How to Eliminate the Checking of the Null value DataTable in IF condition

string mac = GetMac();
           string Qry = "Select VUserid,Password from passtable where VUserid='" + UserName.Text + "' and Flag='A'";
           string qry = "Select VUserid,Password from passtable where Flag='A'";
           string strq = "Select Mac_id from Sysinfo Where Appflag='A'";
           using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["EvalCon"].ConnectionString))
           {
               try
               {

                   SqlCommand cmd = new SqlCommand(Qry, conn);
                   SqlCommand cmd1 = new SqlCommand(qry, conn);
                   SqlCommand cmd2 = new SqlCommand(strq, conn);
                   conn.Open();
                   SqlDataAdapter da = new SqlDataAdapter(cmd);
                   SqlDataAdapter daa = new SqlDataAdapter(cmd1);
                   SqlDataAdapter dap = new SqlDataAdapter(cmd2);
                   DataTable dt = new DataTable();
                   DataTable dtt = new DataTable();
                   DataTable tab = new DataTable();
                   da.Fill(dt);
                   daa.Fill(dtt);
                   dap.Fill(tab);
                   for (int i = 0; i < tab.Rows.Count; i++)
                   {
                       for (int x = 0; x <= dtt.Rows.Count - 1; x++)
                       {
                           if (mac == tab.Rows[i]["Mac_id"].ToString() || tab.Rows.Count != 0)
                           {
                               if (UserName.Text == dtt.Rows[x]["VUserid"].ToString() && Password.Text == dtt.Rows[x]["Password"].ToString())
                               {
                                   Response.Redirect("~/Changepass.aspx");
                                   break;
                               }
                               else
                               {
                                   lblMessage.Visible = true;
                                   lblMessage.ForeColor = System.Drawing.Color.Red;
                                   lblMessage.Text = "Invalid Username or Password !!!";

                               }
                           }
                           else
                           {
                               lblMessage.Visible = true;
                               lblMessage.ForeColor = System.Drawing.Color.Red;
                               lblMessage.Text = "Invalid Access Point for Evaluation !!!";
                           }
                       }
                   }

               }
               finally
               {
                   conn.Close();
                   conn.Dispose();
               }
           }