Jak wywoływać funkcje z systemowych bibliotek dll

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;
using System.Runtime.InteropServices;

// The Blank Page item template is documented at http://go.microsoft.com/fwlink/?LinkId=402352&clcid=0x409

namespace App8
{
    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class MainPage : Page
    {

        [DllImport("user32.dll")]
        public static extern int MessageBox(int h, string m, string c, int type);

        public MainPage()
        {
            this.InitializeComponent();
        }

        private void button_Click(object sender, RoutedEventArgs e)
        {
            MessageBox(0, "Witajcie", "Wiadomość", 0);
        }
    }
}