using System.Runtime.InteropServices;

private const int WM_NCLBUTTONDOWN = 0x00A1;
private  const int HTCAPTION = 2;
[DllImport("user32.dll", CharSet = CharSet.Unicode)]
public static extern IntPtr SendMessage(IntPtr hwnd, int wMsg, IntPtr wParam, IntPtr lParam);
[DllImport("user32.dll", CharSet = CharSet.Unicode)]
public static extern bool ReleaseCapture();

protected override void OnMouseDown( MouseEventArgs e )
{
    base.OnMouseDown( e );
    if (e.Button == MouseButtons.Left)  // 按下的是鼠标左键              
    {
        ReleaseCapture();   // 释放捕获                 
        SendMessage(this.Handle, WM_NCLBUTTONDOWN, (IntPtr)HTCAPTION, IntPtr.Zero);    // 拖动窗体              
    }
}