Question

I have a C++ dll that I need to call from C#. One of the functions in the dll requires a char* for an input parameter, and another function uses a char* as an output parameter.

What is the proper way to call these from C#?

Answer 1

Just using strings will work fine for input parameters, though you can control details about the string with the MarshalAs attribute. E.g.

[DllImport("somedll.dll", CharSet = CharSet.Unicode)]
static extern void Func([MarshalAs(UnmanagedType.LPWStr)] string wideString);

As for returning char* parameters, that’s a little more complex since object ownership is involved. If you can change the C++ DLL you can use CoTaskMemAllocate, with something like:

void OutputString(char*& output)
{
    char* toCopy = "hello...";
    size_t bufferSize = strlen(toCopy);
    LPVOID mem = CoTaskMemAlloc(bufferSize);
    memcpy(mem, toCopy, bufferSize);
    output = static_cast<char*>(mem);
}

The C# side then just uses an ‘out string’ parameter, and the garbage collector can pick up the ownership of the string.

Another way of doing it would be to use a StringBuilder, but then you need to know how big the string will be before you actually call the function.