Wsrtjtyk

I have a C++ dll in 64-Bit, which contains a function that returns an LPStr. I would like to call this function in C#. The function declaration looks like this:

__declspec(dllexport) LPSTR __stdcall function(int16_t error_code);

In my C# code I have tried the following:

 [DllImport(@"<PathToInterface.dll>", EntryPoint = "function")]

 [return: MarshalAs(UnmanagedType.LPStr)]

 public static extern string function(Int16 error_code);

And then in the program:

string ErrorMessage = "";

ErrorMessage = function(-10210);

I know that the function itself is good, as I can call it from another program (written in LabVIEW FWIW). But when I execute the C# Program, it just exits with error code 0x80000003, I can't even try, catch the exeption.

How do I call this function properly?

As a side node: I do have other functions in this dll, that use LPStr as parameters, which I can call without a problem. It is only two functions that return LPStr that make problems

How do I call this function properly?

As interop? you can't ... it is also error prone in plain C++

you should rather do it like

extern "C" __declspec(dllexport)  int __stdcall function(int16_t error_code, 

             LPSTR buffer, size_t size)

{

    LPCSTR format = "error: %i";

    size_t req = _scprintf(format, error_code); // check for require size

    if (req > size) //buffer size is too small

    {

        return req; //return required size

    }

    sprintf_s(buffer, size, format, error_code); //fill buffer 

    return 0;

}

And usage

class Program

{

    static void Main(string args)

    {

        short error_code = -10210;

        var ret = function(error_code, null, 0); // check for required size of the buffer

        var sb = new StringBuilder(ret); // create large enough buffer

        ret = function(error_code, sb, (uint)sb.Capacity + 1); //call again 

        var error_desc = sb.ToString(); //get results

        Console.WriteLine(error_desc);

        Console.ReadKey();

    }



    [DllImport("TestDll.dll", EntryPoint = "function", CharSet = CharSet.Ansi)]

    public static extern int function(short error_code, StringBuilder sb, int size);

}

usage in C++

typedef int (__stdcall *function)(int16_t error_code, LPSTR buffer, size_t size);

int main()

{

    auto handle = LoadLibrary(L"TestDll.dll");

    auto proc = (function)GetProcAddress(handle, "_function@12"); 

    // of course it can be done via linking



    int16_t error_code = 333;

    const int ret = proc(error_code, NULL, 0); // check for size

    CHAR* buffer = new CHAR[ret + 1];

    //CHAR buffer[200]; //eventually allocate on stack 

    //but size have to be constant value and may be too small

    proc(error_code, buffer, ret+1); // call again 

    MessageBoxA(0, buffer, "Return", 0); //show result

    delete buffer; //free buffer!



    FreeLibrary(handle);

    return 0;

}