combobox中有1，2，3三个item
第1次选择1，四个事件都触发了，当我再次选择1时，4没出现，3，2，1均出现
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
MessageBox.Show("1");
}
private void comboBox1_SelectedValueChanged(object sender, EventArgs e)
{
MessageBox.Show("2");
}
private void comboBox1_SelectionChangeCommitted(object sender, EventArgs e)
{
MessageBox.Show("3");
}
private void comboBox1_TextChanged(object sender, EventArgs e)
{
MessageBox.Show("4");
}