If I create a new Win32 Console project (unmanaged C++, Visual Studio 2005), 
and add the following to the main program:

  // Add this above the main routine
  #include <windows.h>  // Add this at the top of the file

  // Add this to the main routine
  MessageBox(NULL, "A", "B", MB_OK);

The compiler (Visual Studio 2005) complains that it can't convert parameter 
2 from 'const char [2]' to 'LPCWSTR'.

I don't understand why the compiler apparently thinks that I am using 
multi-byte character semantics.  The really bizarre part is that I have 
another project that contains calls to MessageBox, and that project compiles 
just fine.  I've looked at the project properties in both projects, but I 
can't see anything that would obviously cause one to compile correctly and 
one to fail to compile.

This all began when I tried to write some code that formats a message and 
sends it to the MessageBox function, like this:

  ostringstream msg;
  msg << "my message expression";
  MessageBox(NULL, msg.str().c_str(), "Title", MB_OK);

In this case, the compiler complains that it can't convert parameter 2 from 
'const char *' to 'LPCWSTR'.  But, as I said, I have this identical code in 
another project and it works just fine.

TIA - Bob