I’m encountering a NullReferenceException in the SetInterval() method of my ApiWorker class when running a unit test. Below are the relevant parts of my code.

public ApiWorkerTests()
{
    _mockLogger = new Mock<ILogger<ApiWorker>>();
    _mockDbService = new Mock<IDatabaseService>();
    _mockApiService = new Mock<IApiService>();
    _mockConfig = new Mock<IConfiguration>();

    _mockConfig.Setup(c => c[$"Authentication:CompanyKey"]).Returns("SomeKey");
    _mockConfig.Setup(c => c[$"Authentication:VehicleKey"]).Returns("SomeKey");
    _mockConfig.Setup(c => c["Interval"]).Returns("00:00:10");

    _apiWorker = new ApiWorker(_mockApiService.Object, _mockLogger.Object, _mockConfig.Object, _mockDbService.Object);
}

[Fact]
public void ApiWorker_Initialization()
{
    // Arrange & Act
    var worker = new ApiWorker(_mockApiService.Object, _mockLogger.Object, _mockConfig.Object, _mockDbService.Object);

    // Assert
    Assert.NotNull(worker);
}

And here’s the ApiWorker:

 public ApiWorker(IApiService apiService, ILogger<ApiWorker> logger, IConfiguration config, IDatabaseService db)
 {
     _apiService = apiService;
     _logger = logger;
     _db = db;
     _companyKey = config[$"Authentication:CompanyKey"] ?? throw new Exception("Company ID cannot be null");
     _vehicleKey = config[$"Authentication:VehicleKey"] ?? throw new Exception("Vehicle ID cannot be null");
     _config = config; 

     SetInterval();
 }

private void SetInterval()
{
    var intervalString = _config.GetValue<string>("Interval");
    
    // Conversion to DateTime     
}

Hard-coding the intervalString to “00:00:10” makes the test pass, but I want to understand why the mock setup isn’t working as expected. The setup of the IConfiguration mock seems to be the issue.