Entity Framework Core thread safe?

Question

I follow Generic Repository Pattern in ASP.NET Core , but on IRepository , I use IQueryable instead of IEnumerable :

public  interface IRepository<T> where T: BaseEntity
{
    IQueryable<T> Table { get; }
    IEnumerable<T> TableNoTracking { get; }
    T Get(long id);
    void Insert(T entity);
    void Update(T entity);
    void Delete(T entity);
}

and implementation class:

    public class EFRepository<T> : IRepository<T> where T : BaseEntity
    {
        private readonly ApplicationDbContext _ctx;
        private DbSet<T> entities;
        string errorMessage = string.Empty;

        public EFRepository(ApplicationDbContext context)
        {
            this._ctx = context;
            entities = context.Set<T>();
        }

        public virtual IQueryable<T> Table => this.entities;
    }

Service class:

public class MovieService : IMovieService
{
        private readonly IRepository<MovieItem> _repoMovie;

        public MovieService(IRepository<MovieItem> repoMovie)
        {
            _repoMovie = repoMovie;
        }

        public async Task<PaginatedList<MovieItem>> GetAllMovies(int pageIndex = 0, int pageSize = int.MaxValue,
               IEnumerable<int> categoryIds = null)
        {
            var query = _repoMovie.Table;

            if (categoryIds != null)
            {
                query = from m in query
                        where categoryIds.Contains(m.CategoryId)
                        select m;
            }

            return await PaginatedList<MovieItem>.CreateAsync(query, pageIndex, pageSize);
        }
}

On Startup.cs:

  public void ConfigureServices(IServiceCollection services)
  {
        // Add framework services.
        services.AddDbContext<ApplicationDbContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

        services.AddMvc();
        services.AddScoped(typeof(IRepository<>), typeof(EFRepository<>));           
        services.AddTransient<IMovieService, MovieService>();
        services.AddTransient<ICategoryService, CategoryService>();
    }

This code throws an error:

InvalidOperationException: A second operation started on this context before a previous operation completed. Any instance members are not guaranteed to be thread safe.

If I switch back to IEnumerable on IRepository , then it runs fine.

Any idea how to get it to work with IQueryable , to make to EF Core to run in the right way ?

query = from m in query
        where categoryIds.Contains(m.CategoryId)
        select m;

Answer 1

Entity Framework DbContext is not thread safe. You can execute only one query at a time otherwise you will get an exception like you did above.

I suppose that you use our repository multiple times during same request in parallel that's why you get the exception. If you don't create a transaction per request you can simply make repository transient. In this case new repository will be created fro each instance of your service and you will avoid concurrency issue.