I was porting an android code to C# and when I build I get this. I have no idea what that caused it. I also added CheeseBaron's SlidingMenuSharp to my project.
Update 1: Removing SlidingMenuSharp library fixes the issue. Anyone know the reason why is this happening?
Update 2: Adding reference to Mono.Android.Support.v4 fixes the issue even when I add the SlidingMenuSharp. So it looks like it was trying to look for the assembly.