using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace UI.test
{
    public partial class FormA : Form
    {
        public FormA()
        {
            InitializeComponent();
        }

        private void FormA_Load(object sender, EventArgs e)
        {

        }
        //A窗体中定义一个可以改变自己文本框内容的方法
        private void SetTXTA(string textBoxB)
        {
            textBoxA.Text = textBoxB;
        }
        //打开窗体B按钮
        private void button1_Click(object sender, EventArgs e)
        {
            FormB fromb = new FormB();
            fromb.Show();
            fromb.SetTXTB_Event += SetTXTA;//为B中的事件绑定A中的方法

        }
    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace UI.test
{
    public partial class FormB : Form
    {
        public FormB()
        {
            InitializeComponent();
        }

        private void FormB_Load(object sender, EventArgs e)
        {

        }
         //委托： Action<T>表示无返回值的委托类型
         //      Funt<T> 表示有返回值的委托类型
        //定义事件
        public event Action<string> SetTXTB_Event;

        private void button1_Click(object sender, EventArgs e)
        {
            //调用事件：
            SetTXTB_Event(textBoxB.Text);

        }
       
    }
}