当相关对象在.Include中为null时，Entityframework Core 2.0错误
我正在使用EF core 2.0和.Net core2.0来开发我的webapi应用程序。
在下面的查询中，当案例没有工作流程（即可空的工作流程）时，我会遇到exception。 我得到的错误是’System.NullReferenceException：对象引用未设置为对象的实例。’
当c.workflow为null时，我怀疑.ThenInclude（）上的exception。
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);
考虑到工作流是空的，任何人都可以帮助如何实现此查询吗？
例外：
System.NullReferenceException: Object reference not set to an instance of an object. at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.IncludeLoadTreeNodeBase.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.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.d__5.MoveNext()","hasFullStack":true,"parsedStack":[{"level":0,"method":"Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler+IncludeLoadTreeNodeBase+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+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+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+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+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+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+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null，则查询工作正常。 当它为null时发生exception，并且当它的前一个实体为null时，它发生在.ThenInclude（）
目前我正在尝试下面的某些事情。 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();
这是核心2.0中的错误修复了beta / patch public 2.0.1，你可以使用sync（不是async作为解决方法）
补丁公共路径： https ： //dotnet.myget.org/F/aspnet-2-0-2-october2017-patch-public/api/v3/index.json