Hello kfir ANKORI,
There is nothing wrong in your code. In fact it works perfectly when run in Silverlight. The problem here is that you've placed the format provider in UserControl's resources. This would be the approach in Silverlight, but here in WPF the format provider has to be present in the visual tree for the binding to work. This is due to a limitation in WPF, where bindings with ElementName don't work when specified within the resources.
So everything you have to do is just move it somewhere in the Grid.
If you meet any other difficulties, feel free to contact us immediately.
the Telerik team
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