.NET Core Unit Testing - Mock IOptions<T>

You need to manually create and populate an IOptions<SampleOptions> object. You can do so via the Microsoft.Extensions.Options.Options helper class. For example:

IOptions<SampleOptions> someOptions = Options.Create<SampleOptions>(new SampleOptions());

You can simplify that a bit to:

var someOptions = Options.Create(new SampleOptions());

Obviously this isn't very useful as is. You'll need to actually create and populate a SampleOptions object and pass that into the Create method.

If you intent to use the Mocking Framework as indicated by @TSeng in the comment, you need to add the following dependency in your project.json file.

   "Moq": "4.6.38-alpha",

Once the dependency is restored, using the MOQ framework is as simple as creating an instance of the SampleOptions class and then as mentioned assign it to the Value.

Here is a code outline how it would look.

SampleOptions app = new SampleOptions(){Title="New Website Title Mocked"}; // Sample property
// Make sure you include using Moq;
var mock = new Mock<IOptions<SampleOptions>>();
// We need to set the Value of IOptions to be the SampleOptions Class
mock.Setup(ap => ap.Value).Returns(app);

Once the mock is setup, you can now pass the mock object to the contructor as

SampleRepo sr = new SampleRepo(mock.Object);   

HTH.

FYI I have a git repository that outlines these 2 approaches on Github/patvin80

You can avoid using MOQ at all. Use in your tests .json configuration file. One file for many test class files. It will be fine to use ConfigurationBuilder in this case.

Example of appsetting.json

{
    "someService" {
        "someProp": "someValue
    }
}

Example of settings mapping class:

public class SomeServiceConfiguration
{
     public string SomeProp { get; set; }
}

Example of service which is needed to test:

public class SomeService
{
    public SomeService(IOptions<SomeServiceConfiguration> config)
    {
        _config = config ?? throw new ArgumentNullException(nameof(_config));
    }
}

NUnit test class:

[TestFixture]
public class SomeServiceTests
{

    private IOptions<SomeServiceConfiguration> _config;
    private SomeService _service;

    [OneTimeSetUp]
    public void GlobalPrepare()
    {
         var configuration = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json", false)
            .Build();

        _config = Options.Create(configuration.GetSection("someService").Get<SomeServiceConfiguration>());
    }

    [SetUp]
    public void PerTestPrepare()
    {
        _service = new SomeService(_config);
    }
}

You can always create your options via Options.Create() and than simply use AutoMocker.Use(options) before actually creating the mocked instance of the repository you're testing. Using AutoMocker.CreateInstance<>() makes it easier to create instances without manually passing parameters

I've changed you're SampleRepo a bit in order to be able to reproduce the behavior I think you want to achieve.

public class SampleRepoTests
{
    private readonly AutoMocker _mocker = new AutoMocker();
    private readonly ISampleRepo _sampleRepo;

    private readonly IOptions<SampleOptions> _options = Options.Create(new SampleOptions()
        {FirstSetting = "firstSetting"});

    public SampleRepoTests()
    {
        _mocker.Use(_options);
        _sampleRepo = _mocker.CreateInstance<SampleRepo>();
    }

    [Fact]
    public void Test_Options_Injected()
    {
        var firstSetting = _sampleRepo.GetFirstSetting();
        Assert.True(firstSetting == "firstSetting");
    }
}

public class SampleRepo : ISampleRepo
{
    private SampleOptions _options;

    public SampleRepo(IOptions<SampleOptions> options)
    {
        _options = options.Value;
    }

    public string GetFirstSetting()
    {
        return _options.FirstSetting;
    }
}

public interface ISampleRepo
{
    string GetFirstSetting();
}

public class SampleOptions
{
    public string FirstSetting { get; set; }
}

Given class Person that depends on PersonSettings as follows:

public class PersonSettings
{
    public string Name;
}

public class Person
{
    PersonSettings _settings;

    public Person(IOptions<PersonSettings> settings)
    {
        _settings = settings.Value;
    }

    public string Name => _settings.Name;
}

IOptions<PersonSettings> can be mocked and Person can be tested as follows:

[TestFixture]
public class Test
{
    ServiceProvider _provider;

    [OneTimeSetUp]
    public void Setup()
    {
        var services = new ServiceCollection();
        // mock PersonSettings
        services.AddTransient<IOptions<PersonSettings>>(
            provider => Options.Create<PersonSettings>(new PersonSettings
            {
                Name = "Matt"
            }));
        _provider = services.BuildServiceProvider();
    }

    [Test]
    public void TestName()
    {
        IOptions<PersonSettings> options = _provider.GetService<IOptions<PersonSettings>>();
        Assert.IsNotNull(options, "options could not be created");

        Person person = new Person(options);
        Assert.IsTrue(person.Name == "Matt", "person is not Matt");    
    }
}

To inject IOptions<PersonSettings> into Person instead of passing it explicitly to the ctor, use this code:

[TestFixture]
public class Test
{
    ServiceProvider _provider;

    [OneTimeSetUp]
    public void Setup()
    {
        var services = new ServiceCollection();
        services.AddTransient<IOptions<PersonSettings>>(
            provider => Options.Create<PersonSettings>(new PersonSettings
            {
                Name = "Matt"
            }));
        services.AddTransient<Person>();
        _provider = services.BuildServiceProvider();
    }

    [Test]
    public void TestName()
    {
        Person person = _provider.GetService<Person>();
        Assert.IsNotNull(person, "person could not be created");

        Assert.IsTrue(person.Name == "Matt", "person is not Matt");
    }
}