how to interact with Custom control in C#.net?

void btnShowAll_Click(object sender, EventArgs e)
        {
            foreach (Control c in Page.Controls)
            {
                if (c is CheckBox)
                    {
                    
                    CheckBox chkbob=(CheckBox)c;
                    chkbob.Checked=true;
                    
                    }
                }
            }