displaying checkbox item in textbox

 protected void CheckBoxList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        string a = string.Empty;

              
        foreach (ListItem li in CheckBoxList1.Items)
        {
            
            if (li.Selected == true)
            {

                
                a += li.ToString() + ",";
                
            }
          
        }
     
        a = a.TrimEnd(',');
        TextBox1.Text = a;
    }
    The order of the item is not maintained. When I deselect any item from checkbox all items rearragens starting from the first item of checkbox. I want the orderj of selection  to be maintanined on deselection.

Thanks.