Username in datagrid

[rcoco]

Hi all,
I have this problem could I get some help? This code works on a plain
webform and I get user name:
string DomainUser =
System.Security.Principal.WindowsIdentity.GetCurrent().Name.Replace("\
\","/");
System.DirectoryServices.DirectoryEntry ADEntry = new
System.DirectoryServices.DirectoryEntry("WinNT://"+DomainUser);
Response.Write(ADEntry.Properties["FullName"].Value.ToString());

But When I try it on a datagrid it does not work why is it so? where
would I be going wrong?
myCommand.Parameters.Add(new SqlParameter("@Name",SqlDbType.Text));
myCommand.Parameters["@Name"].Value =
System.Security.Principal.WindowsIdentity.GetCurrent().Name.Replace("\
\","/");
System.DirectoryServices.DirectoryEntry ADEntry = new
System.DirectoryServices.DirectoryEntry("WinNT://"+st6);
Response.Write(ADEntry.Properties["FullName"].Value.ToString());
I actually get this error: Object reference not set to an instance of
an object.
Thanks.