private void btnProcess_Click(object sender, EventArgs e)
{
  long start, end;
  lstList.Items.Clear();
  // long.TryParse() tries to convert a string to a long, and if
  // successful, it fills in the passed-in variable and returns
  // true; otherwise it returns false.  thus, if both textboxes
  // contain text that can be converted to longs, we can process
  // the numbers, and if not, we inform the user of the error.
  if (long.TryParse(txtStart.Text, out start) && 
      long.TryParse(txtEnd.Text, out end))
    for (long x = start; x <= end; ++x)
      lstList.Items.Add(x);
  else
    MessageBox.Show("You must enter integers in both text boxes.",
                    "Error!", MessageBoxButtons.OK, 
                    MessageBoxIcon.Error);
}

        private void textBox_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (char.IsDigit(e.KeyChar)) { }
            else if (e.KeyChar == '\b') { }
            else { e.Handled = true; }
        }

        private void textBox_TextChanged(object sender, EventArgs e)
        {
            long value;
            if (!long.TryParse(textBox.Text, out value))
                textBox.Text = long.MaxValue.ToString();
        }

// if start is greater than end, swap the two
if(start > end)
{
  long temp = start;
  start = end;
  end = temp;
}
long difference = end - start > 5000 ? 5000 : end - start;
for(long x = 0; x<=difference; ++x)
  lstList.Items.Add((start + x).ToString());