[Winform]{C#} Combobox อยากทราบวิธีนำ combobox มาคำนวณ กับ texbox ครับ

textbox1 = 500
anInteger = Convert.ToDouble(textbox1.Text);

if(combobox.SelectedIndex == 0)
{
textbox2.text  = aninteger * 0.2
}
else if(combobox.SelectedIndex == 1)
{
textbox2.text  = aninteger * 0.3
}
else if(combobox.SelectedIndex == 2)
{
textbox2.text  = aninteger * 0.5
}

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    if (this.comboBox1.SelectedIndex < 0) return;

    double result;

    if (double.TryParse(this.textBox1.Text, out result))
    {
        double percent = 0.0;

        switch (this.comboBox1.SelectedIndex)
        {
            case 0 :
                percent = 0.2;
                break;
            case 1 :
                percent = 0.3;
                break;
            case 2 :
                percent = 0.5;
                break;
        }

        this.BeginInvoke((MethodInvoker)delegate {
            this.comboBox1.Text = string.Format("{0} = {1}", this.comboBox1.Text, (result * percent).ToString("N2"));
        });
    }
}