While XAML is not strictly necessary, it just provides for abstraction and separation between the UI layer code and server side code. XAML is rendered on the client machine while C# code would hit the server to get the required elements. So I would imagine that XAML can be good for saving a few server trips.
However, if you'd still rather write C# instead of XAML, see if this helps:
http://stackoverflow.com/questions/6548528/using-c-sharp-instead-of-xaml-in-silverlight[
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DockPanel[
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try this link as well, hope this helps
Cheers..