How to inject DbContext into repository constructor in ASP.NET core

Question

I am writing my first ASP.NET Core Web API and am trying to work out how to inject my DbContext object into my repository constructor. I followed the EF part of this tutorial where the DbContext is registered to the service collection via services.AddDbContext<DbContext> and services.GetRequiredService<DbContext>() is used to initialize the database values.

 public class Startup { public IConfiguration Configuration { public void ConfigureServices(IServiceCollection services) { services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1); services.AddSingleton<IItemRepository, ItemRepository>(); services.AddSingleton<IUserRepository, UserRepository>(); services.AddSwaggerGen(c => { c.SwaggerDoc("v1", new Info { Title = "My API", Version = "v1" }); }); services.AddDbContext<RAFYCContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection"))); } }

 public class Program { public static void Main(string[] args) { //CreateWebHostBuilder(args).Build().Run(); IWebHost host = CreateWebHostBuilder(args).Build(); using (IServiceScope scope = host.Services.CreateScope()) { IServiceProvider services = scope.ServiceProvider; try { RAFYCContext context = services.GetRequiredService<RAFYCContext>(); DbInitializer.Initialize(context); } catch (Exception ex) { ILogger logger = services.GetRequiredService<ILogger<Program>>(); logger.LogError(ex, "An error occurred while seeding the database."); } } host.Run(); } }

I am able to inject the DbContext into the Controller and then call a repository method to assign it to the UserRepository:

 public class UserController : ControllerBase { IUserRepository _userRepo; public UserController(IUserRepository userRepo, RAFYCContext context) { _userRepo = userRepo; _userRepo.SetDbContext(context); } }

 public class UserRepository : IUserRepository { public RAFYCContext _context; public UserRepository() { } public void SetDbContext(RAFYCContext context) { this._context = context; } }

This works, however I would like to inject the DbContext into the constructor of my repository rather than assigning it in the Controller constructor after instantiation, like below:

 [Route("api/[controller]")] [ApiController] public class UserController : ControllerBase { IUserRepository _userRepo; public UserController(IUserRepository userRepo) { _userRepo = userRepo; } }

 public class UserRepository : IUserRepository { RAFYCContext _context; public UserRepository(RAFYCContext context) { _context = context; } }

With this code I get the following error:

InvalidOperationException: Cannot consume scoped service 'RAFYC.MobileAppService.Models.RAFYCContext' from singleton 'RAFYC.MobileAppService.Repositories.IUserRepository'

Does anyone know if this is possible with ASP.NET Core (2.2)?

The error is accurate as you are trying to inject a scoped dependency into a singleton, which is not allowed. DbContexts are added as scoped. — Nkosi
– Nkosi, Commented May 30, 2019 at 19:03
@Nkosi I'm not trying to imply the error was incorrect, just if there is a way I can design my code (other than the one I suggested in the post) the achieve what I'm after. If not, I'll live with what I've got! — GDiff94
– GDiff94, Commented May 30, 2019 at 19:06
You could consider adding the repositories as scoped as well — Nkosi
– Nkosi, Commented May 30, 2019 at 19:07
Should also take a quick review of this in the docs learn.microsoft.com/en-us/aspnet/core/fundamentals/… which should help explain the problem — Nkosi
– Nkosi, Commented May 30, 2019 at 19:09
@NKosi adding it as scoped did the trick, and thanks for the docs, makes more sense now — GDiff94
– GDiff94, Commented May 30, 2019 at 19:14

Nkosi · Accepted Answer · 2019-05-30 19:15:25Z

7

AddDbContext adds a DbContext as scoped by default, which would cause problems with the singleton repositories.

It's dangerous to resolve a scoped service from a singleton. It may cause the service to have incorrect state when processing subsequent requests.

Reference Dependency injection in ASP.NET Core: Service lifetimes

I would suggest adding the repositories as scoped as well if possible.

services.AddScoped<IItemRepository, ItemRepository>(); services.AddScoped<IUserRepository, UserRepository>();

answered May 30, 2019 at 19:15

Nkosi

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2 Comments

Vočko Over a year ago

This is a correct answer. If you register your repositories as singleton, you will see EF exceptions:

A second operation started on this context before a previous asynchronous operation completed. Use 'await' to ensure that any asynchronous operations have completed before calling another method on this context. Any instance members are not guaranteed to be thread safe.

This is because you only create it once and thus you will be re-using the db context for all your requests (they might be concurrent).

Dunge Over a year ago

Adding the repository as scoped prevent pre-loading some stuff in its constructor (for example, calling EF.CompileQuery), or doing a query to load data that is static for the lifetime of the application but re-used in multiple subsequent queries (rebuilding an object without call to Include()) like we would previously do with a Singleton repository. How would you get around that?

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