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            OleDbConnection cn2 = new OleDbConnection(dbconnect);
            cn2.Open();
            string sql_abfrage2 = "SELECT datum FROM [gesammt_stunden_tag]";
            OleDbCommand cmd2 = new OleDbCommand(sql_abfrage2, cn2);
            OleDbDataReader dr2 = cmd2.ExecuteReader();
            while (dr2.Read())
            {
                // ????????????????????????
            }
            cn2.Close();					

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            monthCalendar1.BoldedDates = new System.DateTime[]
            {
                new System.DateTime(2013, 11, 18), new System.DateTime(2013, 11, 16)
            };					

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using (var connection = new OleDbConnection(dbconnect))
{
    connection.Open();

    // Werte abfragen
}					

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dr2.GetDateTime(0);					

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OleDbConnection cn5 = new OleDbConnection(dbconnect);
            cn5.Open();
            string sql_abfrage5 = "SELECT datum FROM [gesammt_stunden_tag] GROUP BY datum";
            OleDbCommand cmd5 = new OleDbCommand(sql_abfrage5, cn5);
            OleDbDataReader dr5 = cmd5.ExecuteReader();
            while (dr5.Read())
            {
                datum_fett = (dr5["datum"].ToString());
                string[] split1 = datum_fett.Split();
                foreach (string ausgabe_fett in split1)
                {
                    listBox1.Items.Add(ausgabe_fett);
                }
            }
            cn5.Close();					

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List<DateTime> list = new List<DateTime>();

// in Schleife
DateTime dateTime = ...; // wichtig: hier NICHT als String auslesen!
list.Add(dateTime);

// nach der Schleife
monthCalendar.BoldedDates = list.ToArray();					

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(DateTime)dataReader["datum"]					

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dataReader.GetDataTime(dataReader.GetOrdinal("datum"))					

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datum_fett = dr5["datum"].ToString();
listBox1.Items.Add(datum_fett);					

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            OleDbConnection cn5 = new OleDbConnection(dbconnect);
            cn5.Open();
            string sql_abfrage5 = "SELECT datum FROM [gesammt_stunden_tag] GROUP BY datum";
            OleDbCommand cmd5 = new OleDbCommand(sql_abfrage5, cn5);
            OleDbDataReader dr5 = cmd5.ExecuteReader();
            while (dr5.Read())
            {
                List<DateTime> list = new List<DateTime>();

                // in Schleife
                DateTime dateTime = (DateTime)dataReader["datum"]; // wichtig: hier NICHT als String auslesen!
                list.Add(dateTime);

                // nach der Schleife
                monthCalendar1.BoldedDates = list.ToArray();        
            }
            cn5.Close();					

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DateTime dateTime = (DateTime)dr5["datum"];					

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            OleDbConnection cn5 = new OleDbConnection(dbconnect);
            cn5.Open();
            string sql_abfrage5 = "SELECT datum FROM [gesammt_stunden_tag]";
            OleDbCommand cmd5 = new OleDbCommand(sql_abfrage5, cn5);
            OleDbDataReader dr5 = cmd5.ExecuteReader();
            while (dr5.Read())
            {
                List<DateTime> list = new List<DateTime>();
                datum_fett = dr5["datum"].ToString();

                DateTime dateTime = DateTime.ParseExact(datum_fett, "dd.MM.yyyy", CultureInfo.InvariantCulture);
                list.Add(dateTime);

                monthCalendar1.BoldedDates = list.ToArray();
            }
            cn5.Close();					

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            List<DateTime> list = new List<DateTime>();
            OleDbConnection cn5 = new OleDbConnection(dbconnect);
            cn5.Open();
            string sql_abfrage5 = "SELECT datum FROM [gesammt_stunden_tag]";
            OleDbCommand cmd5 = new OleDbCommand(sql_abfrage5, cn5);
            OleDbDataReader dr5 = cmd5.ExecuteReader();
            while (dr5.Read())
            {
                datum_fett = dr5["datum"].ToString();
                DateTime dateTime = DateTime.ParseExact(datum_fett, "dd.MM.yyyy", CultureInfo.InvariantCulture);
                list.Add(dateTime);
            }
            monthCalendar1.BoldedDates = list.ToArray();
            cn5.Close();