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The MessageBox.Show method calls a function of the browser. The MessageBox doesn't belong to Silverlight. As the MessageBox is opened modal by the browser, the browser waits for the response, and so also Silverlight has to wait for it.
The RadWindow components are silverlight components and we cannot implement such functionality. We cannot freeze the main thread.
Please do not hesitate to contact us if you require any further information.
the Telerik team
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