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                string[] newmsgtexta = newmsgtext.Split(new Char[] { '/' });

                Form currentForm = Application.OpenForms[newmsgtexta[1]];
                if (currentForm == null)
                {
                    string ip = "";
                    chatf Chatf = new chatf(ip);
                    Chatf.Name = newmsgtexta[1];
                    Chatf.WindowState = FormWindowState.Minimized;
                    Chatf.Text = newmsgtexta[1];
                    Chatf.textBox1.Text = newmsgtexta[2];  // << das geht
                    Chatf.Show();
                }
                else
                {
                    //currentForm.textbox1.Text = newmsgtexta[2];  << das geht leider nicht^^
                    currentForm.Show();
                }					

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if (currentForm is chatf)
{
  var chatForm = (chatf) currentForm;
  chatForm.textBox1.Text = "foo";
}