I second this question, and to the "why do you need this" posters I say "not everyone uses Visual Studio as their editor; we don't, but when debugging code it is too easy to accidentally modify the source file, which we don't want. I'm not in Visual Studio for any other reason than to debug my code, for reasons that are irrelevant to the question.

Every editor I've ever used has the ability to open files read-only, so why is this question being scorned? I want my debugger to have an option to always open source files read-only, and will manage the edits to the file elsewhere.

Most common example: sometimes the wrong window is active while I'm debugging, and I accidentally type into the source file via the debugger, not into the intended window (be it email or a DOS command prompt or whatever). Does nobody else's active window switch out from under them unexpectedly?