Inject dependencies in interceptors constructors

[facuabdo]

Hi Lawrence, i found your package of great utility. I was wondering if dependencies that need to be injected into the intercerptors using the constructor are considered -in a way that they're resolved automatically- or if you need to resolve them manually.

[lawrence-laz]

Hi facuabdo, I'm glad you find the package helpful. 🙂

Yes, automatic dependency injection is considered for interceptor classes. There are two ways of achieving this.

1. If you are using Microsoft's IServiceProvider as a dependency container, then you can use this package Decor.Extensions.Microsoft.DependencyInjection and follow these instructions.

2. If you use some other dependency container, then you can provide a Factory method with your container's resolve implementation. Pseudo example:

   Decorator decorator = ... ;
   YourDependencyContainer container = ... ;
   decorator.Factory = (type) => (IDecorator)container.Resolve(type);

   After this the Decorator class will be able to resolve constructor dependencies automatically.

Let me know if this helps. Otherwise I could provide a minimal working example if you share which dependency container you are using.

[facuabdo]

Thanks for the reply, it definitely helps. I missed mentioning i was using ASP.NET Core built-in DI. While I was waiting for your reply I ran some tests and found out it works out of the box (all dependencies are resolved).

One thing that is getting me some trouble is that my interceptor has a DbContext dependency. The only way I can get it to work is registering DbContext as transient, which is something I want to avoid since i'm trying to implement Unit of Work using interceptors + DbContext.

This is my ServiceCommandInterceptor code

public class ServiceCommandInterceptor : IDecorator
    {
        private readonly DbContext _unitOfWork;

        public ServiceCommandInterceptor(DbContext unitOfWork)
        {
            _unitOfWork = unitOfWork;
        }

        public Task OnInvoke(Call call)
        {
            using (IDbContextTransaction transaction = _unitOfWork.Database.BeginTransaction())
            {
                try
                {
                    var result = call.Next();
                    _unitOfWork.SaveChanges();
                    transaction.Commit();
                    return result;
                }
                catch
                {
                    transaction.Rollback();
                    throw;
                }
            }
        }
    }

And this are my registrations in the ConfigureServices method

public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllers();

            
            services.AddScoped<DbContext, InmobiliariaCloudDbContext>();
            services.AddScoped(typeof(IRepository<>), typeof(BaseEFRepository<>));

            services.AddScoped<IAccountService, AccountService>();
            services.AddTransient<ServiceCommandInterceptor>();
            services.Decorate<IAccountService>();

            services.AddScoped<IAccountService, AccountService>();
            services.AddTransient<ServiceQueryInterceptor>();
            services.Decorate<IAccountService>();
        }

Calling any method from AccountService throws System.InvalidOperationException: Cannot resolve 'Services.Core.ServiceCommandInterceptor' from root provider because it requires scoped service 'Microsoft.EntityFrameworkCore.DbContext'.

Changing 1st line to

services.AddTransient<DbContext, InmobiliariaCloudDbContext>();

Works just fine.

In this case my AccountService has only one depedency on IRepository which is resolved to BaseEFRepository. BaseEFRepository as well has a dependency on DbContext. If my service had more dependencies on more repositories it would mean that each of them would receive a new instance of DbContext (as a result of being registered as transient) and unit of work would not work at all. Any ideas why it can't be resolved when registered as scoped?

PS: sorry for the many many edits, i couldn't get the code to display correctly

[lawrence-laz]

I was pretty confident scoped dependencies in transient IDecorator shouldn't cause any problems, but added this test just to make sure. The test passes without problems.

The error seems like it could be specific to Asp.NET Core environment. Could you possibly provide me with a minimal part of your project that reproduces this problem?

Oh and I noticed a slight unrelated problem in your OnInvoke method: call.Next() call should be awaited. Otherwise _unitOfWork.SaveChanges(); might get executed before the service finishes its work.