変数を宣言し、正しい値を返していますが、なぜこのエラーがまだ発生するのかわかりません。
  sqlString.Append( " SELECT  Medication.Description + ' - ' + Medication.Strength + ' - ' + Medication.MedicationCode as DrugName, Resident.Title + ' ' + Resident.firstName + ' ' + Resident.Surname AS Resident, MedicationDispenseTimeslot.MedicationDosageBarcode, 0 AS checkBox " );
             sqlString.Append( " FROM Medication INNER JOIN MedicationDispense ON Medication.MedicationID = MedicationDispense.MedicationID INNER JOIN Resident ON MedicationDispense.ResidentID = Resident.ResidentID INNER JOIN MedicationDispenseStatus ON MedicationDispense.MedicationDispenseStatusID = MedicationDispenseStatus.MedicationDispenseStatusID INNER JOIN MedicationDispenseTimeslot ON MedicationDispense.MedicationDispenseID = MedicationDispenseTimeslot.MedicationDispenseID WHERE     (MedicationDispenseStatus.MedicationDispenseStatusID = 2) and  MedicationDispense.MedicationDispenseID in (select MedicationDispenseID from dbo.MedicationDispenseHistory where DATEDIFF(d, 0, DateTimeStamp) =DATEDIFF(d, 0, GETDATE()) and MedicationDispenseStatusID=2) AND MedicationDispense.ResidentID = @residentID " );
             sqlString.Append( " AND   (MedicationDispense.MedicationDispenseStatusID > 1) AND ");
             sqlString.Append( "    1 = (CASE WHEN MedicationDispense.StopDate IS NULL " );
             sqlString.Append( "                 THEN (CASE WHEN MedicationDispense.StartDate <= @periodEndDate THEN 1 ELSE 0 END) " );
             sqlString.Append( "            ELSE " );
             sqlString.Append( "                CASE WHEN (MedicationDispense.StartDate between @periodStartDate and @periodEndDate) " );
             sqlString.Append( "                                OR " );
             sqlString.Append( "                          (MedicationDispense.StopDate between @periodStartDate and @periodEndDate) " );
             sqlString.Append( "                                OR " );
             sqlString.Append( "                          (MedicationDispense.StartDate < @periodStartDate and MedicationDispense.StopDate > @periodEndDate) " );
             sqlString.Append( "    GROUP BY Medication.Description + ' - ' + Medication.Strength + ' - ' + Medication.MedicationCode, Resident.Title + ' ' + Resident.firstName + ' ' + Resident.Surname , MedicationDispenseTimeslot.MedicationDosageBarcode " );
             command = new SqlCommand(sqlString.ToString(), connection);
             command.Parameters.Add( "@residentID", SqlDbType.Int ).Value = residentID.ToString();
             command.Parameters.Add("@periodStartDate", SqlDbType.DateTime).Value = Helper.GetValue(periodStartDate);
             command.Parameters.Add("@periodEndDate", SqlDbType.DateTime).Value = Helper.GetValue(periodEndDate);
編集
  command = new SqlCommand( sqlString.ToString(), connection );
     SqlDataAdapter dataAdapter = new SqlDataAdapter( command );
     dataAdapter.Fill( ds, "dataTable" );
     return ds;