unchecked selectall checkbox when one checkbox is unchecked C# Win Form [on hold]

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:

 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