how to unit test asp.net core application with constructor dependency injection

Although @Kritner's answer is correct, I prefer the following for code integrity and better DI experience:

[TestClass]
public class MatchRepositoryTests
{
    private readonly IMatchRepository matchRepository;

    public MatchRepositoryTests()
    {
        var services = new ServiceCollection();
        services.AddTransient<IMatchRepository, MatchRepositoryStub>();

        var serviceProvider = services.BuildServiceProvider();

        matchRepository = serviceProvider.GetService<IMatchRepository>();
    }
}

A simple way, I wrote a generic dependency resolver helper class and then built the IWebHost in my unit test class.

Generic Dependency Resolver

        using Microsoft.AspNetCore.Hosting;
        using Microsoft.AspNetCore.Mvc;
        using Microsoft.Extensions.Configuration;
        using Microsoft.Extensions.DependencyInjection;
        using Microsoft.Extensions.Hosting;
        public class DependencyResolverHelper
        {
            private readonly IWebHost _webHost;
    
            /// <inheritdoc />
            public DependencyResolverHelper(IWebHost webHost) => _webHost = webHost;
    
            public T GetService<T>()
            {
                var serviceScope = _webHost.Services.CreateScope();
                var services = serviceScope.ServiceProvider;
                try
                {
                  var scopedService = services.GetRequiredService<T>();
                  return scopedService;
                }
                catch (Exception e)
                {
                   Console.WriteLine(e);
                   throw;
                }
            }
        }
    }

Unit Test Project:

      [TestFixture]
        public class DependencyResolverTests
        {
            private DependencyResolverHelper _serviceProvider;

            public DependencyResolverTests()
            {

                var webHost = WebHost.CreateDefaultBuilder()
                    .UseStartup<Startup>()
                    .Build();
                _serviceProvider = new DependencyResolverHelper(webHost);
            }
    
            [Test]
            public void Service_Should_Get_Resolved()
            {
                
                //Act
                var YourService = _serviceProvider.GetService<IYourService>();
    
                //Assert
                Assert.IsNotNull(YourService);
            }
    

        }

Your controllers in .net core have dependency injection in mind from the start, but this does not mean you are required to use a dependency injection container.

Given a simpler class like:

public class MyController : Controller
{

    private readonly IMyInterface _myInterface;

    public MyController(IMyInterface myInterface)
    {
        _myInterface = myInterface;
    }

    public JsonResult Get()
    {
        return Json(_myInterface.Get());
    }
}

public interface IMyInterface
{
    IEnumerable<MyObject> Get();
}

public class MyClass : IMyInterface
{
    public IEnumerable<MyObject> Get()
    {
        // implementation
    }
}

So in your app, you're using the dependency injection container in your startup.cs, which does nothing more than provide a concretion of MyClass to use when IMyInterface is encountered. This does not mean it is the only way of getting instances of MyController however.

In a unit testing scenario, you can (and should) provide your own implementation (or mock/stub/fake) of IMyInterface as so:

public class MyTestClass : IMyInterface
{
    public IEnumerable<MyObject> Get()
    {
        List<MyObject> list = new List<MyObject>();
        // populate list
        return list;
    }        
}

and in your test:

[TestClass]
public class MyControllerTests
{

    MyController _systemUnderTest;
    IMyInterface _myInterface;

    [TestInitialize]
    public void Setup()
    {
        _myInterface = new MyTestClass();
        _systemUnderTest = new MyController(_myInterface);
    }

}

So for the scope of unit testing MyController, the actual implementation of IMyInterface does not matter (and should not matter), only the interface itself matters. We have provided a "fake" implementation of IMyInterface through MyTestClass, but you could also do this with a mock like through Moq or RhinoMocks.

Bottom line, you do not actually need the dependency injection container to accomplish your tests, only a separate, controllable, implementation/mock/stub/fake of your tested classes dependencies.

If you are using the Program.cs + Startup.cs convention and want to get this working quickly you can reuse your existing host builder with a one-liner:

using MyWebProjectNamespace;

public class MyTests
{
    readonly IServiceProvider _services = 
        Program.CreateHostBuilder(new string[] { }).Build().Services; // one liner

    [Test]
    public void GetMyTest()
    {
        var myService = _services.GetRequiredService<IMyService>();
        Assert.IsNotNull(myService);
    }
}

Sample Program.cs file from web project:

using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Hosting;

namespace MyWebProjectNamespace
{
    public class Program
    {
        public static void Main(string[] args) =>
            CreateHostBuilder(args).Build().Run();

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.UseStartup<Startup>();
                });
    }
}

You can use asp.net core DI and inject mocked instance objects in your tests. Here is a full working example :

For the sake of the example :

I only kept the IMatchService dependency from the code snippet of the initial question

I added a DoSomething action in the MatchController so that there is someting to test.

I added an Add method to the IMatchService and the MatchService classes so that there is soemthing to mock.

Please note that the methods that will have a Setup with Moq should be virtual.

