Windows窗体如何确定selectedindex是由用户还是代码更改的

public partial class Form1 : Form
{
    private bool isIndexChangedByCode = false;

    public Form1()
    {
        InitializeComponent();
        comboBox1.SelectedIndexChanged += ComboBox1_SelectedIndexChanged;
    }

    private void ComboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (!isIndexChangedByCode)
        {
            MessageBox.Show("SelectedIndex was changed by user.");
        }
        else
        {
            isIndexChangedByCode = false;
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        isIndexChangedByCode = true;
        comboBox1.SelectedIndex = 1;
    }
}