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 va开发者_Python百科lidation 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:

1. vm property is a string which is validated and updates the backing model.
2. 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

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=''}