test.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using System.Threading.Tasks;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public class Test
{
    public unsafe static int Main(string[] args)
    {
        Console.WriteLine("Hello World!");
        delegate* unmanaged[Cdecl]<int, int> unmanagedPtr = &Callback;
        NativeFunctionWithCallback((IntPtr)unmanagedPtr);

        return args.Length;
    }

    [UnmanagedCallersOnly(CallConvs = new[] { typeof(CallConvCdecl) })]
    public unsafe static int Callback(int i)
    {
        Console.WriteLine ($"-- managed callback called with i: {i}");
        return i * i;
    }

    [DllImport("NativeLib")]
    private unsafe static extern void NativeFunctionWithCallback(IntPtr callback);
    //private unsafe static extern void NativeFunctionWithCallbackUsingFunctionPointers(delegate* unmanaged[Cdecl]<int, int> callback);
}