C++ from C#: C++ function (in a DLL) returning false, but C# thinks it's true!

Question

I'm writing a little C# app that calls a few functions in a C++ API. I have the C++ code building into a DLL, and the C# code calls the API using DllImport. (I am using a .DEF file for the C++ DLL so I don't need extern "C".)

So far the API has one function, which currently does absolutely nothing:

bool Foo()
{
  return false;
}

In C#, I have the following:

public class FooAPI
{
    [DllImport("Foo.dll")]
    public static extern bool Foo();
}

...

bool b = FooAPI.Foo(); 
if (!b) 
{ 
    // Throw an exception 
} 

My problem is that, for some reason, b is always evaluating to TRUE. I have a breakpoint on if (!b) and the debugger reports it as 'true', irrelevant of whatever the C++ function is returning.

Is the C# bool the same as the C++ bool? Though even if this wasn't the case, I still don't get how it would find the return value to be 'true' :)

Can anyone help me with this bizarre discrepancy?

Thanks in advance!

Answer 1

Try [return: MarshalAs (UnmanagedType.I1)]. By default, C# interop marshals C# bool as the Win32 BOOL, which is the same as int, while C++ bool is one byte AFAIR. Thus, C#'s default marshaling expects the return value to be a BOOL in the eax register, and picks up some non-zero garbage because C++ bool is returned in al.