You don't "validate" a DateTimePicker - instead you use the
MinDate[
^] and
MaxDate[
^] properties to restrict the range that it permits and it will validate them itself.
So when you handle the ComboBox changed event, set the MinDate to teh first of that month, and the MaxDate to the last day of the month (Which is simple: Use
DateTime.AddMonths[
^] to add +1 month, then
DateTime.AddDays[
^] to add -1 days to get you to the last day of the month you want)
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