Get Application Runtime Version

In C# I can do the following to get the application version at runtime. In a Xamarin.Android app this only returns 0.0.0.0 everytime.

Version appVersion = System.Reflection.Assembly.GetExecutingAssembly().GetName().Version;
Log.Debug ("app_version", string.Format("{0}.{1}.{2}.{3}", appVersion.Major, appVersion.Minor, appVersion.Build, appVersion.Revision));

How do you get the runtime version of the application in Xamarin.Android?