Q:

WPF (.net 3.5) Binding to number (int, dbl) but allow user to input nothing

If you bind to a view-model property that is a nullable double (or int) how do you allow a user to specify nothing.  
I'm looked into validation but I can't see how I could get that to help.
It seems like if a textbox is blanked out WPF sees it as an empty string and then doesn't put the into  the vm proptery.
So 2 ways around it I can think of is:

vm property is a string which is
validated and updates the backing
model.
special code that indicates no value (in my case the dbl should not
be neg so -9 could be a null...but
don't like the idea of magic
numbers)

Any other ideas?  Am I missing something?
cody

A:

Set TargetNullValue to the empty string on the binding.  When WPF sees this value, it will set the source to null instead of trying to convert the value.  
{Binding TargetNullValue=''}

