try this.
private void FillMyDropDown()
{
int[] datearr = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 };
int daysInMonthNumber = System.DateTime.DaysInMonth(DateTime.Now.Year, DateTime.Now.Month);
object[] currArray = new object[daysInMonthNumber];
Array.Copy(datearr, currArray, daysInMonthNumber);
MyDropdownID.Items.AddRange(currArray);
}