When I was doing C# code reviews at first in VS 2012 I was getting a side by side comparison of the old and new code. However now I am getting all code in the same window with r
There should be a button on the toolbar like this one that will let you select the 4 different "diff modes".
You can also use the following keyboard shortcuts:
Ctrl+\
Ctrl 1
Ctrl 2
Ctrl 3
Ctrl 4
to change the view.