空引用异常和?? 操作者

Question

我在我的Asp.Net MVC应用程序中有一个表单,它在以下行返回Null引用异常,但仅在回发错误之后:

<%= Html.TextArea("Description", Model.Evt.Description ?? "")%>

字符串描述确实是空的,但我的期望是,如果Model.evt.Description是,null那么""将显示在TextArea.为什么它在最初加载表单时工作正常呢？

唯一的区别是ViewUserControl的类型是从初始加载的新对象和回发重新加载的模型对象填充的.

我错过了什么？

堆栈跟踪如下:

[NullReferenceException: Object reference not set to an instance of an object.]
   System.Web.Mvc.HtmlHelper.GetModelStateValue(String key, Type destinationType) +63
   System.Web.Mvc.Html.TextAreaExtensions.TextAreaHelper(HtmlHelper htmlHelper, String name, Boolean useViewData, String value, IDictionary`2 implicitAttributes, IDictionary`2 explicitParameters, IDictionary`2 htmlAttributes) +261
   System.Web.Mvc.Html.TextAreaExtensions.TextArea(HtmlHelper htmlHelper, String name, String value, IDictionary`2 htmlAttributes) +213
   System.Web.Mvc.Html.TextAreaExtensions.TextArea(HtmlHelper htmlHelper, String name, String value) +61
   ASP.views_events_eventform_ascx.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in c:\Documents and Settings\Richard Box\My Documents\Visual Studio 2008\Projects\EventsManager\EventsManager\Views\Events\EventForm.ascx:64
   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

Answer 1

它看起来像Model或Evt是null,实际上不是Description字段.