c# - 在后台执行时，IQueryable上的.ToList导致NullReferenceException。

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What is a NullReferenceException, and how do I fix it?
                                
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同步执行时，我的代码工作正常，但是通过QueueBackgroundWorkItem触发时，我的代码失败。

更新：问题似乎与Sitefinity动态模块管理器有关，我正在使用QueueBackgroundWorkItem创建的线程内要求HTTP上下文，因此这是一个针对Sitefinity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();
}

DataSync类：

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
    }
}

下面类中的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:

最佳答案

原来这是Sitefinity CMS的一个问题，我正在从要求HTTP上下文中查询数据，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（包括@ Igor，@ Hogan，@ PeterBons和@daramasala）的帮助下，我帮助我理解了这个问题，我可以通过为SF授予特权并在线程内模拟HttpContext来解决该问题。

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

在此论坛上，只有我以外的SF文档可以找到：
http://docs.sitefinity.com/bug-tracker-create-the-savebug-action

我的DataSync类中更新了RunSync（）方法：

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);
}

关于c# - 在后台执行时，IQueryable上的.ToList导致NullReferenceException。，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/41525338/