__declspec(dllexport) ::vector<std::string>

Question

I've been trying to work out how to return an array of strings from a c++ dll to a c# application but am stuck on how to do this or find an article at a very basic level.

Suppose I have the code below. How do I fix the bolded line:

extern "C" {
    __declspec(dllexport) int GetANumber();

//unsure on this line:
    **__declspec(dllexport) ::vector<std::string> ListDevices();**

}

extern::vector<std::string> GetStrings()
{
    vector<string> seqs;
    return seqs;
}

extern int GetANumber()
{
    return 27;
}

thanks

Matt

Answer 1

You could use the COM Automation SAFEARRAY type, even without doing full COM (no object, no class, no interface, no TLB, no registry, etc.), just with DLL exports, as .NET supports it natively with P/Invoke, something like this:

C++:

extern "C" __declspec(dllexport) LPSAFEARRAY ListDevices();

LPSAFEARRAY ListDevices()
{
    std::vector<std::string> v;
    v.push_back("hello world 1");
    v.push_back("hello world 2");
    v.push_back("hello world 3");

    CComSafeArray<BSTR> a(v.size()); // cool ATL helper that requires atlsafe.h

    std::vector<std::string>::const_iterator it;
    int i = 0;
    for (it = v.begin(); it != v.end(); ++it, ++i)
    {
        // note: you could also use std::wstring instead and avoid A2W conversion
        a.SetAt(i, A2BSTR_EX((*it).c_str()), FALSE);
    }
    return a.Detach();
}

C#:

static void Main(string[] args)
{ 
    foreach(string s in ListDevices())
    {
        Console.WriteLine(s);
    }
}


[DllImport("MyUnmanaged.dll")]
[return: MarshalAs(UnmanagedType.SafeArray)] 
private extern static string[] ListDevices();