C++ Calling Function from C# COM DLL

Question

I'm trying to call a string function from a C# DLL that I made (to test COM out). I have a function called add which takes two ints and returns the sum. This seems to work. My other function, returnString, takes a string and returns it. This ends up printing out something blank. Here is the code:

C#:

public class Class1 : MyClass
{
    public string returnString(string a)
    {
        return a;
    }

    public int add(int a, int b)
    {
        return a + b;
    }
}

C++:

int main()
{
    CoInitialize(NULL);

    MyClassPtr obj;
    obj.CreateInstance(__uuidof(Class1));

    BSTR string_result = L"\0";
    int int_result = 0;

    HRESULT hr1 = obj->returnString(L"Hello", &string_result); // should set string_result to "Hello"
    HRESULT hr2 = obj->add(5, 7, (long*)&int_result); // should set int_result to 12

    if (hr1 != S_OK)
        std::cout << "hr1: " << hr1 << std::endl;
    else{ 
        _bstr_t str(string_result);
        std::cout << str << std::endl; // prints a blank line rather than "Hello"
    }

    if (hr2 != S_OK)
        std::cout << "hr2: " << hr2 << std::endl;
    else
        std::cout << int_result << std::endl; // prints 12

    CoUninitialize();
}

Here's what I get when I peak the definitions:

virtual HRESULT __stdcall returnString (
    /*[in]*/ BSTR a,
    /*[out,retval]*/ BSTR * pRetVal ) = 0;
  virtual HRESULT __stdcall add (
    /*[in]*/ long a,
    /*[in]*/ long b,
    /*[out,retval]*/ long * pRetVal ) = 0;

So what's wrong with the code? I posted a similar question recently and found out a little bit more, but I still cannot get it to behave properly. Thank you!

I could be totally off (I'm not a C#/COM guy), but can a C# string be mapped correctly to a BSTR? In that, when transferring things to and from C++ it's common to use character pointers, as most string types don't translate, Does COM have similar issues? — John Bargman
– John Bargman, Commented Feb 26, 2016 at 1:41
I'm not a C# guy myself. I just wanted to make a C# DLL compatible with C++ so that I could help someone make a more versatile/portable DLL. So I wouldn't know. — Archie Gertsman
– Archie Gertsman, Commented Feb 26, 2016 at 1:45
I figured out the problem. I had to cast L"Hello" to type "CComBSTR", and now it prints out fine. But I will consider standard linkage, as this seems a bit stressful. — Archie Gertsman
– Archie Gertsman, Commented Feb 26, 2016 at 2:12

Archie Gertsman · Accepted Answer · 2016-02-26 02:21:23Z

1

Here was the problem: L"Hello" needed to be casted to type CComBSTR. This is what the result should look like:

HRESULT hr1 = obj->returnString((CComBSTR)L"Hello", &string_result);

Everything else can remain the same :)

answered Feb 26, 2016 at 2:21

Archie Gertsman

1,6612 gold badges20 silver badges50 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

C++ Calling Function from C# COM DLL

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related