Entity Framework Core A second operation started on this context before a previous operation completed

Question

I m working on a ASP Net Core 2 0 project using Entity Framework Core    lt PackageReference Include  Microsoft EntityFrameworkCore  Version  2 0 1    gt     lt PackageReference Include  Microsoft EntityFrameworkCore Tools  Version  2 0 0  PrivateAssets  All    gt   lt PackageReference Include  Microsoft EntityFrameworkCore Design  Version  2 0 0   gt    And in one of my list methods I m getting this error   InvalidOperationException  A second operation started on this context before a previous operation completed  Any instance members are not guaranteed to be thread safe  Microsoft EntityFrameworkCore Internal ConcurrencyDetector EnterCriticalSection     This is my method        HttpGet   currentPage   pageSize           HttpGet   currentPage   pageSize   search         public ListResponseVM lt ClientVM gt  GetClients  FromRoute  int currentPage  int pageSize  string search                var resp   new ListResponseVM lt ClientVM gt             var items    context Clients              Include i   gt  i Contacts               Include i   gt  i Addresses               Include  ClientObjectives Objective                Include i   gt  i Urls               Include i   gt  i Users               Where p   gt  string IsNullOrEmpty search     p CompanyName Contains search                OrderBy p   gt  p CompanyName               ToPagedList pageSize  currentPage            resp NumberOfPages   items TotalPage           foreach  var item in items                        var client    mapper Map lt ClientVM gt  item                client Addresses   new List lt AddressVM gt                 foreach  var addr in item Addresses                                var address    mapper Map lt AddressVM gt  addr                   address CountryCode   addr CountryId                  client Addresses Add address                              client Contacts   item Contacts Select p   gt   mapper Map lt ContactVM gt  p   ToList                client Urls   item Urls Select p   gt   mapper Map lt ClientUrlVM gt  p   ToList                client Objectives   item Objectives Select p   gt   mapper Map lt ObjectiveVM gt  p   ToList                resp Items Add client                      return resp          I m a bit lost especially because it works when I run it locally  but when I deploy to my staging server  IIS 8 5  it gets me this error and it was working normally  The error started to appear after I increase the max length of one of my models  I also updated the max length of the corresponding View Model  And there are many other list methods that are very similar and they are working   I had a Hangfire job running  but this job doesn t use the same entity  That s all I can think to be relevant  Any ideas of what could be causing this

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I got the same message  But it s not making any sense in my case  My issue is I used a  NotMapped  property by mistake   It probably only means an error of Linq syntax or model class in some cases  The error message seems misleading  The original meaning of this message is you can t call async on same dbcontext more than once in the same request    NotMapped  public int PostId   get  set    public virtual Post Post   get  set      You can check this link for detail  https   www softwareblogs com Posts Details 5 error-a-second-operation-started-on-this-context-before-a-previous-operation-completed

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My situation is different   I was trying to seed the database with 30 users  belonging to specific roles  so I was running this code    for  var i   1  i  lt   30  i          CreateUserWithRole  Analyst     analyst i    UserManager       This was a Sync function  Inside of it I had 3 calls to   UserManager FindByNameAsync username  Result UserManager CreateAsync user  pass  Result UserManager AddToRoleAsync user  roleName  Result   When I replaced  Result with  GetAwaiter   GetResult    this error went away

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If your method is returning something back  you can solve this error by putting  Result to the end of the job and   Wait   if it doesn t return anything

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I had the same error  It happened because I called a method that was constructed as public async void     instead of public async Task

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you can use SemaphoreSlim to block the next thread that will try to execute that EF call  static SemaphoreSlim semSlim   new SemaphoreSlim 1  1    await semSlim WaitAsync    try        something like this here         EmployeeService GetList    or      var result   await  ctx Employees ToListAsync      finally     semSlim Release

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I think this answer still can help some one and save many times  I solved a similar issue by changing IQueryable to List or to array  collection       For example   var list  context table1 where         to  var list  context table1 where      ToList      or ToArray

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Entity Framework Core does not support multiple parallel operations being run on the same DbContext instance  This includes both parallel execution of async queries and any explicit concurrent use from multiple threads  Therefore  always await async calls immediately  or use separate DbContext instances for operations that execute in parallel

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I managed to get that error by passing an IQueryable into a method that then used that IQueryable  list  as part of a another query to the same context    public void FirstMethod            This is returning an IQueryable     var stockItems    dbContext StockItems          Where st   gt  st IsSomething        SecondMethod stockItems      public void SecondMethod IEnumerable lt Stock gt  stockItems        var grnTrans    dbContext InvoiceLines          Where il   gt  stockItems Contains il StockItem            ToList        To stop that happening I used the approach here and materialised that list before passing it the second method  by changing the call to SecondMethod to be SecondMethod stockItems ToList

