Error in Authenticate user usig active directory

Hello all, I new to active directory.. i have the following code to create a new user and assign a group to that user... which create a user in Local User and group section of Administrative tools/computer managment .



      // create user
             DirectoryEntry AD = new DirectoryEntry("WinNT://" + Environment.MachineName + ",computer");
             DirectoryEntry NewUser = AD.Children.Add(textUserName.Text, "user");
             NewUser.Invoke("SetPassword", new object[] {textPassword.Text});
            NewUser.Invoke("Put", new object[] {"Description", "Test User from .NET"});
             NewUser.CommitChanges();
            
            // assign group
            DirectoryEntry grp;
             grp = AD.Children.Find("Guests", "group");
             if (grp != null)
             {grp.Invoke("Add", new object[] {NewUser.Path.ToString()});}
             MessageBox.Show("Account created successfully");



Here is a code to authentcate user... but it gives a error like..."The provider does not support searching and cannot search WinNT://ADMIN,computer." I also confuse when to use "LDAP" protocol and when "WinNT".


DirectoryEntry AD = new DirectoryEntry("WinNT://" + Environment.MachineName + ",computer");
                                 
            Object obj = AD.NativeObject;
            DirectorySearcher directorySearcher = new DirectorySearcher(AD);
            directorySearcher.Filter ="(SAMAccountName = " + textUserName.Text + ")";
            directorySearcher.PropertiesToLoad.Add("cn");
            SearchResult sr = directorySearcher.FindOne();

         
            if (sr == null)
            {
                MessageBox.Show("Not Authenticated!!");
            }
            else
            {
                MessageBox.Show("Authenticated!!");
            }



If anybody has a idea.. plz HELP...