【原创】Asp.net MVC 学习笔记之为什么使用Html.TextBox方法会出现异常？

在上篇文章：【原创】Asp.net MVC学习笔记之-基于类型来绑定Model的属性 中，例举了如何使用基于类型和基于用途2种方式结合来限制Model的绑定，由此引发了一个意想不到的问题。

先看一下前台页面的代码：

<p>
                <label for="Test1">Test1:</label>
                <%= Html.TextBox("Test1") %>
                <%= Html.ValidationMessage("Test1", "*") %>
            </p>
            <p>
                <label for="Test2">Test2:</label>
                <%= Html.TextBox("Test2") %>
                <%= Html.ValidationMessage("Test2", "*") %>
            </p>
            <p>
                <label for="Test3">Test3:</label>
                <%= Html.TextBox("Test3") %>
                <%= Html.ValidationMessage("Test3", "*") %>
            </p>
            <p>
                <label for="Test4">Test4:</label>
                <%= Html.TextBox("Test4") %>
                <%= Html.ValidationMessage("Test4", "*") %>
            </p>
            <p>
                <label for="Test5">Test5:</label>
                <%= Html.TextBox("Test5") %>
                <%= Html.ValidationMessage("Test5", "*") %>
            </p>

 当页面输入的数据在Controller里经过数据绑定之后，只剩下了Test3一项有值，其他几个都是null,然后当程序走到

Html.TextBox的时候就出错了，报错如下：

堆栈跟踪: 


[NullReferenceException: 未将对象引用设置到对象的实例。]
   System.Web.Mvc.HtmlHelper.GetModelStateValue(String key, Type destinationType) +63
   System.Web.Mvc.Html.InputExtensions.InputHelper(HtmlHelper htmlHelper, InputType inputType, String name, Object value, Boolean useViewData, Boolean isChecked, Boolean setId, Boolean isExplicitValue, IDictionary`2 htmlAttributes) +519
   System.Web.Mvc.Html.InputExtensions.TextBox(HtmlHelper htmlHelper, String name, Object value, IDictionary`2 htmlAttributes) +34
   System.Web.Mvc.Html.InputExtensions.TextBox(HtmlHelper htmlHelper, String name) +60
   ASP.views_dinners_create_aspx.__RenderContent2(HtmlTextWriter __w, Control parameterContainer) in e:\学习资料\我的学习文件\工作学习\【12】-网页方向\MVC\一步一步学asp.net_mvc\Projects\NerdDinner\NerdDinner\Views\Dinners\Create.aspx:30
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +256
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
   System.Web.UI.Control.Render(HtmlTextWriter writer) +10
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   ASP.views_shared_site_master.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in e:\学习资料\我的学习文件\工作学习\【12】-网页方向\MVC\一步一步学asp.net_mvc\Projects\NerdDinner\NerdDinner\Views\Shared\Site.Master:26
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +256
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
   System.Web.UI.Control.Render(HtmlTextWriter writer) +10
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +134
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
   System.Web.UI.Page.Render(HtmlTextWriter writer) +29
   System.Web.Mvc.ViewPage.Render(HtmlTextWriter writer) +59
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1266

 

找了半天也没找到原因，于是我把数据绑定的限制去掉试了一下，就没问题了，于是我想问题可能出在这个方法上面，祭出

Reflector,直接打开C盘下的System.Web.MVC.Dll，看个究竟：

通过HtmlHelper.TextBox方法，很容易跟到这个函数：

private static string InputHelper(this HtmlHelper htmlHelper, InputType inputType, string name, object value, bool useViewData, bool isChecked, bool setId, bool isExplicitValue, IDictionary<string, object> htmlAttributes)
{
    ModelState state;
    if (string.IsNullOrEmpty(name))
    {
        throw new ArgumentException(MvcResources.Common_NullOrEmpty, "name");
    }
    TagBuilder builder = new TagBuilder("input");
    builder.MergeAttributes<string, object>(htmlAttributes);
    builder.MergeAttribute("type", HtmlHelper.GetInputTypeString(inputType));
    builder.MergeAttribute("name", name, true);
    string str = Convert.ToString(value, CultureInfo.CurrentCulture);
    bool flag = false;
    switch (inputType)
    {
        case InputType.CheckBox:
        {
            bool? modelStateValue = htmlHelper.GetModelStateValue(name, typeof(bool)) as bool?;
            if (modelStateValue.HasValue)
            {
                isChecked = modelStateValue.Value;
                flag = true;
            }
            break;
        }
        case InputType.Password:
            if (value != null)
            {
                builder.MergeAttribute("value", str, isExplicitValue);
            }
            goto Label_0152;

        case InputType.Radio:
            break;

        default:
        {
            string str3 = (string) htmlHelper.GetModelStateValue(name, typeof(string));
            builder.MergeAttribute("value", str3 ?? (useViewData ? htmlHelper.EvalString(name) : str), isExplicitValue);
            goto Label_0152;
        }
    }
    if (!flag)
    {
        string a = htmlHelper.GetModelStateValue(name, typeof(string)) as string;
        if (a != null)
        {
            isChecked = string.Equals(a, str, StringComparison.Ordinal);
            flag = true;
        }
    }
    if (!flag && useViewData)
    {
        isChecked = htmlHelper.EvalBoolean(name);
    }
    if (isChecked)
    {
        builder.MergeAttribute("checked", "checked");
    }
    builder.MergeAttribute("value", str, isExplicitValue);
Label_0152:
    if (setId)
    {
        builder.GenerateId(name);
    }
    if (htmlHelper.ViewData.ModelState.TryGetValue(name, out state) && (state.Errors.Count > 0))
    {
        builder.AddCssClass(HtmlHelper.ValidationInputCssClassName);
    }
    if (inputType == InputType.CheckBox)
    {
        StringBuilder builder2 = new StringBuilder();
        builder2.Append(builder.ToString(TagRenderMode.SelfClosing));
        TagBuilder builder3 = new TagBuilder("input");
        builder3.MergeAttribute("type", HtmlHelper.GetInputTypeString(InputType.Hidden));
        builder3.MergeAttribute("name", name);
        builder3.MergeAttribute("value", "false");
        builder2.Append(builder3.ToString(TagRenderMode.SelfClosing));
        return builder2.ToString();
    }
    return builder.ToString(TagRenderMode.SelfClosing);
}

 

 

这里注意Switch应该是走到default里，继续跟，发现这个函数有问题：

internal object GetModelStateValue(string key, Type destinationType)
{
    ModelState state;
    if (this.ViewData.ModelState.TryGetValue(key, out state))
    {
        return state.Value.ConvertTo(destinationType, null);
    }
    return null;
}

 

其中这句话怎么看怎么不爽：return state.Value.ConvertTo(destinationType, null);

于是我在Controller里自己试了一把：

  System.Web.Mvc.ModelState state;
            if (this.ViewData.ModelState.TryGetValue("Test1", out state))
            {
                var result = state.Value.ConvertTo(typeof(string), null);

            }

监视器打开，问题找到了

至于这后面隐藏了MVC的什么内部实现，等我搞清楚了再回头看看。今天的问题算是告一段落了。