举一个简单的例子，窗体1（父窗体）点击按钮2弹出窗体2（子窗体），窗体2点击按钮1隐藏窗体1的按钮1

    //窗体1
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void button2_Click(object sender, EventArgs e)
        {
            Form2 frm = new Form2();
            frm.CloseBtn += new Close(closebtn);
            frm.Show();
        }
        private void closebtn()
        {
            button1.Visible = false;
        }
    }


    //窗体2
    public delegate void Close();//定义委托
    public partial class Form2 : Form
    {
       
        public Form2()
        {
            InitializeComponent();
        }

        public event Close CloseBtn;//自定义事件
        private void button1_Click(object sender, EventArgs e)
        {
            CloseBtn();
        }
    }