read multiple proxy result data in edit mode

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Silverlight4

need to bind data for multiple combobox controls in edit mode.but due to asynchronous calls I'm not able to read the proxy result .

I can do this in constructor itself but those controls were dependency(one controlid is need for another control for that specific data)so I can't bind this in constructor.

When user clicks on edit based on the data I need to get the data again from service and bind to my controls but proxy won't hit at this moment.