IQueryable 上的 .ToList 在后台执行时会导致 NullReferenceException

Question

我的代码在同步执行时工作正常，但在通过 QueueBackgroundWorkItem 触发时失败。

更新：问题似乎与我正在使用的 Sitefinity 动态模块管理器有关，需要在 QueueBackgroundWorkItem 创建的线程中使用 HTTP 上下文，因此这是 Sitefinity 特定的问题。我找到了一篇似乎指向解决方案的文章：http://www.sitefinity.com/developer-network/forums/sitefinity-sdk/errors-with-managers-when-multi-threading

我的代码的相关部分

启动后台作业的 ApiController 操作：

// GET: api/Sync/MFP
[HttpGet]
public IHttpActionResult MFP()
{
    HostingEnvironment.QueueBackgroundWorkItem(ct => startMFPSync());
    return Ok("Sync Started!");
}

private void startMFPSync()
{
    var sourcesConfig = Config.Get<SyncSourcesSettingsConfig>();

    MFPApi api = new MFPApi(new MFPConfig
    {
        Url = sourcesConfig.MFPUrl,
        Key = sourcesConfig.MFPKey,
        Password = sourcesConfig.MFPPassword,
    });

    DataSync dataSync = new DataSync(api);
    dataSync.RunSync();
}

数据同步class:

public class DataSync
{
    IDataSource dataSource;

    public DataSync(IDataSource source)
    {
        dataSource = source;
    }

    public void RunSync()
    {
        dataSource.GetResponse();

        List<SyncContent> dataToSync = dataSource.GetDataForSync();
        SFDynamicModuleSync destinationSync = new SFDynamicModuleSync(dataToSync);

        // function call where exception occurs
        destinationSync.CacheModuleData();

        // other sync operations
    }
}

下面class中的CacheModuleData()函数是发生异常的地方：

public class SFDynamicModuleSync : IDataDestinationSync
{
    /// <summary>
    /// List of SyncContent objects to sync
    /// </summary>
    private List<SyncContent> dataToSync;

    /// <summary>
    /// Used to store results of CacheModuleData
    /// </summary>
    private List<List<DynamicContent>> modulesItems;

    /// <summary>
    /// Sitefinity dynamic module manager
    /// </summary>
    private DynamicModuleManager dynamicModuleManager;

    /// <summary>
    /// Initializes a new instance of the <see cref="SFDynamicModuleSync"/> class
    /// </summary>
    /// <param name="dataToSync">List of SyncContent objects to sync</param>
    public SFDynamicModuleSync(List<SyncContent> dataToSync)
    {
        this.dataToSync = dataToSync;
        this.modulesItems = new List<List<DynamicContent>>();
        this.dynamicModuleManager = DynamicModuleManager.GetManager();
    }

    /// <summary>
    /// Retrieves all data from dynamic modules and places in modulesItems
    /// </summary>
    public void CacheModuleData()
    {
        foreach (string contentType in this.dataToSync.Select(e => e.ContentTypeName))
        {
            Type type = TypeResolutionService.ResolveType(contentType);

            IQueryable<DynamicContent> moduleItems = this.dynamicModuleManager.GetDataItems(type)
                .Where(i => i.Status == ContentLifecycleStatus.Master);

            if(moduleItems != null)
            {
                // The .ToList() here causes a NullReferenceException when code is triggered by background job
                List<DynamicContent> moduleItemsList = moduleItems.ToList();
                this.modulesItems.Add(moduleItemsList);
            }
        }
    }

    // other sync methods - not included here for abbrevity 
}

堆栈跟踪：

System.NullReferenceException was unhandled by user code
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=Unity_ILEmit_DynamicClasses
  StackTrace:
       at DynamicModule.ns.Wrapped_OpenAccessDynamicModuleProvider_81d3fcbe95dd4a47b8c1cb1cc5a692ab.ApplyFilters(IDataItem item)
       at Telerik.Sitefinity.Security.FieldsPermissionsApplierEnumerator`1.Demand(T forItem)
       at Telerik.Sitefinity.Security.PermissionApplierEnumeratorBase`1.MoveNext()
       at Telerik.Sitefinity.Data.Linq.DataItemEnumerator`1.MoveNext()
       at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
       at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
       at TeamSI.Sitefinity.DataSync.DataDestinations.SFDynamicModuleSync.CacheModuleData() in C:\Projects\SIEQ\TeamSI.Sitefinity.DataSync\DataDestinations\SFDynamicModuleSync.cs:line 75
       at TeamSI.Sitefinity.DataSync.DataSync.RunSync() in C:\Projects\SIEQ\TeamSI.Sitefinity.DataSync\DataSync.cs:line 28
       at SitefinityWebApp.Mvc.Controllers.SyncController.startMFPSync() in C:\Projects\SIEQ\Main_Site\Mvc\Controllers\SyncController.cs:line 72
       at SitefinityWebApp.Mvc.Controllers.SyncController.<MFP>b__1_0(CancellationToken ct) in C:\Projects\SIEQ\Main_Site\Mvc\Controllers\SyncController.cs:line 52
       at System.Web.Hosting.HostingEnvironment.<>c__DisplayClass91_0.<QueueBackgroundWorkItem>b__0(CancellationToken ct)
       at System.Web.Hosting.BackgroundWorkScheduler.<RunWorkItemImpl>d__7.MoveNext()
  InnerException:

Answer 1

原来这是 Sitefinity CMS 的一个问题，我正在查询需要 HTTP 上下文的数据，这当然在线程中丢失了。 NullReferenceException 发生在 dynamicModuleManager.GetDataItems() 方法中。

在朋友和很棒的 SO 贡献者的帮助下，包括@Igor、@Hogan、@PeterBons 和@daramasala，他们帮助我理解了这个问题，我能够通过提升 SF 的权限并在线程中模拟 HttpContext 来解决它。

http://www.sitefinity.com/developer-network/forums/sitefinity-sdk/errors-with-managers-when-multi-threading

我只能在论坛之外找到关于使用此方法的 SF 文档： http://docs.sitefinity.com/bug-tracker-create-the-savebug-action

更新了我的 DataSync 中的 RunSync() 方法 class:

public void RunSync()
{
    SystemManager.RunWithElevatedPrivilegeDelegate worker = new SystemManager.RunWithElevatedPrivilegeDelegate(args => {

        dataSource.GetResponse();
        List<SyncContent> dataToSync = dataSource.GetDataForSync();
        var destinationSync = new SFDynamicModuleSync(dataToSync);

        destinationSync.CacheModuleData();

        // complete sync operations for each content type
        for (int i = 0; i < dataToSync.Count; i++)
        {
            destinationSync.DeleteOldItems(i);
            destinationSync.AddItems(i);
            destinationSync.UpdateItems(i);
        }
    });

    SystemManager.RunWithElevatedPrivilege(worker);
}

IQueryable 上的 .ToList 在后台执行时会导致 NullReferenceException

.ToList on IQueryable causes NullReferenceException when executed in background

c#

linq

sitefinity

asp.net-apicontroller