Sharing an enum from C#, C++/CLI, and C++

问题:

I have a library that consists of three parts. First is native C++, which provides the actual functionality. Second is a C++/CLI wrapper/adaptor for the C++ library, to simplify the C# to C++ transition. Finally I have a C# library, which invokes the C++ library through the C++/CLI adaptor.

Right now there I have two sets of parallel enum definitions, one stored in a .cs file and the other in a .h file. This poses a double problem:

1. I have dual maintenance. I must always synchronize changes of an enum in both file locations.
2. The namespace used by both enums should be identical but the C++/CLI wrapper, which views both sets of enums and translates between them, incurs a naming collision.

Right now I'm not sure a solution such as this or that would solve both problems. Thoughts?

回答1:

Just put your #include "Enum.cs" directive inside an outer namespace to resolve the naming collision.

EDIT: A variation suggested by Brent is to use #define to substitute one of the namespaces (or even the enum name itself) declared in the .cs file. This also avoids the naming collision, without making the namespace hierarchy deeper.

回答2:

Even if you include the C# enum in your native C++ (as suggested in your first link), both enums are not "the same", the C++ enum is nothing but a list of named integers, while the C# enum is derived from Enum. As a consequence, you get a collision in C++/CLI when trying to use them both.

A possible solution is to use the preprocessor so that your C++/CLI assembly sees both enums in different namespaces:

// shared_enum.h

#undef ENUMKEYWORD
#undef ENUMNAMESPACE

#ifdef MANAGED
#define ENUMKEYWORD public enum class
#define ENUMNAMESPACE EnumShareManaged
#else
#define ENUMKEYWORD enum
#define ENUMNAMESPACE EnumShare
#endif

namespace ENUMNAMESPACE
{
    ENUMKEYWORD MyEnum
    {
        a = 1,
        b = 2,
        c = 3,
    };
}

In your C++/CLI code, make an inclusion like that:

#undef MANAGED
#include "shared_enum.h"
#define MANAGED
#include "shared_enum.h"

This gives you the availability to distinguish between those two kind of enums EnumShare::MyEnum or EnumShareManaged::MyEnum in your C++/CLI code.

EDIT: just found this SO post showing the correct way to cast between unmanaged and managed enums, this surely will work here, too. For example, in the C++/CLI, the transition from managed to unmanaged enum can be done like this:

void MyWrapperClass::MyWrapperFunction(EnumShareManaged::MyEnum mx)
{
    EnumShare::MyEnum nx = static_cast<EnumShare::MyEnum>(mx);
    // call a native function "func"
    func(nx);
}

回答3:

Consider writing code generator program, which reads native h-file file with enumerations and generates another h-file, converting enum to C++/CLI enum class. Such code generator can be used in C++/CLI project on the Custom Build Step, producing required CLI enumerations.

I use this approach to generate native wrapper classes to get Enum::GetNames and Enum::GetName functions in unmanaged C++.