System.Diagnostics.Debug.WriteLine("")
You can debug the native code in the windows phone Runtime Component by selecting the native debug engine for the UI Task. in this case you will also get the debug output.
to select the native debug engine for UI Task
1. Go to debug property page by right clicking on the project and selecting the "properties" menu followed by selecting the "Debug" tab.
2. Under the "UI Task" there is a drop down. select "Native Only"