It looks like DateTimePicker raises ValueChanged twice:

• once with MaxDate
• once with current Value

on DropDown opened, if Value is not the MaxDate.

To reproduce drop DateTimePicker to the form, put

dateTimePicker1.MinDate = new DateTime(2024, 5, 31);
dateTimePicker1.MaxDate = new DateTime(2024, 6, 2);
dateTimePicker1.Value = dateTimePicker1.MaxDate.AddHours(-1);

to the Load event handler.

Wire up ValueChanged event and add some debug output:

private void dateTimePicker1_ValueChanged(object sender, EventArgs e)
{
   System.Diagnostics.Debug.WriteLine(DateTime.Now.ToString("mm:ss:ms") + " ::  " + dateTimePicker1.Value.ToShortDateString() + " __ " + dateTimePicker1.Value.ToShortTimeString());
}

Each time the DropDown being opened the event ValueChanged fired twice (except Value is MaxDate, in this case no ValueChanged being raised as expected).

Is there any setting, which triggers this odd behavior, or is it a “by design” bug?