Uncheck Select All Checkbox when one checkbox is Unchecked C# in windows form

I have a checkbox when you click on it it select all checkbox the problem here that when unchecked any check box checked didn't remove from select all I tried this code:

private void checkBox1_CheckedChanged(object sender, EventArgs e)
{
    if (checkBox1.Checked == false) 
        checkBox9.Checked = false;
}

but when i checked any check box it remove all checkbox i want to unchecked select all only not all checkbox