c#中怎样将FORM1里的listbox里选中的项显录在FORM2里的RichTextbox里

Form1里边有一个ListBox叫listBox

public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
form2 = new Form2();
form2.Show();
}

private Form2 form2;

private void listBox_SelectedIndexChanged(object sender, EventArgs e)
{
form2.SetRBText(listBox.SelectedItem.ToString());
}
}

Form2里边有一个RichTextBox，叫richTextBox
public partial class Form2 : Form
{
public Form2()
{
InitializeComponent();
}
public Form2 SetRBText(string str)
{
richTextBox.Text = str;
return this;
}
}

form1 f1 =new form1();
foreach(item i in f1.listbox)
{ichtextbox.add(i);}