在不使用Windows窗体中的事件的情况下按下检测Shift键？

using System;
using System.Windows.Forms;

namespace DetectShiftKey
{
    class Program
    {
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
        }
    }

    public class Form1 : Form
    {
        public Form1()
        {
            this.KeyPreview = true;
            this.KeyDown += new KeyEventHandler(Form1_KeyDown);
        }

        private void Form1_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.ShiftKey)
            {
                MessageBox.Show("Shift键被按下了！");
            }
        }
    }
}