TextBox.Watermark in Silverlight 4

While browsing MSDN documentation, you may come across this gem: TextBox.Watermark.

“Awesome! I’ve been wanting a built-in way to do watermarking on my text boxes! This is great, let me go ahead and set that in XAML!”

Unfortunately, if you run this you won’t get what you expect:


And the detail:


What is this? Well, look at the MSDN documentation closely:


That’s right. It’s supported in Silverlight 4, but it also says “Do not use in a Silverlight 4 application”. If you do use it, you receive a System.NotImplemented exception. To verify, here is the code for the property decompiled via Reflector:

There it is – it throws an exception any time it’s not in design mode. This makes no sense right? Why would Microsoft do this?

Unfortunately I haven’t found any definitive answer yet, however if I had to guess it’s because Microsoft is planning on implementing a Watermark behavior on the TextBox control in a future version (perhaps v5) and wanted to effectively reserve this property so third party control creators don’t subclass TextBox and create their own Watermark property. I know of at least one control vendor, ComponentOne, who has a control that inherits from TextBox and provides a Watermark property. To me, it seems this is Microsoft’s way of discouraging people from using this property name on their own TextBox subclasses.

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