下拉菜单数据绑定

@FormHelper.DropdownList(a => a.UserRoleId, model.Roles)

public List<SelectListItem> Roles { get; set; } = new List<SelectListItem>();

 this.Roles.Add(new SelectListItem() { Text = Resources.NonUserRole, Value = "0" });
 this.Roles.AddRange(SelectListItems.FromQuery(context.Set<UserRole>(),
                    r => new { text = r.Name, value = r.Id }, r => r.text, r => r.value));