鼠标中键向下移动的事件

看一看

• Control.MouseDown Event
• MouseButton Enumeration
• Detecting Mouse Button Events in C#

你也可以试试这个

private void Form1_MouseMove(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Middle)
    {
        label1.Text = String.Format("{0} :: {1}", e.X, e.Y);
    }
}