从C＃COM DLL调用C ++

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 int s 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!

Answer 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 :)