Validating Two DateTime Pickers

[joe_pool_is]

I have a Windows VS2005 Form where my Clients can select a Start Date and an End Date.

 

Obviously, the Start Date must occur before the End Date, or there will be problems, so I need to validate the entries.

 

Where should I validate these controls, and how should this be done?

 

I have existing code that validates the control and give the Client a friendly message box when it is incorrect when the DateTimePickerControl's ValueChanged event is fired:

void DateTime_ValueChanged(object sender, EventArgs e) {
 if (sender is DateTimePicker) {
   if (dtEnd.Value < dtStart.Value) {
     string strCtrl = string.Empty;
     if (sender.Equals(dtStart) == true) {
       strCtrl = lblStartDate.Text;
       DateTime newVal = dtStart.Value.AddDays(-1);
       while (dtEnd.Value < newVal) {
         newVal = newVal.AddDays(-1);
       }
       dtStart.Value = newVal;
       dtStart.Focus();
     } else if (sender.Equals(dtEnd) == true) {
       strCtrl = lblEndDate.Text;
       DateTime newVal = dtEnd.Value.AddDays(1);
       while (newVal < dtStart.Value) {
         newVal = newVal.AddDays(1);
       }
       dtEnd.Value = newVal;
       dtEnd.Focus();
     }
     string fmt = string.Format("{0} must be less than or equal to {1}.\r\n\r\nYour {2} has been modified.",
                                 lblStartDate.Text, lblEndDate.Text, strCtrl);
     MessageBox.Show(fmt, "Invalid Date", MessageBoxButtons.OK, MessageBoxIcon.Information, 0);
   }
 }
}

 

If someone drops the Calendar down and tries to scroll to a month or year that is out of range, the GUI goes nuts with message boxes from my code until the system runs out of memory.

 

I tried rewriting the code to only respond when the calendar scrolls back up, but then validation would fail whenever a date was manually entered in the textbox.

 

How should I address this?

[PlausiblyDamp]

If you use the Validating Event rather than the ValueChanged event it should prevent the problem with the event triggering itself repeatedly.

 

Also you might consider an ErrorProvider rather than a MessageBox as a less intrusive way of notifying the user.

[joe_pool_is]

That helped a lot! Thank you.