you must reset the binding after you add new items at run-time. working code example:
ar inrange0 = new List<int>
{
1, 2, 3, 4, 5, 6, 7, 8
};
comboBox1.DataSource = inrange0;
inrange0.Add(9);
comboBox1.DataSource = null;
comboBox1.DataSource = inrange0;
If you are not using binding: then, all you need to do is add new items:
comboBox1.Items.Clear();
comboBox1.Items.AddRange(inrange0.Cast<object>().ToArray());
comboBox1.Items.Add(9);