Browser {
    public class FormRewriterControlAdapter :
            System.Web.UI.Adapters.ControlAdapter {
        protected override void Render(HtmlTextWriter writer) {
            base.Render(new RewriteFormHtmlTextWriter(writer));
        }
    }

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

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

        public override void WriteAttribute(string name, string value, bool fEncode) {
            if (name == "action") {
                HttpContext context = HttpContext.Current;

                if (context.Items["ActionAlreadyWritten"] == null) {
                    value = context.Request.ServerVariables["HTTP_X_REWRITE_URL"] ?? context.Request.RawUrl;
                    context.Response.Write(value);  //这里执行了，源代码里有,是Request.RawUrl的值。 HTTP_X_REWRITE_URL 这个是空的.
                    context.Items["ActionAlreadyWritten"] = true;
                }
            }

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

<browser refID="Default">
        <controlAdapters>
            <adapter controlType="System.Web.UI.HtmlControls.HtmlForm"
                     adapterType="DOTA.Browser.FormRewriterControlAdapter" />
        </controlAdapters>
    </browser>