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.