字符串拼接

  int ArtID;
    string str; 
  protected void LinkButton9_Click(object sender, EventArgs e)
    {
        foreach (RepeaterItem rep in rptList.Items)
        {
            CheckBox cb = (CheckBox)(rep.FindControl("cboCheck"));
            if (cb.Checked == true)
            {
                boolean = true;
                ArtID = int.Parse(des.Decrypt((((HiddenField)(rep.FindControl("hiddenFieldId"))).Value)));
            }
        }
        str = ArtID + "," + ArtID;
        Response.Redirect("Default.aspx?idd=" + str + "");
    }
想得到str的值，应该是1,2,3
str = "1,2,3";
如何得到

  int ArtID;
    string str=""; 
  protected void LinkButton9_Click(object sender, EventArgs e)
    {
        foreach (RepeaterItem rep in rptList.Items)
        {
            CheckBox cb = (CheckBox)(rep.FindControl("cboCheck"));
            if (cb.Checked == true)
            {
                boolean = true;
                ArtID = int.Parse(des.Decrypt((((HiddenField)(rep.FindControl("hiddenFieldId"))).Value)));
            }
            str += ArtID + ",";
        }
         
        Response.Redirect("Default.aspx?idd=" + str.TrimEnd(',') + "");
    }