I had the same problem and fixed it with this solution: 1) create a solution to any project you want (winforms, etc.), 2) find the project folder and find the .sln file. 3) open it using notepad. 4) copy the first 4 lines as shown here .
5) find your .sln file for the project you want to downgrade. for example: foo.sln (its found where your project is located). 6) open it with a notebook and insert lines instead of the first 4 lines 7) save and open sln. he must open it under visual studio 2015
Hope this works for you.
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