如何强制用户将建议的条目带入ComboBox？

AutoCompleteStringCollection autoCompleteData = new AutoCompleteStringCollection();
autoCompleteData.AddRange(new string[] { "条目1", "条目2", "条目3" });
comboBox1.AutoCompleteSource = AutoCompleteSource.CustomSource;
comboBox1.AutoCompleteCustomSource = autoCompleteData;
comboBox1.AutoCompleteMode = AutoCompleteMode.SuggestAppend;

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    if (comboBox1.SelectedIndex == -1)
    {
        MessageBox.Show("请选择一个建议的条目！");
        comboBox1.Text = "";
    }
}

private void button1_Click(object sender, EventArgs e)
{
    if (comboBox1.SelectedIndex == -1)
    {
        errorProvider1.SetError(comboBox1, "请选择一个建议的条目！");
    }
    else
    {
        errorProvider1.SetError(comboBox1, "");
    }
}