asp a send the "page" object to class and change "text-box" control

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ASP.NET

ok , i send refernce of page to some class i check in the constrctor if i cant change text box
control and it worked.

object myTxt = pageSender.Page.FindControl("txtPresent");
            if (myTxt != null)
            {
                ((System.Web.UI.WebControls.TextBox)myTxt).Text = "2222";

            }

now the problem
i start a new thred and run it :

web.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(pageLoadedComlited);
            Application.Run();

new in the documet comleted function i tray to change the control and its not work:

void client_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
        {
            object myTxt = pageSender.Page.FindControl("txtPresent");
            if (myTxt != null)
            {
                ((System.Web.UI.WebControls.TextBox)myTxt).Text = e.ProgressPercentage.ToString();

            }
        }

I am able to identify the control But it does not work.
please help