Multi-platform DLLImport

Program.cs

using System;
using System.Runtime.InteropServices;

namespace MySample
{
    internal class NativeMethods
    {
        // declare a delegate that fits 
        public delegate int MessageBoxFn(IntPtr hWnd, string text, string caption, int options);

        // storage for the delegate
        private static MessageBoxFn _messageBox;

        // property to choose the platform-appropriate delegate at runtime (single-time initialization)
        internal static MessageBoxFn MessageBox => _messageBox ?? (_messageBox = Environment.OSVersion.Platform == PlatformID.Win32NT ? (MessageBoxFn) MessageBoxWindows: MessageBoxLinux);

        // a more complex example of choosing (still single-time init)
        internal static MessageBoxFn MessageBox2
        {
            get
            {
                if (_messageBox != null)
                {
                    return _messageBox;
                }

                // choose the per-platform implementation
                switch (Environment.OSVersion.Platform)
                {
                    case PlatformID.MacOSX:
                        return _messageBox = MessageBoxLinux;

                    case PlatformID.Unix:
                        return _messageBox = MessageBoxLinux;

                    case PlatformID.Win32NT:
                        return _messageBox = MessageBoxWindows;

                    // not available on this platform
                    default:
                        return _messageBox = (hWnd, text, caption, options) =>
                        {
                            // fall back to complaining.
                            Console.WriteLine("MessageBox Missing for this platform");
                            return -1;
                        };
                }
            }
        }

        // delcare the imported function that binds to the Windows version
        [DllImport("user32.dll", CharSet = CharSet.Auto, EntryPoint = "MessageBoxW")]
        private static extern int MessageBoxWindows(IntPtr hWnd, string text, string caption, int options);

        // delcare the imported function that binds to the Linux version
        [DllImport("user32.so", CharSet = CharSet.Auto, EntryPoint = "MyMessageBoxW")]
        private static extern int MessageBoxLinux(IntPtr hWnd, string text, string caption, int options);
    }


    public class Program
    {
        private static void Main(string[] args)
        {
            // test the fuction
            NativeMethods.MessageBox(IntPtr.Zero, "Helllo", "World", 0);

            // or the more complicated selection
            NativeMethods.MessageBox2(IntPtr.Zero, "Helllo", "World", 0);
        }
    }
}