先检查一下，然后与你的代码进行比较..

http://msdn.microsoft.com/en-us/library/windowsphone/develop/hh202956%28v=vs.105%29.aspx?cs-save-lang=1&cs -lang = csharp＃code-snippet-3 [ ^ ]

Check this one first and then compare with your code..
http://msdn.microsoft.com/en-us/library/windowsphone/develop/hh202956%28v=vs.105%29.aspx?cs-save-lang=1&cs-lang=csharp#code-snippet-3[^]