Program.cs

#if CS

using System.Runtime.InteropServices;

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Managed");
        var r = Compute(12, 12);
        Console.WriteLine($"Result: {r}");

        Console.WriteLine("Native");
        r = ComputeCompiled(12, 12);
        Console.WriteLine($"Result: {r}");
    }

    [DllImport("ComputeCompiled.dll")]
    static extern int ComputeCompiled(int a, int b);
#elif CPP

// cl /LD /Fe:ComputeCompiled.dll /Fd:ComputeCompiled.pdb /Zi /ZH:SHA_256 /D"CPP" /TpProgram.cs

extern "C"
{
    static int Compute(int,int);

    __declspec(dllexport) int ComputeCompiled(int a, int b)
    {
        return Compute(a, b);
    }
#endif

    static int Compute(int a, int b)
    {
        int c = a + b;
        int d = b + c;
        int e = d * c;
        return e * 12;
    }
}