MaskedTextBox - Testing For Dirty (Returns Error If Blank)

Using VB.Net 2010 (Pro)

I have a form that contains various textboxes, comboboxes and a couple of maskedtextboxes.  These last ones are giving me fits.

The value in the maskedtextboxes are all formatted as shortdate.  It's possible for them to be empty (no date).  Occassionally there will be a date that the user must remove (make it blank).

I'd like to show if a form is "dirty", that is if the user has changed the data from the last time it was loaded or saved.  To that end, I have a structure where I keep all the values from the various controls on the form.  Everything works fine except for those darned maskedtextboxes, when they are empty.  During the checkForDirty() routine, I get to a line such as:
bDirty = bDirty Or CDate(txtRegExpiry.Text) <> IIf(IsNothing(.Reg_Expr), "", .Reg_Expr)

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which, if the maskedtextbox is empty will throw the error:
Conversion from string "  /  /" to type 'Date' is not valid.
Note: It is the control (CDate(txtRegExpiry.Text)) that is specfically throwing the error.

I tried removing the CDate() but that didn't help.

So, the question is, how do I compare a maskedtextbox to a variable that can either be a date or nothing without throwing an error?

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You should be able to test the value before evaluating it, using the MaskFull property:

If (myMaskedTextBox.MaskFull) Then
    ' Filled out....
    ' Not filled out....
End If

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return Nothing
return CDate(txtValue.Text)
ClifAuthor Commented:
I'm not sure how that will allow me to compare the value in txtRegExpiry with the value in VehicleList(i).Reg_Expr
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The problem occurrs when you try to compare a blank value to a date, correct?
ClifAuthor Commented:

Your example gives basically the same error.

The value of an empty MaskedTextBox that is formatted as shortdate is: "/  /"  (at least that is what is it in this case)
ClifAuthor Commented:

The problem occurs (at this point) while I'm comparing the value in a MaskedTextBox named txtRegExpiry with a value in a variable named VehicleList(i).Reg_Expr and the MaskedTextBox is "empty" (which appears, based on my prior post, to mean the prompt is still there, but there is no other values).
Right MaskFull will tell you if it's been fiilled out or not. You can't compare a date to a mom-date so first you determine whether it's a date at all. If it is, then you do your comparison.
Mom-date.... Oops. Typing on a phone. Non-date
see what i wanted to tell you was check for the length of value in masked text box, if it is greater than 0 then only do the comparison otherwise do some default action as per your system requirements...

//do some nice stuff here
//move out of here

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He's saying the same thing. Just different way.
ClifAuthor Commented:
That's all well and good, except that a partial fill (ie, month and day but no year) in the MaskedTextBox will show up as equal to the empty variable (that is, not dirty), which is not correct.
use DateTime.TryParse to see if the value is valid datetime or not....
see an example here:-
ClifAuthor Commented:
Rather than going for length, I decided to go for literal.  I'm testing for "/  /".  I'm sure if we ever go international, I'll have to revisit this, but for the time begin it now works.

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