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In some cases  this error occurs when calling an async method without the await keyword  which can simply be solved by adding await before the method call  however  the answer might not be related to the mentioned question but it can help solving a similar error

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I have a background service that performs an action for each entry in a table  The problem is  that if I iterate over and modify some data all on the same instance of the DbContext this error occurs    One solution  as mentioned in this thread is to change the DbContext s lifetime to transient by defining it like  services AddDbContext lt DbContext gt  ServiceLifetime Transient     but because I do changes in multiple different services and commit them at once using the SaveChanges   method this solution doesnt work in my case   Because my code runs in a service  I was doing something like  using  var scope   Services CreateScope         var entities   scope ServiceProvider GetRequiredService lt IReadService gt    GetEntities       var writeService   scope ServiceProvider GetRequiredService lt IWriteService gt        foreach  Entity entity in entities              writeService DoSomething entity             to be able to use the service like if it was a simple request  So to solve the issue i just split the single scope into two  one for the query and the other for the write operations like so   using  var readScope   Services CreateScope    using  var writeScope   Services CreateScope         var entities   readScope ServiceProvider GetRequiredService lt IReadService gt    GetEntities       var writeService   writeScope ServiceProvider GetRequiredService lt IWriteService gt        foreach  Entity entity in entities              writeService DoSomething entity             Like that  there are effevtively two different instances of the DbContext being used   Another possible solution would be to make sure  that the read operation has terminated before starting the iteration  That is not very pratical in my case because there could be a lot of results that would all need to be loaded into memory for the operation which I tried to avoid by using a Queryable in the first place

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I faced the same issue but the reason was none of the ones listed above  I created a task  created a scope inside the task and asked the container to obtain a service  That worked fine but then I used a second service inside the task and I forgot to also asked for it to the new scope  Because of that  the 2nd service was using a DbContext that was already disposed   Task task   Task Run      gt                using  var scope   serviceScopeFactory CreateScope                          var otherOfferService   scope ServiceProvider GetService lt IOfferService gt                    everything was ok here  then I did               productService DoSomething        from the main scope  and this failed because the db context associated to that service was already disposed                                    I should have done this   var otherProductService   scope ServiceProvider GetService lt IProductService gt     otherProductService DoSomething

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I just managed to make it work again  It makes not much sense but it worked    Remove Hangfire from StartUp  I was creating my job there   Deleted the hangfire database  Restarted the server   I ll investigate further later but the method I called with hangfire receives a DBContext and that is the possible cause

