I have the famous General access denied but I can't find someone in a similar
situation.

I am trying to do something very simple: let each user in the company update
his own mobile phone number on a web page.  The web application has Anonymous
off and Integrated Windows authentication.  It works fine retrieving the
user's own data so it means that the user is authenticated and logged in.

However, .CommitChanges() will only work if I instantiate the DirectoryEntry
with explictly supplied credentials.  WHY?  WHY?  WHY?  This is not
acceptable in my situation as no one would trust a web page that asks for a
password.

My test code is as follows:

   protected void Page_Load(object sender, EventArgs e)
   {
       string samid = Request.ServerVariables["AUTH_USER"];
      //converts domain\userid to userid
       samid = samid.Substring(samid.IndexOf(@"\") + 1);

       DirectoryEntry searchRoot =
           new DirectoryEntry(@"LDAP://DC=mydomain,DC=local",
                                       
"thisuser","password",AuthenticationTypes.Secure);

      //The following WON'T WORK at the CommitChanges() line:
      //DirectoryEntry searchRoot =
      //     new DirectoryEntry(@"LDAP://DC=mydomain,DC=local");

       using (searchRoot)
       {
           DirectorySearcher searcher =
             new DirectorySearcher(searchRoot, "(sAMAccountName=" + samid +
")",
                                               new string[] {
"displayName", "mobile" });
           using (DirectoryEntry result =
searcher.FindOne().GetDirectoryEntry())
           {
               Label1.Text = result.Properties["displayName"].Value as
string;
               txtboxMobile.Text = result.Properties["mobile"][0].ToString();
               result.Properties["mobile"][0] = "00000000";
               result.CommitChanges();
           }
       }
}

The user definitely has rights to change mobile number as GALMOD32 works fine.

Thanks in advance