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

namespace TestMouseMove

{
    public partial class Form4 : Form
    {
        public Form4()
        {
            InitializeComponent();
        }

        #region 子窗口刷新父窗口的值

        private string strLabel1 = "";

        public string StrLabel1
        {
            get
            {
                return strLabel1;
            }
            set
            {
                strLabel1 = value;
                this.label1.Text = strLabel1;
            }
        }
        #endregion

        private void button1_Click(object sender, EventArgs e)
        {
            Form5 form5 = new Form5(this);//這里注意傳個(gè)this
            form5.Show();
        }
    }
}

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

namespace TestMouseMove
{
    public partial class Form5 : Form
    {
        Form4 form4 = new Form4();

        public Form5(Form4 formFrm)//這個(gè)構(gòu)造方法里有參數(shù)
        {
            form4 = formFrm; //這個(gè)必須要有
            InitializeComponent();
        }

   private void button1_Click(object sender, EventArgs e)

        {
            form4.StrLabel1 = this.textBox1.Text;
        }
    }
}