ASP.NET Core Dependency Injection error: Unable to

Question

I created an .NET Core MVC application and use Dependency Injection and Repository Pattern to inject a repository to my controller. However, I am getting an error:

InvalidOperationException: Unable to resolve service for type 'WebApplication1.Data.BloggerRepository' while attempting to activate 'WebApplication1.Controllers.BlogController'.

Model (Blog.cs)

namespace WebApplication1.Models
{
    public class Blog
    {
        public int BlogId { get; set; }
        public string Url { get; set; }
    }
}

DbContext (BloggingContext.cs)

using Microsoft.EntityFrameworkCore;
using WebApplication1.Models;

namespace WebApplication1.Data
{
    public class BloggingContext : DbContext
    {
        public BloggingContext(DbContextOptions<BloggingContext> options)
            : base(options)
        { }
        public DbSet<Blog> Blogs { get; set; }
    }
}

Repository (IBloggerRepository.cs & BloggerRepository.cs)

using System;
using System.Collections.Generic;
using WebApplication1.Models;

namespace WebApplication1.Data
{
    internal interface IBloggerRepository : IDisposable
    {
        IEnumerable<Blog> GetBlogs();

        void InsertBlog(Blog blog);

        void Save();
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using WebApplication1.Models;

namespace WebApplication1.Data
{
    public class BloggerRepository : IBloggerRepository
    {
        private readonly BloggingContext _context;

        public BloggerRepository(BloggingContext context)
        {
            _context = context;
        }

        public IEnumerable<Blog> GetBlogs()
        {
            return _context.Blogs.ToList();
        }

        public void InsertBlog(Blog blog)
        {
            _context.Blogs.Add(blog);
        }

        public void Save()
        {
            _context.SaveChanges();
        }

        private bool _disposed;

        protected virtual void Dispose(bool disposing)
        {
            if (!_disposed)
            {
                if (disposing)
                {
                    _context.Dispose();
                }
            }
            _disposed = true;
        }

        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }
    }
}

Startup.cs (relevant code)

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

    services.AddScoped<IBloggerRepository, BloggerRepository>();

    services.AddMvc();

    // Add application services.
    services.AddTransient<IEmailSender, AuthMessageSender>();
    services.AddTransient<ISmsSender, AuthMessageSender>();
}

Controller (BlogController.cs)

using System.Linq;
using Microsoft.AspNetCore.Mvc;
using WebApplication1.Data;
using WebApplication1.Models;

namespace WebApplication1.Controllers
{
    public class BlogController : Controller
    {
        private readonly IBloggerRepository _repository;

        public BlogController(BloggerRepository repository)
        {
            _repository = repository;
        }

        public IActionResult Index()
        {
            return View(_repository.GetBlogs().ToList());
        }

        public IActionResult Create()
        {
            return View();
        }

        [HttpPost]
        [ValidateAntiForgeryToken]
        public IActionResult Create(Blog blog)
        {
            if (ModelState.IsValid)
            {
                _repository.InsertBlog(blog);
                _repository.Save();
                return RedirectToAction("Index");
            }
            return View(blog);
        }
    }
}

I'm not sure what I'm doing wrong. Any ideas?

Answer 1

I was having a different problem, and yeah the parameterized constructor for my controlleror was already added with the correct interface. What I did was something straightforward. I just go to my startup.cs file, where I could see a call to register method.

public void ConfigureServices(IServiceCollection services)
{
   services.Register();
}

In my case, this Register method was in a separate class Injector. So I had to add my newly introduced Interfaces there.

public static class Injector
{
    public static void Register(this IServiceCollection services)
    {
        services.AddTransient<IUserService, UserService>();
        services.AddTransient<IUserDataService, UserDataService>();
    }
}

If you see, the parameter to this function is this IServiceCollection

Hope this helps.

Answer 2

The exception says it cannot resolve the service for WebApplication1.Data.BloggerRepository because the constructor on your controller is asking for the concrete class instead of the interface. So just change that:

public BlogController(IBloggerRepository repository)
//                    ^
//                    Add this!
{
    _repository = repository;
}

Answer 3

You need to add a new service for DBcontext in the startup

Default

services.AddDbContext<ApplicationDbContext>(options =>
                options.UseSqlServer(
                    Configuration.GetConnectionString("DefaultConnection")));

Add this

services.AddDbContext<NewDBContext>(options =>
                options.UseSqlServer(
                    Configuration.GetConnectionString("NewConnection")));

Answer 4

In my case I was trying to do dependency injection for an object which required constructor arguments. In this case, during Startup I just provided the arguments from the configuration file, for example:

var config = Configuration.GetSection("subservice").Get<SubServiceConfig>();
services.AddScoped<ISubService>(provider => new SubService(config.value1, config.value2));

Answer 5

I ran into this issue because in the dependency injection setup I was missing a dependency of a repository that is a dependency of a controller:

services.AddScoped<IDependencyOne, DependencyOne>();    <-- I was missing this line!
services.AddScoped<IDependencyTwoThatIsDependentOnDependencyOne, DependencyTwoThatIsDependentOnDependencyOne>();

Answer 6

If you are using AutoFac and getting this error, you should add an "As" statement to specify the service that the concrete implementation implements.

Ie. you should write:

containerBuilder.RegisterType<DataService>().As<DataService>();

instead of

containerBuilder.RegisterType<DataService>();