Console application to show how to use DLLImport to import methods exposed by an unmanaged dynamic-link library (DLL). This example shows how to call the MessageBox Win32 API Call.

interopdemo.cs

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

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            InteropDemo.MessageBox("app", "hello");
        }
    }

    public sealed class InteropDemo
    {
        private InteropDemo() { }

        //adds an ok button to the message box
        private const uint MB_OK = (uint)0x00000000L;

        //windows handle 
        private static IntPtr? windowHandle;

        // Use DllImport to import the Win32 MessageBox function.
        [DllImport("user32.dll", CharSet = CharSet.Unicode)]
        private static extern int MessageBox(IntPtr hWnd, string text, string caption, uint type);

        // Use to get the window handle
        [DllImport("user32.dll", SetLastError = true)]
        private static extern IntPtr FindWindow(string className, string windowName);

        /// <summary>
        /// this is the internal api call
        /// </summary>
        /// <param name="caption"></param>
        /// <param name="text"></param>
        public static void MessageBox(String caption, String text)
        {
            //or use windowHandle if allocated
            IntPtr hWnd = windowHandle ?? IntPtr.Zero;
            MessageBox(hWnd, caption, text, MB_OK);
        }

    }
}