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First  upvote  at the least  alsami s answer   That got me on the right path   But for those of you doing IoC  here is a little bit of a deeper dive   My error  same as others      One or more errors occurred   A second operation started on this   context before a previous operation completed  This is usually caused   by different threads using the same instance of DbContext  For more   information on how to avoid threading issues with DbContext  see   https   go microsoft com fwlink  linkid 2097913     My code setup    Just the basics      public class MyCoolDbContext  DbContext      public DbSet  lt MySpecialObject gt  MySpecialObjects          get         set           and  public interface IMySpecialObjectDomainData     and  note MyCoolDbContext is being injected   public class MySpecialObjectEntityFrameworkDomainDataLayer  IMySpecialObjectDomainData      public MySpecialObjectEntityFrameworkDomainDataLayer MyCoolDbContext context               HERE IS WHERE TO SET THE BREAK POINT  HOW MANY TIMES IS THIS RUNNING               this entityDbContext   context    throw new ArgumentNullException  MyCoolDbContext is null    Exception null             and  public interface IMySpecialObjectManager     and  public class MySpecialObjectManager  IMySpecialObjectManager       public const string ErrorMessageIMySpecialObjectDomainDataIsNull    IMySpecialObjectDomainData is null       private readonly IMySpecialObjectDomainData mySpecialObjectDomainData       public MySpecialObjectManager IMySpecialObjectDomainData mySpecialObjectDomainData            this mySpecialObjectDomainData   mySpecialObjectDomainData    throw new ArgumentNullException ErrorMessageIMySpecialObjectDomainDataIsNull   Exception null             And finally   my multi threaded class  being called from a Console App Command Line Interface app       public interface IMySpecialObjectThatSpawnsThreads     and  public class MySpecialObjectThatSpawnsThreads  IMySpecialObjectThatSpawnsThreads       public const string ErrorMessageIMySpecialObjectManagerIsNull    IMySpecialObjectManager is null        private readonly IMySpecialObjectManager mySpecialObjectManager       public MySpecialObjectThatSpawnsThreads IMySpecialObjectManager mySpecialObjectManager            this mySpecialObjectManager   mySpecialObjectManager    throw new ArgumentNullException ErrorMessageIMySpecialObjectManagerIsNull   Exception null             and the DI buildup    Again  this is for a console application  command line interface    which exhibits slight different behavior than web-apps       private static IServiceProvider BuildDi IConfiguration configuration           this is being called early inside my command line application   console application           string defaultConnectionStringValue   string Empty     get this value from configuration             setup our DI     IServiceCollection servColl   new ServiceCollection                AddLogging loggingBuilder   gt  loggingBuilder AddConsole                THE BELOW TWO ARE THE ONES THAT TRIPPED ME UP               AddTransient lt IMySpecialObjectDomainData  MySpecialObjectEntityFrameworkDomainDataLayer gt         AddTransient lt IMySpecialObjectManager  MySpecialObjectManager gt            so the  ServiceLifetime Transient  below                is what you will find most commonly on the internet search results           if  MY ORACLE           AddDbContext lt ProvisioningDbContext gt  options   gt  options UseOracle defaultConnectionStringValue   ServiceLifetime Transient          endif         if  MY SQL SERVER           AddDbContext lt ProvisioningDbContext gt  options   gt  options UseSqlServer defaultConnectionStringValue   ServiceLifetime Transient          endif      servColl AddSingleton  lt IMySpecialObjectThatSpawnsThreads         MySpecialObjectThatSpawnsThreads gt          ServiceProvider servProv   servColl BuildServiceProvider         return servProv      The ones that surprised me were the  change to  transient for           AddTransient lt IMySpecialObjectDomainData  MySpecialObjectEntityFrameworkDomainDataLayer gt         AddTransient lt IMySpecialObjectManager  MySpecialObjectManager gt      Note  I think because IMySpecialObjectManager was being injected into  MySpecialObjectThatSpawnsThreads   those injected objects needed to be Transient to complete the chain   The point being       it wasn t just the  My DbContext that needed  Transient   but a bigger chunk of the DI Graph   Debugging Tip   This line   this entityDbContext   context    throw new ArgumentNullException  MyCoolDbContext is null    Exception null     Put your debugger break point there   If your MySpecialObjectThatSpawnsThreads is making N number of threads  say 10 threads for example       and that line is only being hit once   that s your issue   Your DbContext is crossing threads   BONUS   I would suggest reading this below url article  oldie but goodie  about the differences web-apps and console-apps  https   mehdi me ambient-dbcontext-in-ef6   Here is the header of the article in case the link changes      MANAGING DBCONTEXT THE RIGHT WAY WITH ENTITY FRAMEWORK 6  AN IN-DEPTH   GUIDE Mehdi El Gueddari   I hit this issue with WorkFlowCore  https   github com danielgerlag workflow-core     lt ItemGroup gt       lt PackageReference Include  WorkflowCore  Version  3 1 5    gt     lt  ItemGroup gt    sample code below   to help future internet searchers   namespace MyCompany Proofs WorkFlowCoreProof BusinessLayer Workflows MySpecialObjectInterview Workflows               using System          using MyCompany Proofs WorkFlowCoreProof BusinessLayer Workflows MySpecialObjectInterview Constants          using MyCompany Proofs WorkFlowCoreProof BusinessLayer Workflows MySpecialObjectInterview Glue          using MyCompany Proofs WorkFlowCoreProof BusinessLayer Workflows WorkflowSteps           using WorkflowCore Interface          using WorkflowCore Models           public class MySpecialObjectInterviewDefaultWorkflow   IWorkflow lt MySpecialObjectInterviewPassThroughData gt                        public const string WorkFlowId    MySpecialObjectInterviewWorkflowId                public const int WorkFlowVersion   1               public string Id   gt  WorkFlowId               public int Version   gt  WorkFlowVersion               public void Build IWorkflowBuilder lt MySpecialObjectInterviewPassThroughData gt  builder                                builder                               StartWith context   gt                                                Console WriteLine  Starting workflow                               return ExecutionResult Next                                                       bunch of other Steps here that were using IMySpecialObjectManager   here is where my DbContext was getting cross-threaded                           Then lastContext   gt                                                Console WriteLine                             bool wroteConcreteMsg   false                          if  null    lastContext  amp  amp  null    lastContext Workflow  amp  amp  null    lastContext Workflow Data                                                        MySpecialObjectInterviewPassThroughData castItem   lastContext Workflow Data as MySpecialObjectInterviewPassThroughData                              if  null    castItem                                                                Console WriteLine  MySpecialObjectInterviewDefaultWorkflow complete      0    - gt   1    castItem PropertyOne  castItem PropertyTwo                                   wroteConcreteMsg   true                                                                                   if   wroteConcreteMsg                                                        Console WriteLine  MySpecialObjectInterviewDefaultWorkflow complete   Data did not cast                                                        return ExecutionResult Next                                                  OnError WorkflowCore Models WorkflowErrorHandling Retry  TimeSpan FromSeconds 60                                     and   ICollection lt string gt  workFlowGeneratedIds   new List lt string gt                     for  int i   0  i  lt  10  i                                          MySpecialObjectInterviewPassThroughData currentMySpecialObjectInterviewPassThroughData   new MySpecialObjectInterviewPassThroughData                        currentMySpecialObjectInterviewPassThroughData MySpecialObjectInterviewPassThroughDataSurrogateKey   i                             private readonly IWorkflowHost workflowHost                      string wfid   await this workflowHost StartWorkflow MySpecialObjectInterviewDefaultWorkflow WorkFlowId  MySpecialObjectInterviewDefaultWorkflow WorkFlowVersion  currentMySpecialObjectInterviewPassThroughData                       workFlowGeneratedIds Add wfid

