ASP.NET Core Dependency Injection error: Unable to resolve service for type while attempting to activate

kimbaudi picture kimbaudi · Dec 1, 2016 · Viewed 349.7k times · Source

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

DavidG picture DavidG · Dec 1, 2016

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;
}