ASP.NET

private static string GetUserName( string loginName ) 
{
     string strName;
     string dcName = "";
     string dName = "DomainName";

     char[] sep = {'.'};
     string[] str = dName.Split(sep);
     int i, len = str.Length;

     string appPath = "LDAP://";

     for ( i = 0; i < len; i++ )
     {
        dcName = "dc = "+str[i]+" ";
        appPath = appPath + dcName + ","; 
     }

     appPath = appPath.Remove( appPath.Length-1,1 ); 

     DirectoryEntry entry = new DirectoryEntry(appPath);

     DirectorySearcher mySearcher = new DirectorySearcher(entry);

     try
     {
         mySearcher.Sort.PropertyName = "CN";

         mySearcher.Filter = 
            ( "&(objectClass=user)(SAMAccountName="+ loginName +")" );

//( "&(ObjectCategory=user)(SAMAccountName="+ loginName +")" ); 
// ==> for this too same kind of exception was thrown

         mySearcher.PageSize = 25;
         i = 0;

         SearchResult result = mySearcher.FindOne();

         strName = result.GetDirectoryEntry().Name;
         //string ak = result.GetDirectoryEntry().Username.ToString(); 

         strName = strName.Remove(0, 3);

         return strName;
      }
      catch(Exception exe)
      {
         throw(exe);
      }
      finally
      {
        if (entry != null)
        { 
          entry.Dispose(); 
        }

        if (mySearcher != null)
        {
          mySearcher.Dispose();
        }

        Console.ReadLine();
       }
    }