How to implement string passing in C++ dll, change it and return it to .Net C# project?

Question

C++ / C#

There is a project in C #, a dll goes to it on the pluses. It is necessary to pass a string from C # to a function, in which to change it (add / delete a character) and return it back. I collected information on the network on this topic, I did not find a solution. Here I found this toster.ru/q/58640, but this is just a transfer of a string from a lib to C#.
It would be great if the answer to my question is a code example in which, on the C# side, we call a function either with two parameters (strings), in C++ we combine them, return them to C# and display them on the screen.

Answer 1

To describe and call functions from regular dlls in c/c++, the Marshal class is used.
You can allocate memory, convert a string, structure to unmanaged memory and vice versa.
The example calls the GetComputerName function, which fills the memory allocated in the program, and then the result is converted to a string and printed to the console.

class Program
{
    static void Main(string[] args)
    {
        // Выделение памяти из неуправляемой области
        // Получаем обычный указатель для использования в c/c++ функциях
        var pComputerName = Marshal.AllocHGlobal(256);
        // Вызываем описанную внешнюю функцию
        // Она будет работать непосредственно с памятью по указателю
        int size = 256;
        GetComputerName(pComputerName, ref size);
        // Переводим результат в управляемый вид
        var str = Marshal.PtrToStringUni(pComputerName);
        Console.WriteLine(str);
        // Обязательно освобождаем выделенную память
        Marshal.FreeHGlobal(pComputerName);
        Console.ReadLine();
    }

    [DllImport("kernel32.dll", CharSet = CharSet.Unicode)]
    public static extern void GetComputerName(IntPtr pComputerName, ref int size); 
}