Calling C# function from C++/CLI - Convert Return C# string to C String

Question

I have a C# function that I made into a DLL:

public static string Test(string name)
{
    return "Hello " + name;
}

In C++/CLI project I successfully import that DLL, now I want to have a way to call that function and make it available for normal unmanaged C++. So I want to export the C++/CLI function like this:

extern "C" __declspec(dllexport)
void __stdcall Example(char* name, char* greet) {
    // name will be passed to C# Test(...) function
    // and greet will contains the returned value

    // call to the C# function here:
    ...
}

I dont care what the C++/CLI function looks like, as long as i can export it to normal unmanaged C++.

** Edit: as someone complain about my question, I only need to know how you can call the C# function, given a C string, and how to retrieve the returned result and store it in another C string. It's not like a "problem", it's like a newbie who don't know how to code, and come here to ask... Thank you **

** Edit2: now i noticed, someone has edited my post (i dont know, a moderator or someone...). Now when i re-read my post, even i don't know what the post trying to ask... Please, i think you shouldn't do that **

Answer 1

With C++/CLI you have everything you need at your disposal.

You could do it this way:

#include <string>
#include <msclr\marshal_cppstd.h>

extern "C" __declspec(dllexport)
void __stdcall Example(char* name, char* greet) {
    // name will be passed to C# Test(...) function
    // and greet will contains the returned value

    // Create new System::String^ from char*
    System::String^ clrString = msclr::interop::marshal_as<System::String^>(name);

    // Call C# function
    System::String^ result = Test(clrString);

    // Create new std::string from System::String^
    std::string cppString = msclr::interop::marshal_as<std::string>(result);

    // Copy C++-string to the destination
    strcpy(greet, cppString.c_str());
}

This solution uses std::string . You can also use a marshal_context to directly convert between System::String and char[] , but I prefer to use std::string since it saves you some typing and there is less to go wrong.

Of course, one can shorten this down to:

strcpy(greet, marshal_as<string>(Test(marshal_as<String^>(name))).c_str());

Or even further, since System::String has a constructor accepting char* :

strcpy(greet, marshal_as<string>(Test(name)).c_str());

Have a look here for more info about marshalling: http://msdn.microsoft.com/en-us/library/bb384865.aspx

Important: C# uses dynamic strings and C# code often happily generates very long strings, resulting in a crash or worse if the memory pointed to by greet is not large enough to contain the string. A common way to deal with this is to change the signature of Example to something like this:

void __stdcall Example(char* name, char* greet, size_t destBufferSize)

And check if the destBufferSize is large enough to contain the resulting string or truncate the value using strncpy or similar methods.