Not able to store the checkbox values in seperate rows

protected void Submit(object sender, EventArgs e)
{
var Trolley = txttrolley.Text;
string Id = "";
if (btnsubmit.Text == "Add")
{

Id = " 0";
}
else
{
Id = ViewState["ID"].ToString();
}

string Attribute = "";
try
{

foreach (ListItem item in DropDownCheckBoxes1.Items)
{
if (item.Selected)
{
Attribute += item.Value + ",";
}
}
Attribute = Attribute.Remove(Attribute.Length - 1);
}
catch (Exception ex)
{

}