From the last sentence is not exactly clear which type of type you are using: RadMaskedNumericInput or RadMaskedTextInput. Can you confirm which type you are using?
If we have correctly understood your scenario you are trying to set the mask to zero instead of null when you clear the mask. If this is the case, you can use the MaskedInputExtensions class and set the AllowNull attached property to False. This way when the ClearCommand of the mask has executed the value will be set to zero. Basically, binding a button to the ClearCommand of the mask will not break the MVVM pattern. Here is the sample code:
You can take a look at the MaskedInputExtensions
help article where this class is further described.
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