Így nem kell a textBox1-nek publicnak lennie:
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
public Form1(string text):this()
{
textBox1.Text = text;
}
private void button1_Click(object sender, EventArgs e)
{
Form1 f = new Form1(textBox1.Text);
f.ShowDialog();
}
}