how to add value of for loop into a textbox

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private void Window_Loaded_1(object sender, RoutedEventArgs e)
   {
      int ymax = 500;
      parentchart.Axes.Add(new LinearAxis() { Minimum = 0, Orientation=AxisOrientation.Y, Maximum = ymax > 0 ? ymax : 300 });
      txty.Text = ymax.ToString();
      List<int> inp = new List<int>();
      for (int i = 0; i <= ymax; i++)
         {
            inp.Add(i);
            textbox.Text = inp.ToString();
            cmbb.ItemsSource = inp;
            cmbb.SelectedIndex = 0;
         }
   }

textbox.Text += inp.ToString();



StringBuilder sb = new StringBuilder;
string sep = "";
for (int i = 0; i <= ymax; i++)
{
inp.Add(i);
sb.AppendFormat("{0}{1}", sep, inp);
sep = ",";
cmbb.ItemsSource = inp;
cmbb.SelectedIndex = 0;
}
textbox.Text = sb.ToString();