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The exception means that  context is being used by two threads at the same time  either two threads in the same request  or by two requests   Is your  context declared static maybe  It should not be   Or are you calling GetClients multiple times in the same request from somewhere else in your code   You may already be doing this  but ideally  you d be using dependency injection for your DbContext  which means you ll be using AddDbContext   in your Startup cs  and your controller constructor will look something like this   private readonly MyDbContext  context    not static  public MyController MyDbContext context         context   context      If your code is not like this  show us and maybe we can help further

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In my case I use a template component in Blazor     lt BTable ID  Table1  TotalRows  MyList Count    gt    The problem is calling a method  Count  in the component header  To resolve the problem I changed it like this    int total   MyList Count      and later     lt BTable ID  Table1  TotalRows  total  gt

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I know this issue has been asked two years ago  but I just had this issue and the fix I used really helped   If you are doing two queries with the same Context - you might need to remove the AsNoTracking  If you do use AsNoTracking you are creating a new data-reader for each read  Two data readers cannot read the same data

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I got the same problem when I try to use  FirstOrDefaultAsync    in the async method in the code below  And when I fixed FirstOrDefault   - the problem was solved    context Issues Add issue           await  context SaveChangesAsync             int userId    context Users              Where u   gt  u UserName    Options UserName               FirstOrDefaultAsync                Id

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Solve my problem using this line of code in my Startup cs file                Adding a transient service means that each time the service is requested  a new instance is created when you are working with       Dependency injection         services AddDbContext lt Context gt  options   gt                          options UseSqlServer  configuration GetConnectionString  ContextConn                  ServiceLifetime Transient

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I had the same problem and it turned out that parent service was a singelton  So the context automatically became singelton too  Even though was declared as Per Life Time Scoped in DI   Injecting service with different lifetimes into another   Never inject Scoped  amp  Transient services into Singleton service    This effectively converts the transient or scoped service into the singleton    Never inject Transient services into scoped service   This converts the transient service into the scoped

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In my case I was using a lock which does not allow the use of await and does not create compiler warning when you don t await an async   The problem   lock  someLockObject           do stuff     context SaveChangesAsync          some other code somewhere else doing await context SaveChangesAsync   shortly after the lock gets the concurrency error   The fix  Wait for the async inside the lock by making it blocking with a  Wait    lock  someLockObject           do stuff     context SaveChangesAsync   Wait

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I am not sure if you are using IoC and Dependency Injection to resolve your DbContext where ever it might be used  If you do and you are using native IoC from  NET Core  or any other IoC-Container  and you are getting this error  make sure to register your DbContext as Transient  Do  services AddTransient lt MyContext gt       OR  services AddDbContext lt MyContext gt  ServiceLifetime Transient     instead of  services AddDbContext lt MyContext gt       AddDbContext adds the context as scoped  which might cause troubles when working with multiple threads    Also async   await operations can cause this behaviour  when using async lambda expressions   Adding it as transient also has its downsides  You will not be able to make changes to some entity over multiple classes that are using the context because each class will get its own instance of your DbContext   The simple explanation for that is  that the DbContext implementation is not thread-safe  You can read more about this here

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Another possible case  if you use the connection direct  don t forget to close if  I needed to execute arbitrary SQL query  and read the result  This was a quick fix  I did not want to define a data class  not set up  quot normal quot  SQL connection  So simply I reused EFC s database connection as var connection   Context Database GetDbConnection   as SqlConnection  Make sure you call connection Close   before you do Context SaveChanges

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