public class Form1 : System.Windows.Forms.Form { private System.Windows.Forms.Button button1; public Form1() { InitializeComponent(); } private void button1_Click(object sender, System.EventArgs e) { Form2 f2 = new Form2(); f2.SetText("Hello world"); f2.ShowDialog(this); } //Reste de code de ta fenêtre principale, dont le Main } public class Form2 : System.Windows.Forms.Form { private System.Windows.Forms.Label label1; public Form1() { InitializeComponent(); } public void SetText(string Text) { label1.Text = Text; } //Reste de ta fenêtre secondaire }