strSQL = "select sum(netto) as Netto, sum(mwst)as MwSt, Sum(Brutto) as Brutto from Rechnung";
strSQL += " where Monat = @Monat and jahr = @Jahr";
DataSet ds = new DataSet();
SqlDataAdapter adapter = new SqlDataAdapter();
adapter.SelectCommand = new SqlCommand(strSQL, mySqlConnection);
adapter.SelectCommand.Parameters.Add("@Monat", SqlDbType.Int).Value = txtMonat.Text;
adapter.SelectCommand.Parameters.Add("@Jahr", SqlDbType.Int).Value = txtJahr.Text;
adapter.Fill(ds);
Counter = ds.Tables[0].Rows.Count;