Cannot access a disposed object for DbContext in .NET Core for Async method

Question

I am having a weird problem in one of my microservice web api. My async GET methods throw a Cannot access a disposed object exception for my DbContext except for the very first time they are invoked. I tried looking online for an answer but nothing worked. I made sure my methods are not async void and I await the necessary calls. Since my POST and DELETE methods work fine, I am fairly certain that the real culprit is the IMapper instance. I think it might always point to the first instance of the DbContext and that is why the works the first time but not the ones after. Any help or pointers would be gladly appreciated

Here are some snapshots of the code.

Startup.cs

...
// Add AutoMapper
        services.AddAutoMapper(new Assembly[] { typeof(AutoMapperProfile).GetTypeInfo().Assembly });

// Add DbContext using NoSQL Server Provider
services.AddDbContext<ProfileDbContext>(options =>
            options.UseMongoDb(Configuration.GetConnectionString("TeamJobProfilesDatabase")));
...

MyController.cs

    // GET api/profiles
    [HttpGet]
    [ProducesResponseType(StatusCodes.Status200OK)]
    public async Task<ActionResult<ProfilesListViewModel>> GetAll()
    {
        return Ok(await Mediator.Send(new GetAllProfilesQuery()));
    }

    // GET api/profiles/{id}
    [HttpGet("{id}")]
    [ProducesResponseType(StatusCodes.Status200OK)]
    [ProducesResponseType(StatusCodes.Status404NotFound)]
    public async Task<ActionResult<ProfileViewModel>> Get(int id)
    {
        return Ok(await Mediator.Send(new GetProfileQuery { Id = id }));
    }

GetAllProfilesQueryHandler.cs

public class GetAllProfilesQueryHandler : IRequestHandler<GetAllProfilesQuery, ProfilesListViewModel>
{
    private readonly ProfileDbContext _context;
    private readonly IMapper _mapper;

    public GetAllProfilesQueryHandler(ProfileDbContext context, IMapper mapper)
    {
        _context = context;
        _mapper = mapper;
    }

    public async Task<ProfilesListViewModel> Handle(GetAllProfilesQuery request, CancellationToken cancellationToken)
    {
        return new ProfilesListViewModel
        {
            Profiles = await _context.Profiles.ProjectTo<ProfileLookupModel>(_mapper.ConfigurationProvider).ToListAsync(cancellationToken)
        };
    }
}

ProfileDbContext.cs

[MongoDatabase("profileDb")]
public class ProfileDbContext : DbContext
{
    public ProfileDbContext(DbContextOptions<ProfileDbContext> options)
        : base(options)
    {
    }

    public DbSet<Domain.Entities.Profile> Profiles { get; set; }
}

Exception message

{ "error": [ "Cannot access a disposed object. A common cause of this error is disposing a context that was resolved from dependency injection and then later trying to use the same context instance elsewhere in your application. This may occur if you are calling Dispose() on the context, or wrapping the context in a using statement. If you are using dependency injection, you should let the dependency injection container take care of disposing context instances.\\r\\nObject name: 'ProfileDbContext'." ], "stackTrace": " at Microsoft.EntityFrameworkCore.DbContext.CheckDisposed()\\r\\n at Microsoft.EntityFrameworkCore.DbContext.get_InternalServiceProvider()\\r\\n at Microsoft.EntityFrameworkCore.DbContext.get_ChangeTracker()\\r\\n at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompilationContextFactory.get_TrackQueryResults()\\r\\n at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompilationContextFactory.Create(Boolean async)\\r\\n at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](QueryModel queryModel)\\r\\n at Blueshift.EntityFrameworkCore.MongoDB.Storage.MongoDbDatabase.<>c__DisplayClass11_0 1.<CompileAsyncQuery>b__0(QueryContext queryContext)\\r\\n at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteAsync[TResult](Expression query)\\r\\n at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.ExecuteAsync[TResult](Expression expression)\\r\\n at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable 1.System.Collections.Generic.IAs yncEnumerable.GetEnumerator()\\r\\n at System.Linq.AsyncEnumerable.Aggregate_[TSource,TAccumulate,TResult](IAsyncEnumerable 1 source, TAccumulate seed, Func 3 accumulator, Func 2 resultSelector, CancellationToken cancellationToken) in D:\\\\a\\\\1\\\\s\\\\Ix.NET\\\\Source\\\\System.Interactive.Async\\\\Aggregate.cs:line 118\\r\\n at Profile.Application.Profiles.Queries.GetAllProfiles.GetAllProfilesQueryHandler.Handle(GetAllProfilesQuery request, CancellationToken cancellationToken) in C:\\\\Users\\\\Adam\\\\Repositories\\\\TeamJob\\\\TeamJob\\\\src\\\\Services\\\\Profile\\\\Profile.Application\\\\Profiles\\\\Queries\\\\GetAllProfiles\\\\GetAllProfilesQueryHandler.cs:line 24\\r\\n at MediatR.Pipeline.RequestPostProcessorBehavior 2.Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate 1 next)\\r\\n at MediatR.Pipeline.RequestPreProcessorBehavior 2.Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate 1 next)\\r\\n at MediatR.Pipeline.RequestPreProcessorBehavior 2.Handle(TRequ est request, CancellationToken cancellationToken, RequestHandlerDelegate 1 next)\\r\\n at Profile.API.Controllers.ProfilesController.GetAll() in C:\\\\Users\\\\Adam\\\\Repositories\\\\TeamJob\\\\TeamJob\\\\src\\\\Services\\\\Profile\\\\Profile.API\\\\Controllers\\\\ProfilesController.cs:line 19\\r\\n at lambda_method(Closure , Object )\\r\\n at Microsoft.Extensions.Internal.ObjectMethodExecutorAwaitable.Awaiter.GetResult()\\r\\n at Microsoft.AspNetCore.Mvc.Internal.ActionMethodExecutor.AwaitableObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)\\r\\n at System.Threading.Tasks.ValueTask 1.get_Result()\\r\\n at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeActionMethodAsync()\\r\\n at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeNextActionFilterAsync()\\r\\n at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Rethrow(ActionExecutedContext context)\\r\\n at Microsoft.AspNetCore.Mvc.Internal.ControllerAct ionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)\\r\\n at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeInnerFilterAsync()\\r\\n at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextExceptionFilterAsync()" }

