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 delegate void SetVisiableHandler();  

        public partial class Form2 : Form

        {

            public Form2()

            {

                InitializeComponent();

            }

            private void button1_Click(object sender, EventArgs e)

            {

                Form3 frm = new Form3(new SetVisiableHandler(SetVisiable));

                frm.Show();

            }  

            private void SetVisiable()

            {

                SetVisiable(this.label1, !this.label1.Visible);

            }  

            private void SetVisiable(Control control, bool visiable)

            {

                if (this.Controls.Contains(control))

                {

                    control.Visible = visiable;

                }

            }  

        }

    }  
Form3
C#代码：

    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 Form3 : Form

        {

            private SetVisiableHandler m_setVisible;  

            public Form3(SetVisiableHandler setvisible)

            {

                InitializeComponent();

                this.m_setVisible = setvisible;

            }

            private void btnVisible_Click(object sender, EventArgs e)

            {

                if (this.m_setVisible != null)

                {

                    this.m_setVisible();

                }

            }  

        }

    }  
第二种方法: 
用变量,Form4和Form5是同一组
Form4 代码：

    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);//这里注意传个this

                form5.Show();

            }

        }

    }  
Form5 代码：

    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)//这个构造方法里有参数

            {

                form4 = formFrm; //这个必须要有

                InitializeComponent();

            }  

            private void button1_Click(object sender, EventArgs e)

            {

                form4.StrLabel1 = this.textBox1.Text;

            }

        }

    }