private System.Windows.Forms.TextBox textBox1;

     //声明委托 和 事件
    public delegate void TransfDelegate(String value);
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

        public event TransfDelegate TransfEvent; 
        private void button1_Click(object sender, EventArgs e)
        {
            //触发事件
            TransfEvent(textBox1.Text);
            this.Close();
        }
    }

    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 frm = new Form2();
            //注册事件
            frm.TransfEvent += frm_TransfEvent;
            frm.ShowDialog();
        }

        //事件处理方法
        void frm_TransfEvent(string value)
        {
            textBox1.Text = value;
        }
    }