Windows Headers and MC++
Sometimes, mixing the Windows SDK headers and managed code can end up in unexpected results.

Consider this simple example:

#using <mscorlib.dll>
#using <System.Windows.Forms.dll>
#include <windows.h>

using namespace System;
using namespace System::Windows::Forms;

int main()
{
   MessageBox::Show(S"This is a MessageBox");
}

If you compile it, you'll get this error:

t.cpp(10) : error C2653: 'MessageBoxA' : is not a class or namespace name
t.cpp(10) : error C2065: 'Show' : undeclared identifier

Here, what you see is the #define's inserted by the Windows headers all over the place getting in the way. Specifically, the MessageBox() function in the Win32 API is no function at all, really, but rather a pair of them: MessageBoxA() and MessageBoxW(). These are, respectively, ANSI and Unicode variants of the method. The actual MessageBox name is just a #define that maps to one or the other depending on whether the UNICODE macro is defined or not.

What happens in the sample above is that the use of the MessageBox .NET class is replaced by the preprocessor into MessageBoxA, thus leading to an error because there's no class with this name. In other words, what the compiler actually sees in the code above is:

MessageBoxA::Show(S"This is a MessageBox");

The workaround is to temporarily undefine the MessageBox macro around the problem code, by adding this after #include'ing windows.h:

#undef MessageBox

The same can be done for other problematic macro definitions.