private void dataGridView1_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
    {
      TextBox tb = (dataGridView1.EditingControl as TextBox);
      if (tb == null)
      {
        System.Diagnostics.Debugger.Break(); //you changed it to a non TextBox control. Add more support
        return;
      }
      editingBox = tb;
      tb.KeyPress += new KeyPressEventHandler(tb_KeyPress);
      tb.TextChanged += new EventHandler(tb_TextChanged);
    }

void tb_KeyPress(object sender, KeyPressEventArgs e)
    {
      if (!char.IsNumber(e.KeyChar) == Tasto F6)
       Visualizza Form;
    }