Using Visual Studio 2015 and a Visual Studio Asp.Net Project I am increasingly getting 2 errors:-

CSC.exe has stopped working (APPCRASH):-

And CPU hitting 100% due to VBCSCompiler showing multiple times taking loads of memory too.

Fix:-

Close all instances of Visual Studio 2015 and if they don't do it automatically disappear, end tasks for all the VBCSCompiler processes.  In your Visual Studio project under Bin folder you will see a roslyn folder (new for VS 2015).

Delete the roslyn folder and all contents within.

Restart Visual Studio 2015, re-compile your project and all should be well again.