Clear all texboxes on a winform c#

Hi all,

I have a peice of code to clear all textboxes on a winform. This works fine but when I have a form with a control on it who can contain controls itself it doesn't work.

Can anyone tell me how to expand this so that it checks textboxes in controls? Like a form with tabpages wich has groupbox with controls in it.
Below my code.

Greetings

public static void ClearAllTextbox(Form frm)
        {
          foreach (Control ctrl in frm.Controls)
          {
            if (ctrl is TextBox || ctrl is RichTextBox)
            {
              if (ctrl.Text.Length > 0)
              {
                ctrl.Text = string.Empty;
              }
            }
          }
        }

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