C# DllImport with C++ boolean function not returning correctly

Question

I have the following function in a C++ DLL

extern \"C\" __declspec(dllexport) bool Exist(const char* name)
{
 //if (g_Queues.find(name) != g_Queues.end())
 /         


        

Answer 1

This is actually caused by EAX not being fully cleared out by typical C++ code that returns a bool. It's typical for EAX to contain some bogus value when entering a function, and to return false the compiler would typically emit xor al, al. This clears out only the LSB of EAX, and causes C# code to interpret the resulting non-zero value as true instead of false.