public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            this.button1.Enter += button_Enter;
            this.button1.Leave += button_Leave;
            this.button2.Enter += button_Enter;
            this.button2.Leave += button_Leave;
            //继续写其他的
        }
        void button_Leave(object sender, EventArgs e)
        {
            (sender as Button).BackColor = SystemColors.Control;
        }
        void button_Enter(object sender, EventArgs e)
        {
            (sender as Button).BackColor = Color.Red;
        }
    }