Request.ServerVariables抛出NullReferenceException

Question

我有一个MVC3应用程序,它突然给了我一些奇怪的行为.首先是一些背景(尽管我会尝试尽可能简短).

在我的控制器操作中,我有这个代码:

public ActionResult Grid(ApplicationViewModel search = null)
{
    return this.ListView(
        this.Find<Entity>(),
        this.CreateViewModel,
        mixins: new Dictionary<string, Func<EntityViewModel, object>>()
        {
            { "Icon", vm => Url.Content("~\Images\entityType.png") },
            { "Link", vm => Url.Action("Details", vm.ControllerId) }
        });
}

EntityViewModel CreateViewModel(Entity entity);

// Inherited base class methods
protected IQueryable<T> Find<T>(); // Find T entities in DB

protected ListViewResult<TModel, TViewModel> ListView<TModel, TViewModel>(
    IQueriable<TModel> entityQuery, 
    Func<TModel, TViewModel> materializeViewModel,
    IDictionary<string, Func<TViewModel, object>> mixins);

此控制器操作隐藏了大量复杂逻辑,因为它ListViewResult是ActionResult专门为格式化JSON列表而设计的自定义结果.

public class ListViewResult<TModel, TViewModel> :
    ActionResult
{
    public IQueryable<TModel> ViewData { get; set; }
    public Func<TModel, TViewModel> Materialize { get; set; }
    public Dictionary<string, Func<TViewModel, object>> Mixins { get; private set; }

    ...

    public override void ExecuteResult(ControllerContext context)
    {
        // Perform sorting / paging / formatting on IQueryable

       ...

        var viewModels = this.ViewData.Select(this.Materialize);
        try
        {
            // another custom ActionResult for formatting JSON responses
            new JsonNetResult()
            {
                Data = viewModels.ToArray(),
                SerializerSettings = new JsonSerializerSettings()
                {
                    ContractResolver = new MixinContractResolver() 
                    {
                        Mixins = this.Mixins
                    }
                }
            }.ExecuteResult(context);
        }
        catch (Exception e)
        {
            context.HttpContext.Response.StatusCode = 500;
            context.HttpContext.Response.StatusDescription = e.Message;
        }
    }

    private class MixinContractResolver :
        CamelCasePropertyNamesContractResolver
    {
        public Dictionary<string, Func<TViewModel, object>> Mixins { get; set; }

        private List<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
        {
            List<JsonProperty> props = // get base properties
            foreach (var pair in this.Mixins)
            {
                props.Add(new JsonProperty()
                {
                    Ignored = false,
                    NullValueHandling = NullValueHandling.Include,
                    Readable = true,
                    PropertyName = Inflector.Camelize(pair.Key),
                    PropertyType = typeof(object),
                    ValueProvider = new DelegateValueProvider<TViewModel, object>(pair.Value),
                    Writable = false,
                });
            }
        }
    }

    private class DelegateValueProvider<T, R> :
        Newtonsoft.Json.Serialization.IValueProvider
    {
        private readonly Func<T, R> func;

        public DelegateValueProvider(Func<T, R> func)
        {
            this.func = func;
        }

        public object GetValue(object target)
        {
            return (R)this.func((T)target);
        }

        public void SetValue(object target, object value)
        {
            throw new NotSupportedException();
        }
    }
}

现在,似乎偶尔NullReferenceException的被在该行抛出vm => Url.Content(...)和vm => Url.Action(...).这并不总是发生,但如果我刷新几次,我可以可靠地重现它.

更新

在源代码中挖掘了一段时间之后,我相信我已经发现了有问题的代码.问题似乎与UrlRewriterHelper.WasThisRequestRewritten调用的方法有关ServerVariables.Get("IIS_WasUrlRewritten").看来,方法使用名为私有字段的_request是null在代码中的这点(出于某种原因).我最好的猜测是,在操作返回和执行结果之间的某个时间,ServerVariables集合要么丢失它的内部引用_request- 或者 - ServerVariables正在重新创建集合而没有有效的引用_request.

我也意识到这可能是IIS Express的一个问题.在Visual Studio开发服务器下运行时,我无法复制该问题.我已经更新了标签,以反映出最可能的原因.

Answer 1

我刚刚在 IIS Express 上遇到了同样的问题，发现这是因为我安装了 IIS 的 URL 重写扩展，然后将其卸载，但它在 IIS Express 的 applicationhost.config 文件中留下了一些配置。(C:\Users\\Documents\IISExpress\config\applicationhost.config)

我只是注释掉了该文件中的相关行，终止了 IIS Express 并重新运行，但我没有再次看到该问题。

我注释掉的两行在下面的代码片段中进行了注释。

<configuration>
    ...
    <system.webServer>
        ...
        <globalModules>

           <!--<add name="RewriteModule" image="%IIS_BIN%\rewrite.dll" />-->
           ...
        </globalModules>
    </system.webServer>
    <location path="" overrideMode="Allow">
        <system.webServer>
          <modules>
              <!--<add name="RewriteModule" />--> 
              ...
          </modules>
        </system.webServer>
    </location>
</configuration>