Calling C++ function with LPStr return value from C#

Question

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 `function` is implemented? Most important is how you are allocating return value — Selvin, Nov 07 '18 at 21:47
I read that you will need to defined export with an extern "C" block like this: extern "C" __declspec(dllexport) int captureCamera(int& captureId) { ... } — Brian Clink, Nov 07 '18 at 22:00
0x80000003 is E_INVALIDARG. Now, i obviously don't know which piece of the native code executed by that DLL function causes the error; but start with verifying whether the function you call actually "understands" the value `-10210` (i.e., does a corresponding error message/text string exist for this particular value?) — , Nov 07 '18 at 22:01

Selvin · Accepted Answer · 2018-11-08T11:02:13.533

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;
}

Thanks for your answer. It is a shame, that this cannot be handled differently as not everyone has that much control over their Dlls. (It could be a third party dll, in which case that would be it). We followed your instructions and managed to make it work that way. — Pete, Nov 12 '18 at 18:45

Calling C++ function with LPStr return value from C#

1 Answers1

Linked