Binding with StringFormat on Windows Phone 7?

I'm doing some Windows Phone 7 development and want to format a string I've bound to.

For some reason I cannot use the StringFormat like shown below. The option just isn't there.

 <TextBlock Text="{Binding Distance, StringFormat=\{0\}km}" />

开发者_开发问答Why isn't the StringFormat available for Windows Phone 7?

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StringFormat was added with 7.1 / Mango since Mango uses SL4. An example from Visual Studio Magazine:

<TextBlock Text="{Binding CurrentDate,StringFormat='dddd, d MMMM yyyy'}" />