//1-ый вариант
foreach (Control c in this.Controls)
            {
                if (c is TextBox)
                    if ((c as TextBox).Name == "t1")
                        MessageBox.Show((c as TextBox).Text);
            }

//2-ой вариант
var cont = this.Controls.Find("t1",true);

//3-ий вариант
Controls["textBox2"].Text = "OK";