Android.Net.Uri class not found

I have been facing a different/strange problem, I am new to Xamarin.Android. I would like to use Uri Class which is available under Android.Net, but it was not identified by Xamarin when I type in.. even it is giving compile time error, Did I miss anything? I am pretty sure that I included "using Android.Net"

Compile error: initially it was "ambiguity between System.Net.Uri and Android.Net. Uri So I have used like below "Android.Net.Uri contentUri = Android.Net.Uri.FromFile (_file);"

But it says now, " Net" does not exist in the namespace, Are you missing an assembly reference?