c# - 相关对象在 .Include 中为空时出现 Entityframework Core 2.0 错误

Question

我正在使用 EF core 2.0 和 .Net core2.0 开发我的 webapi 应用程序。

在以下查询中，当案例没有工作流(即可为空的工作流)时出现异常。我得到的错误是“System.NullReferenceException:对象引用未设置为对象的实例。”

我怀疑当 c.workflow 为空时 .ThenInclude() 上出现异常。

案例实体:

public class Case 
{
    [Key]
    public int CaseId { get; set; }
    public int? WorkflowId { get; set; }
    public Workflow Workflow { get; set; }
}

查询:

var item = await _context.Cases
    .Include(c => c.CaseDetail)
    .Include(c => c.Workflow).ThenInclude(wf => wf.CurrentWorkflowLevel)
    .Include(c => c.Alerts)
    .FirstOrDefaultAsync(c => c.CaseId == id);

考虑到工作流为空，任何人都可以帮助实现此查询吗？

异常(exception):

System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.IncludeLoadTreeNodeBase.<_AwaitMany>d__8.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.<_IncludeAsync>d__18`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.TaskLiftingExpressionVisitor.<_ExecuteAsync>d__8`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.AsyncSelectEnumerable`2.AsyncSelectEnumerator.<MoveNext>d__3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
   at System.Linq.AsyncEnumerable.SelectEnumerableAsyncIterator`2.<MoveNextCore>d__7.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
   at System.Linq.AsyncEnumerable.AsyncIterator`1.<MoveNext>d__10.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.<MoveNext>d__5.MoveNext()
Application Insights Telemetry (unconfigured): {"name":"Microsoft.ApplicationInsights.Dev.Exception","time":"2017-11-16T16:16:29.4502900Z","tags":{"ai.operation.id":"b6360c2b-4dabca2bb91d06ac","ai.application.ver":"1.0.0.0","ai.internal.nodeName":"UK2207793W1","ai.cloud.roleInstance":"UK2207793W1","ai.location.ip":"::1","ai.operation.name":"GET Cases/GetCases","ai.operation.parentId":"|b6360c2b-4dabca2bb91d06ac.","ai.internal.sdkVersion":"aspnet5c:2.1.1"},"data":{"baseType":"ExceptionData","baseData":{"ver":2,"properties":{"DeveloperMode":"true","Exception":"System.NullReferenceException: Object reference not set to an instance of an object.\r\n   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.IncludeLoadTreeNodeBase.<_AwaitMany>d__8.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.<_IncludeAsync>d__18`1.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()\r\n   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.TaskLiftingExpressionVisitor.<_ExecuteAsync>d__8`1.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.AsyncSelectEnumerable`2.AsyncSelectEnumerator.<MoveNext>d__3.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()\r\n   at System.Linq.AsyncEnumerable.SelectEnumerableAsyncIterator`2.<MoveNextCore>d__7.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()\r\n   at System.Linq.AsyncEnumerable.AsyncIterator`1.<MoveNext>d__10.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()\r\n   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.<MoveNext>d__5.MoveNext()","AspNetCoreEnvironment":"Development","error":"System.NullReferenceException: Object reference not set to an instance of an object.\r\n   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.IncludeLoadTreeNodeBase.<_AwaitMany>d__8.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.<_IncludeAsync>d__18`1.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()\r\n   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.TaskLiftingExpressionVisitor.<_ExecuteAsync>d__8`1.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.AsyncSelectEnumerable`2.AsyncSelectEnumerator.<MoveNext>d__3.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()\r\n   at System.Linq.AsyncEnumerable.SelectEnumerableAsyncIterator`2.<MoveNextCore>d__7.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()\r\n   at System.Linq.AsyncEnumerable.AsyncIterator`1.<MoveNext>d__10.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()\r\n   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.<MoveNext>d__5.MoveNext()","CategoryName":"Microsoft.EntityFrameworkCore.Query","contextType":"FDA.Operational.Core.Workflow.DBContext.WorkflowDBContext","{OriginalFormat}":"An exception occurred in the database while iterating the results of a query for context type '{contextType}'.{newline}{error}"},"exceptions":[{"id":22294293,"typeName":"System.NullReferenceException","message":"An exception occurred in the database while iterating the results of a query for context type 'FDA.Operational.Core.Workflow.DBContext.WorkflowDBContext'.\r\nSystem.NullReferenceException: Object reference not set to an instance of an object.\r\n   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.IncludeLoadTreeNodeBase.<_AwaitMany>d__8.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.<_IncludeAsync>d__18`1.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()\r\n   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.TaskLiftingExpressionVisitor.<_ExecuteAsync>d__8`1.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.AsyncSelectEnumerable`2.AsyncSelectEnumerator.<MoveNext>d__3.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()\r\n   at System.Linq.AsyncEnumerable.SelectEnumerableAsyncIterator`2.<MoveNextCore>d__7.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()\r\n   at System.Linq.AsyncEnumerable.AsyncIterator`1.<MoveNext>d__10.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()\r\n   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.<MoveNext>d__5.MoveNext()","hasFullStack":true,"parsedStack":[{"level":0,"method":"Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler+IncludeLoadTreeNodeBase+<_AwaitMany>d__8.MoveNext","assembly":"Microsoft.EntityFrameworkCore, Version=2.