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();
  }
}

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();
  }
}