The title of the forum states that the SelectionLength returns null, but the content of your reply assumes that the SelectionStart returns 0. Can you please clear which one is your issue? However, the SelectionLength property is updated before the HandlePaste() method is executed and this is why when you try to get it is incorrect.
Can you please tell me why you need to handle the paste of the masked input control? If it is related to a paste bug workaround you can take a look at the following bug item
in our feedback portal and see if this is you case. If so, note that this big is fixed and the fix will be available in our next official release by the end of the next week. Otherwise, can you give us more information on your scenario? This way we might think of better approach to implement the desired result.
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