[Route("api/[controller]")]
public class MatchController : AuthorizedController
{
  private readonly IMatchService _matchService;

  public MatchController(IMatchService matchService)
  {
    _matchService = matchService;
  }

  public virtual int DoSomething()
  {
    return _matchService.Add(1, 2);
  }
}

public interface IMatchService
{
  int Add(int a, int b);
}

public class MatchService : IMatchService
{
  public virtual int Add(int a, int b)
  {
    return a + b;
  }
}

It's always possible to get the Mock by calling the Mock.Get method. For conveniance for each dependency, I create two properties like MatchService and MockedMatchService.

public class MyTests
{
  protected IMatchService MatchService { get; set; }

  protected Mock<IMatchService> MockedMatchService => Mock.Get(MatchService);

  private IServiceProvider ServicesProvider { get; set; }

  [SetUp]
  public void SetupBeforeEachTest()
  {
    // Configure DI container
    ServiceCollection services = new ServiceCollection();
    ConfigureServices(services);
    ServicesProvider = services.BuildServiceProvider();

    // Use DI to get instances of IMatchService
    MatchService = ServicesProvider.GetService<IMatchService>();
  }

  // In this test I mock the Add method of the dependency (IMatchService) so that it returns a value I choose
  [Test]
  public void TestMethod()
  {
    // Prepare
    var matchController = ServicesProvider.GetService<MatchController>();
    int expectedResult = 5;
    MockedMatchService.Setup(x => x.Add(It.IsAny<int>(), It.IsAny<int>())).Returns(expectedResult);

    // Act - This will call the real DoSomething method because the MatchController has comes from a Mock with CallBase = true
    int result = matchController.DoSomething();

    // Check
    Assert.AreEqual(expectedResult, result);
  }

  private static void ConfigureServices(IServiceCollection services)
  {
    services.AddScoped<IMatchService>();
    services.AddScoped<MatchController>();
  }
}

I worked over @madjack and @Kritner's answers and made my

Basic Inheritable Base Test Class for Dependency Injection

Just register your services inside of it and inherite.

public class BaseTester 
{
    protected IProductService _productService; 
    protected IEmployeeService _employeeService; 

    public BaseTester()
    {
        var services = new ServiceCollection();

        services.AddTransient<IProductService, ProductService>();
        services.AddTransient<IEmployeeService, EmployeeService>();

        var serviceProvider = services.BuildServiceProvider();

        _productService = serviceProvider.GetService<IProductService>();
        _employeeService = serviceProvider.GetService<IEmployeeService>();
    }
}

Why would you want to inject those in a test class? You would usually test the MatchController, for example, by using a tool like RhinoMocks to create stubs or mocks. Here's an example using that and MSTest, from which you can extrapolate:

[TestClass]
public class MatchControllerTests
{
    private readonly MatchController _sut;
    private readonly IMatchService _matchService;

    public MatchControllerTests()
    {
        _matchService = MockRepository.GenerateMock<IMatchService>();
        _sut = new ProductController(_matchService);
    }

    [TestMethod]
    public void DoSomething_WithCertainParameters_ShouldDoSomething()
    {
        _matchService
               .Expect(x => x.GetMatches(Arg<string>.Is.Anything))
               .Return(new []{new Match()});

        _sut.DoSomething();

        _matchService.AssertWasCalled(x => x.GetMatches(Arg<string>.Is.Anything);
    }

Improved solution

I improved madjack's solution by wrapping it in single abstract class and adding four methods (including two async equivalents) with callbacks as parameters. GetRequiredScopedService<TSvc>() is using private static property services for caching now, so derived classes don't create new instances over and over. Another optimization is making host static, so we don't build it every time in derived classes. I also removed pointless try/catch:

    public abstract class TestWithDependencyInjection
    {
        private static readonly IHost host =
            Program.CreateHostBuilder(Constants.CommandArgs).Build();
        private static readonly IList<object> services =
            new List<object>();

        private IServiceScope svcScope;

        protected async Task<TResult> UseSvcAsync<TSvc, TResult>(
            Func<TSvc, Task<TResult>> callback, 
            bool shouldBeDisposed = true)
        {
            var scopedSvc = GetRequiredScopedService<TSvc>();
            TResult result = await callback(scopedSvc);
            if(shouldBeDisposed) 
                svcScope.Dispose();
            return result;
        }

        protected async Task UseSvcAsync<TSvc>(
            Func<TSvc, Task> callback)
        {
            var scopedSvc = GetRequiredScopedService<TSvc>();
            await callback(scopedSvc);
            svcScope.Dispose();
        }

        protected TResult UseSvc<TSvc, TResult>(
            Func<TSvc, TResult> callback, bool shouldBeDisposed = true)
        {
            var scopedSvc = GetRequiredScopedService<TSvc>();
            TResult result = callback(scopedSvc);
            if(shouldBeDisposed)
                svcScope.Dispose();
            return result;
        }

        protected void UseSvc<TSvc>(Action<TSvc> callback)
        {
            var scopedSvc = GetRequiredScopedService<TSvc>();
            callback(scopedSvc);
            svcScope.Dispose();
        }

        private TSvc GetRequiredScopedService<TSvc>()
        {
            var requiredScopedSvc = (TSvc)services.SingleOrDefault(
                svc => svc is TSvc);
            if (requiredScopedSvc != null)
                return requiredScopedSvc;
            svcScope = host.Services.CreateScope();
            requiredScopedSvc = svcScope.ServiceProvider
                .GetRequiredService<TSvc>();
            services.Add(requiredScopedSvc);
            return requiredScopedSvc;
        }
    }

Example of returning async result from used injected service:

            int foobarsCount = await UseSvcAsync<IFoobarSvc, int>(
                    foobarSvc => foobarSvc.GetCountAsync());

Additional information

I added optional shouldBeDisposed argument set on true to methods returning TResult and Task<TResult> in case, when you want to use same instance of service outside of callback's body:

            IFoobarSvc foobarSvc = UseSvc<IFoobarSvc, IFoobarSvc>(
                    foobarSvc => foobarSvc, false);