C# / C++ application crashes when run from Windows, but not from Visual Studio

Question

I'm developing a C# application that calls into an unmanaged (C++) dll. I've found a certain user operation that consistently leads to the application crashing when run from Windows Explorer. However, when launched from the Visual Studio debugger, the crash does not occur. I therefore cannot step into the code at the time of the crash and debug exactly what's going on.

What might cause the binary to crash when run from Explorer, but not from Visual Studio? Note that I'm using a Release build; Debug builds crash neither in Visual Studio nor in Explorer.

(If it's relevant, I can say that the crash has to do with manipulating a malloc-allocated array in the C++ DLL. I tracked it down by painstakingly commenting out blocks of code, rebuilding, running from Windows, & checking whether or not the crash occurs. However, I've reached a point where it's become very difficult to proceed without being able to properly break in the debugger).

I'm just interested in being able to recreate the crash within Visual Studio.

Answer 1

When a program works in debug but crash in release, the problem is really often buffer overflow, so you should look for something like an incorrect buffer length variable.

About why it is not crashing when debugging, here is a small article about the side effects of debugger. As you can read, the heap can have a different behavior when a program is launched by a debugger. Buffer overflows often happen on the heap, and you said it might happen with a malloc-ed buffer, so that would be the reason.

Now, to make your program crash while debugging, the only way is probably to attach after launch. If setting breakpoints in the DLL project doesn't work, try to start the debugging with the DLL project instead, and specify your executable as DLL host. If you can't hit breakpoints like that, well, you can always set breakpoints in assembly code, that should always work, but isn't really practical. You can add a compiled assembly breakpoint using __debugbreak() on MSVC.

Note that differences might still happen even when attaching, a debugger will always slightly alter the target behavior. Though only undefined behaviors should change.

Edit: I missed that, but the article says that you can disable debug heap by setting an environment variable _NO_DEBUG_HEAP = 1. That might be the simplest solution to debug your problem (if it works).