Answer 1

The problem is in Mediator.Send method. Mediator class stores request handlers in a static ConcurrentDictionary

private static readonly ConcurrentDictionary<Type, object> _requestHandlers = new ConcurrentDictionary<Type, object>();

and when the Send method is invoked, it uses GetOrAdd method on that dictionary.

var handler = (RequestHandlerWrapper<TResponse>)_requestHandlers.GetOrAdd(requestType, t => Activator.CreateInstance(typeof(RequestHandlerWrapperImpl<,>).MakeGenericType(requestType, typeof(TResponse))));

This means, if the request handler instance doesn't already exist in the dictionary, it creates a new instance (using Activator ) and adds it to the dictionary, but if the request handler instance already exists in the dictionary, it uses the existing one (and this is the cause of your problem).

So, what exactly is causing your error?

The _requestHandlers dictionary is static , which means it lives through multiple requests , ie doesn't get disposed/garbage collected at the end of the request. Your ProfileDbContext , when registered using AddDbContext method, has a scoped lifetime , which means it is created once per request (and disposed at the end of the request). This means you can end up in a situation where _requestHandlers dictionary contains an instance of GetAllProfilesQueryHandler that has a reference on an exposed instance of ProfileDbContext .

Here is what happens:

1. First request arrives.
2. Mediator.Send(new GetProfileQuery { Id = id }) gets called.
3. Mediator.Send(new GetProfileQuery { Id = id }) doesn't find GetAllProfilesQueryHandler in _requestHandlers dictionary, so it instantiates it and also resolves its ProfileDbContext dependency.
4. The request ends. _context field ( ProfileDbContext ) in your GetAllProfilesQueryHandler indsance gets disposed (because it has a scoped lifetime), but the _requestHandlers dictionary (containing GetAllProfilesQueryHandler instance) doesn't get disposed (because it is static).
5. Another request arrives.
6. Mediator.Send(new GetProfileQuery { Id = id }) gets called again.
7. This time Mediator.Send(new GetProfileQuery { Id = id }) finds GetAllProfilesQueryHandler instance in _requestHandlers dictionary and uses the existing instance, whose _context field is disposed at the end of the previous request .
8. GetAllProfilesQueryHandler tries to access the disposed _context field, and gets the "Cannot access a disposed object" error.

Possible solution

Don't let Mediator.Send resolve GetAllProfilesQueryHandler s dependencies.

Maybe pass IServiceProvider serviceProvider to your GetAllProfilesQueryHandler and let it resolve its dependencies as needed:

public class GetAllProfilesQueryHandler : IRequestHandler<GetAllProfilesQuery, ProfilesListViewModel>
{
    private readonly IServiceProvider _serviceProvider;

    public GetAllProfilesQueryHandler(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public async Task<ProfilesListViewModel> Handle(GetAllProfilesQuery request, CancellationToken cancellationToken)
    {
        return new ProfilesListViewModel
        {
            ProfileDbContext context = (ProfileDbContext)this._serviceProvider.GetService(typeof(ProfileDbContext));
            IMapper mapper = (IMapper)this._serviceProvider.GetService(typeof(IMapper));

            Profiles = await context.Profiles.ProjectTo<ProfileLookupModel>(mapper.ConfigurationProvider).ToListAsync(cancellationToken)
        };
    }
}

Edit:

As @Lucian Bargaoanu pointed out in comments, you can resolve handlers through DI, as in https://github.com/jbogard/MediatR.Extensions.Microsoft.DependencyInjection