Use dllimport for class extended

One solution would be to use Bridge pattern.
Software design pattern - Wikipedia, the free encyclopedia[^]
Bridge pattern - Wikipedia, the free encyclopedia[^]

Thus you create a bridge that would allow you to simulate a managed C# class deriving from an unmanaged C++ class. Of course, you cannot directly do that. But it is not hard to create a managed interface in C++/CLI that mimic your original interface and then have also a class that derive from your original interface and forward all calls to the managed interface.

As it required a few lines of code, I won't write it. If you know how to hold managed data inside a native class, then it should be relatively trivial to implement.

On the other hand, do you really need to implement a native C++ abstract class using managed C# class? In fact, mixing managed and native code does required some knowledge about interop.

By the way a book like this one might be very useful if you really want to write such code: Manning | C++/CLI in Action[^]

P.S. See also my comments under question and second solution.

Take a look at this example: How to create dll in C++ for using in C# - Stack Overflow[^]

C++ Code (DLL), eg. math.cpp, compiled to HighSpeedMath.dll:

extern "C"
{
    __declspec(dllexport) int __stdcall math_add(int a, int b)
    {
        return a + b;
    }
}

C# Code, eg. Program.cs:

namespace HighSpeedMathTest
{
    using System.Runtime.InteropServices;

    class Program
    {
        [DllImport("HighSpeedMath.dll", EntryPoint="math_add", CallingConvention=CallingConvention.StdCall)]
        static extern int Add(int a, int b);

        static void Main(string[] args)
        {
            int result = Add(27, 28);
        }
    }
}