用LinQ对两级下拉菜单的绑定

FamilyFinanceSystemDataContext ffsd = new FamilyFinanceSystemDataContext();// 定义一个数据库变量
            var father = from n in ffsd.ExpenditreFather                                                 //对父表查询
                         select n;
            ddlExpenditreFather_Id.DataSource = father;
            ddlExpenditreFather_Id.DataValueField = "ExpenditreFather_Id";
            ddlExpenditreFather_Id.DataTextField = "ExpenditureFather_Name";
            ddlExpenditreFather_Id.DataBind();                                                            //对下拉菜单进行绑定
            if (father.Count() > 0)
            {
                Guid gd = new Guid(ddlExpenditreFather_Id.SelectedItem.Value);       //选择下拉菜单的第一项为绑定项（父表id）
                var child = from m in ffsd.ExpenditreChild                                               //根据父表id查询子表
                            where m.ExpenditreFather_Id == gd
                            select m;
                ddlExpenditreChild_Id.DataSource = child;
                ddlExpenditreChild_Id.DataValueField = "ExpenditreChild_Id";
                ddlExpenditreChild_Id.DataTextField = "ExpenditreChild_Name";
                ddlExpenditreChild_Id.DataBind();                                                          //对下拉菜单进行绑定
            }