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60"},{"level":1,"method":"System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":2,"method":"System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":3,"method":"Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler+<_IncludeAsync>d__18`1.MoveNext","assembly":"Microsoft.EntityFrameworkCore, Version=2.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60"},{"level":4,"method":"System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":5,"method":"System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":6,"method":"System.Runtime.CompilerServices.TaskAwaiter`1.GetResult","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":7,"method":"Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.TaskLiftingExpressionVisitor+<_ExecuteAsync>d__8`1.MoveNext","assembly":"Microsoft.EntityFrameworkCore, Version=2.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60"},{"level":8,"method":"System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":9,"method":"System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":10,"method":"Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor+AsyncSelectEnumerable`2+AsyncSelectEnumerator+<MoveNext>d__3.MoveNext","assembly":"Microsoft.EntityFrameworkCore, Version=2.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60"},{"level":11,"method":"System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":12,"method":"System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":13,"method":"System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1+ConfiguredTaskAwaiter.GetResult","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":14,"method":"System.Linq.AsyncEnumerable+SelectEnumerableAsyncIterator`2+<MoveNextCore>d__7.MoveNext","assembly":"System.Interactive.Async, Version=3.0.3000.0, Culture=neutral, PublicKeyToken=94bc3704cddfc263"},{"level":15,"method":"System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":16,"method":"System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":17,"method":"System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1+ConfiguredTaskAwaiter.GetResult","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":18,"method":"System.Linq.AsyncEnumerable+AsyncIterator`1+<MoveNext>d__10.MoveNext","assembly":"System.Interactive.Async, Version=3.0.3000.0, Culture=neutral, PublicKeyToken=94bc3704cddfc263"},{"level":19,"method":"System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":20,"method":"System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":21,"method":"System.Runtime.CompilerServices.TaskAwaiter`1.GetResult","assembly":"System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e"},{"level":22,"method":"Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider+ExceptionInterceptor`1+EnumeratorExceptionInterceptor+<MoveNext>d__5.MoveNext","assembly":"Microsoft.EntityFrameworkCore, Version=2.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60"}]}],"severityLevel":"Error"}}}
Exception thrown: 'System.NullReferenceException' in System.Private.CoreLib.dll
Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker:Information: Executed action FDA.Operational.Core.Workflow.WebAPI.Controllers.CasesController.GetCases (FDA.Operational.Core.Workflow.WebAPI) in 1562.4148ms
Application Insights Telemetry (unconfigured): {"name":"Microsoft.ApplicationInsights.Dev.Message","time":"2017-11-16T16:16:29.6814886Z","tags":{"ai.operation.id":"b6360c2b-4dabca2bb91d06ac","ai.application.ver":"1.0.0.0","ai.internal.nodeName":"UK2207793W1","ai.cloud.roleInstance":"UK2207793W1","ai.location.ip":"::1","ai.operation.name":"GET Cases/GetCases","ai.operation.parentId":"|b6360c2b-4dabca2bb91d06ac.","ai.internal.sdkVersion":"aspnet5c:2.1.1"},"data":{"baseType":"MessageData","baseData":{"ver":2,"message":"Executed action FDA.Operational.Core.Workflow.WebAPI.Controllers.CasesController.GetCases (FDA.Operational.Core.Workflow.WebAPI) in 1562.4148ms","severityLevel":"Information","properties":{"DeveloperMode":"true","ElapsedMilliseconds":"1562.4148","ActionName":"FDA.Operational.Core.Workflow.WebAPI.Controllers.CasesController.GetCases (FDA.Operational.Core.Workflow.WebAPI)","AspNetCoreEnvironment":"Development","CategoryName":"Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker","{OriginalFormat}":"Executed action {ActionName} in {ElapsedMilliseconds}ms"}}}}
Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware:Error: An unhandled exception has occurred while executing the request

如果 case.workflowid 不为空，查询工作正常。当它为 null 时发生异常，并且当它的前一个实体为 null 时发生在 .ThenInclude() 处

目前我正在尝试像下面这样的东西。不确定这是正确的方法:

    var unassignedCases = await _context.Cases
                              .Include(c => c.CaseDetail)
                              .Include(c => c.Alerts)
                              .Where(c=>c.Workflow == null)
                              .ToListAsync();

    var assignedCases = await _context.Cases
                              .Include(c => c.CaseDetail)
                              .Include(c => c.Workflow).ThenInclude(wf => wf.WorkflowRule)
                              .Include(c => c.Workflow).ThenInclude(wf => wf.CurrentWorkflowLevel).ThenInclude(wfl => wfl.FromRules)
                              .Include(c => c.Alerts)
                              .Where(c => c.Workflow != null)
                              .ToListAsync();

Answer 1

这是核心 2.0 中的一个错误已在 beta/patch public 2.0.1 中修复，您可以使用同步(不是异步作为解决方法)

公开补丁的路径: https://dotnet.myget.org/F/aspnet-2-0-2-october2017-patch-public/api/v3/index.json