Visual studio was consistently crashing when exiting, with 2 specifics solutions…
I had to close all documents, collapse all sub projects, and select the solution in the Solution explorer to avoid VS to crash…

Yesterday, I found the cause: one child class library project was set to compile in asp.Net 2.0,
while all other projects were set to compile for asp.Net 4.0…

To change the target framework of a class library project,
open its properties,
select the tab “compile”,

go to the advanced options :

And select the target network:

That’s all folks