不可为空类型 'System.DateTime' ，ASP.NET MVC

我有一个注册页面，由于内容问题，我们必须要求并强制申请人提供出生日期。所以按理说这个字段不能为空。

我使用 jQuery 在文本框上加水印，告诉他们可以单击它并获取 jQuery UI 日历对象来选择日期。选择日期效果很好，这不是问题。

在测试中，如果我尝试在不选择日期的情况下提交表单，则会收到以下错误...

The parameters dictionary contains a null entry for parameter 'birthdate' of non-nullable type 'System.DateTime' for method 'System.Web.Mvc.ActionResult Register(System.String, System.String, System.String, System.String, Boolean, System.DateTime)' in 'Controllers.MembershipController'. To make a parameter optional its type should be either a reference type or a Nullable type.
Parameter name: parameters 

我不想对日期进行硬编码，它为空的目的是强制验证，以便他们必须选择它。有任何想法吗？我包括了负责区域的代码。真正令人沮丧的是，异常在到达 Register(parameters) 方法之前就被抛出了。 ModelState.IsValid 永远不会被调用。我尝试过 try/catch 块但无济于事。

        <p>             
            <label for="birthday">Birthdate:</label><br />
            <%= Html.TextBox("birthdate", "Select month, year, and date last." , new { @class = "text watermarkOn", @tabindex = "5" }) %>
        </p>

    public ActionResult Register()
    {
        return View();
    } 

    [CaptchaValidator]
    [AcceptVerbs(HttpVerbs.Post)]
    public ActionResult Register(string name, string email, string password, string validation, bool agreement, DateTime birthdate)
    {

        // attempt to validate the registration state, and if it is invalid, 
        // populate the ruleviolations and redisplay the content with the errors.
        if (ModelState.IsValid)
        {
        }

        return View();
    }

    private bool ValidateRegistration(string name, string email, string password, string validation, DateTime birthdate)
    {
        if (String.IsNullOrEmpty(name))
        {
            ModelState.AddModelError("name", "You must specify a name.");
        }
        if (String.IsNullOrEmpty(email))
        {
            ModelState.AddModelError("email", "You must specify an email address.");
        }
        if (password == null || !Text.RegularExpressions.Password(password) )
        {
            ModelState.AddModelError("password",
                String.Format(System.Globalization.CultureInfo.CurrentCulture,
                     "You must specify a password of {0} or more characters, without any whitespace characters.",6));
        }
        if (!String.Equals(password, validation, StringComparison.Ordinal))
        {
            ModelState.AddModelError("_FORM", "The new password and confirmation password do not match.");
        }
        return ModelState.IsValid;
    }
}

让birthdate参数为可为空的DateTime，即DateTime？，然后检查它是否为空，设置模型错误，并在为空时重新渲染带有错误的视图。

[CaptchaValidator]
[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Register(string name, string email, string password, string validation, bool agreement, DateTime? birthdate)
{
    ValidateRegistration( name, email, password, validation, agreement, birthdate );           
    if (ModelState.IsValid)
    {
    }
    return View();
}

private bool ValidateRegistration(string name, string email, string password, string validation, DateTime? birthdate)
{
     if (!birthdate.HasValue)
     {
         this.ModelState.AddModelError( "birthdate", "You must supply a birthdate." );
     }

     ...