Leaving Masked text box control with no value written into it

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Visual-Studio

data-binding

Hello
I am using VB 2008 & ms access for creating WinForms application, I am also using Typed Dataset & databound controls.

for some of my data field in table i am using Masked text box control to write and read Date values and accordingly i have set the masked to short date format.

Now because the value tobe entered in table through Masked control is Optional, user may or may not write date into it. If i write nothing into it & leave the control after focus then it allows me to leave however, if i try to write some date in it and then decide not to enter any value, then the control doesnt allow me to leave the control until i write any valid date value.

same thing happens with text box control used to read and write numeric values, for text boxes i found the solution by using following code
TextBox.databinding(0).nullvalue=string.empty

now i want to know what do i set for Masked Text box used to read and write date values.

sandeep