Unable to resolve service for type while attempting to activate

In my ASP.NET Core application, I get the following error:

> InvalidOperationException: Unable to resolve service for type 'Cities.Models.IRepository' while attempting to activate 'Cities.Controllers.HomeController'.

I the HomeController I am trying to pass the Cities getter to the view like so:

public class HomeController : Controller
{
    private readonly IRepository repository;

    public HomeController(IRepository repo) => repository = repo;

    public IActionResult Index() => View(repository.Cities);
}

I have one file Repository.cs that contains an interface and its implementation like so:

public interface IRepository
{
    IEnumerable<City> Cities { get; }
    void AddCity(City newCity);
}

public class MemoryRepository : IRepository
{
    private readonly List<City> cities = new List<City>();

    public IEnumerable<City> Cities => cities;

    public void AddCity(City newCity) => cities.Add(newCity);
}

My Startup class contains the default-generated code from the template. I have made any changes:

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

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

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        ...
    }
}

For the Dependency Injection framework to resolve IRepository, it must first be registered with the container. For example, in ConfigureServices, add the following:

services.AddScoped<IRepository, MemoryRepository>();

For .NET 6+, which uses the new hosting model by default, add the following in Program.cs instead:

builder.Services.AddScoped<IRepository, MemoryRepository>();

AddScoped is just one example of a service lifetime:

> For web applications, a scoped lifetime indicates that services are created once per client request (connection).

See the docs for more information on Dependency Injection in ASP.NET Core.

We are getting this error in Entity frame work core database first approach. I followed below steps and error got resolved

Step 1: Check Your context class constructor should be like this

public partial class ZPHSContext : DbContext
{
    public ZPHSContext(DbContextOptions<ZPHSContext> dbContextOptions)
        : base(dbContextOptions)
    {
    }
}
    

Step 2: In Startup file

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();
    services.AddDbContext<ZPHSContext>(options =>
        options.UseSqlServer(
            Configuration.GetConnectionString("BloggingDatabase")));
}
    

Step 3: Connection string in appsettings

"ConnectionStrings": {
    "BloggingDatabase": "Server=****;Database=ZPHSS;Trusted_Connection=True;"
}

Step 4: Remove default code in OnConfiguring method in context class

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
}

Other answers are CORRECT, however I was spinning up a new asp.net core 2.1.x project and got this error.

Ended up being a typo by ME.

So in my Controller instead of Correctly using the Interface like this

public HomeController(IApplicationRepository applicationRepository)
{
    _applicationRepository = applicationRepository;
}

My typo had me using ApplicationRepository instead of its interface IApplicationRepository Notice below, and so with NO ERRORS spotting the missing "I" was fun :/

public HomeController(IApplicationRepository applicationRepository)
{
    _applicationRepository = applicationRepository;
}

Thus the controller was not resolving the DI...

A method like this needs to be added to your Startup:

    // This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
    //...

    // Add application services.
    services.AddTransient<IRepository, MemoryRepository>();

    //..
}

Services should be registered before used.

UPDATE: If you do not want to use DI on your application, just create and instance of MemoryRepository on the constructor of HomeController, like this:

public class HomeController : Controller
    {
        private IRepository repository;

        public HomeController()
        {
            repository = new MemoryRepository();
        }

        public IActionResult Index()
        {
            return View(repository.Cities);
        }

    }

You have to add your implementation to DI (Dependeny Injection) section. For .Net Core Mvc, it would be like this:

 public void ConfigureServices(IServiceCollection services)
 {
   services.AddDbContext<ApplicationDbContext>(options =>
    options.UseInMemoryDatabase()
   );
   services.AddScoped<IRepository, MemoRepostory>();

 }

This may not be helpful for your code sample but in my case the same error was a result of a circular dependency.

you have to register your repository like this

services.AddSingleton<IRepository, MemoryRepository>();

In my case, I was trying to access context through constructor. like here;

 private readonly Context _context;

 public ImageController(Context context)
 {
    _context = context;
 }

But When I tried to access the context just by creating an instance of class, it worked like here;

 Context c = new Context();