本次示例效果如下：
Form1为父窗体(包含textBox1、button1)
Form2为子窗体(包含textBox2、button2)

父窗体给子窗体传值
==================
1.点击Form1的button1 打开Form2
  父窗体给子窗体传值 可以调用重载子窗体的构造函数 直接传入相关数值

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

private void button1_Click(object sender, EventArgs e)
        {
            Form2 frm2 = new Form2(this.textBox1.Text);
            frm2.Show();
        }
    }

public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

public Form2(string strTextBox1Text)
        {
            InitializeComponent();
            this.textBox2.Text = strTextBox1Text;
        }
    }

2.点击Form1的button1 打开Form2
  并调用子窗体Form2的公开属性或方法 将Form1的textBox1的值设置给Form2的textBox2
 
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

private void button1_Click(object sender, EventArgs e)
        {
            Form2 frm2 = new Form2();
            frm2.TextBox2Text = this.textBox1.Text;
            frm2.Show();
        }
    }

public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

public string TextBox2Text
        {
            set { this.textBox2.Text = value; }
            get { return this.textBox2.Text; }
        }       
    }
  
3.点击Form1的button1 打开Form2
  在Form2_Load调用父窗体Form1的公开属性或方法 将Form1的textBox1的值设置给Form2的textBox2

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

public string TextBox1Text
        {
            set { this.textBox1.Text = value; }
            get { return this.textBox1.Text;  }
        }

private void button1_Click(object sender, EventArgs e)
        {
            Form2 frm2 = new Form2();
            frm2.Show(this);//或 frm2.ShowDialog(this);

////或者
            //Form2 frm2 = new Form2();
            //frm2.Owner = this;
            //frm2.Show();//或 frm2.ShowDialog();
        }
    }
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

private void Form2_Load(object sender, EventArgs e)
        {
            Form1 frm1 = (Form1)this.Owner;
            this.textBox2.Text = frm1.TextBox1Text;
        }
    }

子窗体给父窗体传值
==================
4.点击Form1的button1 打开Form2
  再点击Form2的button2 
    在button2_Click事件中 通过this.Owner将Form2的textBox2的值设置给Form1的textBox1
    并关闭Form2

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

private void button1_Click(object sender, EventArgs e)
        {
            Form2 frm2 = new Form2();
            frm2.Show(this);//或 frm2.ShowDialog(this);

////或者
            //Form2 frm2 = new Form2();
            //frm2.Owner = this;
            //frm2.Show();//或 frm2.ShowDialog();
        }
    }

public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

private void button2_Click(object sender, EventArgs e)
        {
            Form1 frm1 = (Form1)this.Owner;
     //注意 如果textBox1是放在panel1中的 则先找panel1 再找textBox1
            ((TextBox)frm1.Controls["textBox1"]).Text = this.textBox2.Text;
            this.Close();
        }
    }

5.点击Form1的button1 打开Form2
  再点击Form2的button2 
    在button2_Click事件中 通过this.Owner及调用父窗体Form1的公开属性或方法 
                          将Form2的textBox2的值设置给Form1的textBox1
    并关闭Form2
 
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

public string TextBox1Text
        {
            set { this.textBox1.Text = value; }
            get { return this.textBox1.Text;  }
        }

private void button1_Click(object sender, EventArgs e)
        {
            Form2 frm2 = new Form2();
            frm2.Show(this);//或 frm2.ShowDialog(this);

////或者
            //Form2 frm2 = new Form2();
            //frm2.Owner = this;
            //frm2.Show();//或 frm2.ShowDialog();
        }
    }

public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

private void button2_Click(object sender, EventArgs e)
        {
            Form1 frm1 = (Form1)this.Owner;
            frm1.TextBox1Text = this.textBox2.Text;
            this.Close();
        }
    }