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OleDbConnection cn2 = new OleDbConnection(dbconnect);
            cn2.Open();
            foreach (DataGridViewRow row in dataGridView1.Rows)
            {
                string grid_kunde = row.Cells[0].Value.ToString();
                string grid_taetigkeiten = row.Cells[1].Value.ToString();
                string grid_stunden = row.Cells[2].Value.ToString();
                string grid_datum = row.Cells[3].Value.ToString();
                string grid_kw = row.Cells[4].Value.ToString();
                string grid_tag = row.Cells[5].Value.ToString();
                string grid_monat = row.Cells[6].Value.ToString();
                string sql_abfrage2 = "INSERT INTO [data_kunden_stunden] (kunde, taetigkeit, stunden, datum, kw, tag, jahr, monat) VALUES ('" + grid_kunde + "', '" + grid_taetigkeiten + "', '" + grid_stunden + "', '" + grid_datum + "', '" + grid_kw + "', '" + grid_tag + "', '" + jahr + "', '" + grid_monat + "')";
                OleDbCommand cmd2 = new OleDbCommand(sql_abfrage2, cn2);
                cmd2.ExecuteNonQuery();
            }
            cn2.Close();                    

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uint[] aWeekdays = { 0, 0, 0, 0, 0, 0, 0 };    // Array mit Anzahl der Wochentage (Index 0 = alle Sonntage, 1 = alle Montage etc.)                    

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public enum DayType { WorkDay, Weekend, Holiday }

public Dictionary<DateTime, DayType> GetListOfDayTypes(DateTime from, DateTime to)  
{
// TODO
}                    

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public Dictionary<DateTime, DayType> GetListOfDayTypes(DateTime from, DateTime to)
{
    Dictionary<DateTime, DayType> result = new Dictionary<DateTime, DayType>();  
    while (from <= to)
    {
        if (from.IsWeekend())
            result.Add(from, DayType.Weekend);
        else if (from.IsFeiertag())
            result.Add(from, DayType.Holiday);
        else
            result.Add(from, DayType.WorkDay);
        from = from.AddDays(1);
    }
    return result;
}                    

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foreach (var tuple in GetListOfDayTypes(from, to))
{
    switch (tuple.Value)
    {
        case DayType.WorkDay:
            // Arbeitstag für tuple.Key eintragen
            break;
        case DayType.Weekend:
            // Wochenende für tuple.Key eintragen
            break;
        case DayType.Holiday:
            // Urlaub für tuple.Key eintragen
            break;
        default:
            break;
    }
}                    

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        public Dictionary<DateTime, DayType> GetListOfDayTypes(DateTime dtBeginDate, DateTime dtEndDate)
        {
            Dictionary<DateTime, DayType> result = new Dictionary<DateTime, DayType>();
            while (dtBeginDate <= dtEndDate)
            {
                if (dtBeginDate.DayOfWeek == DayOfWeek.Saturday || dtBeginDate.DayOfWeek == DayOfWeek.Sunday)
                    result.Add(dtBeginDate, DayType.Weekend); // Wochenende
                else if (dtBeginDate.IsFeiertag())
                    result.Add(dtBeginDate, DayType.Holiday); // Feiertag
                else
                    result.Add(dtBeginDate, DayType.WorkDay); // Arbeitstag
                dtBeginDate = dtBeginDate.AddDays(1);
            }
            return result;
        }                    

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        //------------------------------------------------------------------------------------------------------------------------------------
        public bool IsFeiertag(DateTime dtDate)
        //------------------------------------------------------------------------------------------------------------------------------------
        {
            foreach (Feiertage feiertag in this._feiertagsMatrix[this._bundesLand])
            {
                if (dtDate == GetFeiertagDatum(feiertag, dtDate.Year))
                    return( true );
            }
            return (false);
        }                    

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else if (IsFeiertag(dtBeginDate))
    result.Add(dtBeginDate, DayType.Holiday); // Feiertag                    

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public enum DayType { WorkDay, Weekend, Holiday }                    

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        public Dictionary<DateTime, DayType> GetListOfDayTypes(DateTime dtBeginDate, DateTime dtEndDate)
        {
            Dictionary<DateTime, DayType> result = new Dictionary<DateTime, DayType>();
            while (dtBeginDate <= dtEndDate)
            {
                if (dtBeginDate.DayOfWeek == DayOfWeek.Saturday || dtBeginDate.DayOfWeek == DayOfWeek.Sunday)
                    result.Add(dtBeginDate, DayType.Weekend); // Wochenende
                else if (IsFeiertag(dtBeginDate))
                    result.Add(dtBeginDate, DayType.Holiday); // Feiertag
                else
                    result.Add(dtBeginDate, DayType.WorkDay); // Arbeitstag
                dtBeginDate = dtBeginDate.AddDays(1);
            }
            return result;
        }                    

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            foreach (var tuple in GetListOfDayTypes(dtBeginDate, dtEndDate))
            {
                switch (tuple.Value)
                {
                    case DayType.WorkDay:
                        // Arbeitstag für tuple.Key eintragen
                        break;
                    case DayType.Weekend:
                        // Wochenende für tuple.Key eintragen
                        break;
                    case DayType.Holiday:
                        // Urlaub für tuple.Key eintragen
                        break;
                    default:
                        break;
                }
            }