Action<DateTime> addItem = dateTime =>
dropDownList.Items.Add(new ListItem(dateTime.ToString("MMMM"), dateTime.ToString("O")));
addItem(dateTime1);
addItem(dateTime2);
addItem(dateTime3);
addItem(dateTime4);
или просто добавьте ментод
private void AddItem(DateTime dateTime)
{
dropDownList.Items.Add(new ListItem(dateTime.ToString("MMMM"), dateTime.ToString("O")));
}
protected void Page_Load(object sender, EventArgs e)
{
AddItem(dateTime1);
AddItem(dateTime2);
AddItem(dateTime3);
AddItem(dateTime4);
}