What exactly is the "Multi-threaded Debug DLL" Runtime Library option doing in VS 2008?

Answer 1

The main difference between the two options is in the libraries that your code will be linked at later. for the debug version for example this will include LIBCMTD.LIB and a few others. if your library is going to be built as debug the you should always link with MDd. failing to do so will result in lots of unresolved external linker errors at best. and sometimes the code compiles normally but crashes at runtime. if this happens in vb.net then a catch can easily hide the error. I guess you should make sure you build setting is correct. for more detailed information check this.

Answer 2

All DLL's/debug code generation must match across everything that uses them. There may be another referenced library or object or dll or some code in there that is built using the wrong options; or specific options for an individual element that override the global project options.

The only way of figuring it out is to meticulously check all of the options for each file, checking the included and referenced libraries (.lib and .dll) and object files. Check the linker options too.

The reason why it doesn't work is probably because the debug version adds extra guard blocks around memory to allow detection of errors.