Asp.net mvc 3 Asp.net MVC3下拉列表验证

型号：

控制器：

public class Product
{
    public string Title { get; set; }

    [Required]
    public int CategoryId { get; set; } 
    //or
    public int? CategoryId { get; set; } 
}

@Html.DropDownListFor(m => m.CategoryId , new SelectList(ViewBag.Categories , "CategoryId ", "Title"),"")
@Html.ValidationMessageFor(model => model.CategoryId )

视图：

我有客户端验证

但在Microsoft默认脚手架中：

控制器：

public class Product
{
    public string Title { get; set; }

    [Required]
    public int CategoryId { get; set; } 
    //or
    public int? CategoryId { get; set; } 
}

@Html.DropDownListFor(m => m.CategoryId , new SelectList(ViewBag.Categories , "CategoryId ", "Title"),"")
@Html.ValidationMessageFor(model => model.CategoryId )

视图：

---

这是一个非常好和干净的代码。但是客户端验证不起作用。如何使用Microsoft默认脚手架语法和客户端验证只需使CategoryId属性为空即可

@Html.DropDownList("CategoryId" ,"")

[Required]
public int? CategoryId { get; set; }