First of all, even if you implement such behavior, it cannot be called "validator". A "validator" is something which validates data after it is entered, but you want to filter out the input (selection) itself.
I fear to say such behavior is too far from expected UI behavior and would be very confusing. Implementation of it would be problematic, too.
There is a well-known equivalent UI presentation with the same logic:
a set of radio buttons. If you control
Enabled property of each radio button, you can enable or disable some of them depending on other data, effectively filtering the input the way you want.
See
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.radiobutton.aspx[
^].
—SA
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