stille selectedindex i combobox

ComboBox er fra Winforms ...

Følgende virker: ( der er en combobox og button på min form )
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication3
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

            List<ComboItem> list = new List<ComboItem>();
            for (int i = 0; i < 20; i++)
            {
                list.Add(new ComboItem() { Name = i.ToString(), Value = i.ToString() });
            }

            comboBox1.DataSource = list;
            comboBox1.DisplayMember = "Name";
            comboBox1.ValueMember = "Value";

            comboBox1.SelectedValue = "10";
        }

        private void button1_Click(object sender, EventArgs e)
        {
            comboBox1.SelectedValue = "15";
        }
    }

    public class ComboItem
    {
        public string Value { get; set; }
        public string Name { get; set; }
    }
}

// ouT