Mongodb c# Unit testing: Object reference not set to an instance of an object

Question

I'm trying to unit test a data repository for my Workorders.

The repository takes a DatabaseContext instance which accesses the mongodb database and exposes the IMongoCollection for the workorders as you will see posted below.

I use Moq to make mocks of both the IMongoCollection and DatabaseContext. In the Moq setup for the DatabaseContext i choose to return the IMongoCollection when the exposed Workorder property is called. And in turn, the mocked IMongoCollection returns a DeleteResult.Acknowledged(1) task when DeleteOneAsync is called.

In this way i hoped for some unit testing without needing the database running, but i got stuck with an error telling me "System.NullReferenceException : Object reference not set to an instance of an object.", pointing at a row when i am trying to do "return result.DeleteCount == 1, in which result is the returned DeleteResult object.

It seems like my result isn't created properly, below is my code.

The error itself Occurred when running the tests

Test Name:  MESAPITests.RepositoryTests.WorkorderRepositoryTests.DeleteWorkorderById_ReturnsBooleanTrue
Test FullName: MESAPITests.RepositoryTests.WorkorderRepositoryTests.DeleteWorkorderById_ReturnsBooleanTrue
Test Source:    C:\Users\Zacke\Documents\Repositories\MES-API\MES-API Tests\RepositoryTests\WorkorderRepositoryTests.cs : line 22
Test Outcome:   Failed
Test Duration:  0:00:00.354

Result StackTrace:  
at MESAPI.Repositories.WorkorderRepository.<DeleteWorkorderById>d__2.MoveNext() in C:\Users\Zacke\Documents\Repositories\MES-API\MES-API\Repositories\WorkRepositories\WorkorderRepository.cs:line 28
--- End of stack trace from previous location where exception was thrown ---    
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at MESAPITests.RepositoryTests.WorkorderRepositoryTests.<DeleteWorkorderById_ReturnsBooleanTrue>d__1.MoveNext() in C:\Users\Zacke\Documents\Repositories\MES-API\MES-API Tests\RepositoryTests\WorkorderRepositoryTests.cs:line 34
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
Result Message: System.NullReferenceException : Object reference not set to an instance of an object.

DataRepositoryTests

public class WorkorderRepositoryTests
{
    private WorkorderRepository _repo;

    [Fact]
    public async void DeleteWorkorderById_ReturnsBooleanTrue()
    {
        var mockCollection = new Mock<IMongoCollection<Workorder>>();
        mockCollection
            .Setup(_ => _.DeleteOneAsync(It.IsAny<Expression<Func<Workorder, bool>>>(),
                default(CancellationToken)))
            .ReturnsAsync(await Task.Run<DeleteResult>(() => new DeleteResult.Acknowledged(1)));

        var mockContext = new Mock<IDatabaseContext>();
        mockContext.Setup(_ => _.Workorders).Returns(mockCollection.Object);

        _repo = new WorkorderRepository(mockContext.Object);
        var id = ObjectId.GenerateNewId().ToString();
        var result = await _repo.DeleteWorkorderById(id);
        Assert.True(result);
    }
}

WorkorderRepository Error occurred on second line in DeleteWorkorderById

public class WorkorderRepository : IWorkorderRepository
{
    private readonly IMongoCollection<Workorder> _workorders;

    public WorkorderRepository(IDatabaseContext context)
    {
        _workorders = context.Workorders;
    }

    public async Task<bool> DeleteWorkorderById(string id)
    {
        var result = await _workorders.DeleteOneAsync(w => w.Id == id);
        return result.DeletedCount == 1;
    }

    public async Task<List<Workorder>> GetAllWorkordersAsList()
    {
        return await _workorders.FindAsync(new BsonDocument()).Result.ToListAsync();
    }

    public async Task<Workorder> GetWorkorderById(string id)
    {
        return await _workorders.FindAsync(w => w.Id == id).Result.FirstOrDefaultAsync();
    }

    public async Task<Workorder> PostNewWorkroder(WorkorderPost workorderPost)
    {
        var newWorkorder = new Workorder(workorderPost);
        await _workorders.InsertOneAsync(newWorkorder);
        return await _workorders.FindAsync(w => w.Id == newWorkorder.Id).Result.FirstOrDefaultAsync();
    }

    public async Task<bool> UpdateWorkorder(Workorder workorder)
    {
        var result = await _workorders.ReplaceOneAsync(w => w.Id == workorder.Id, workorder);
        return result.MatchedCount != 0;
    }
}

IDatabaseRepository

public interface IDatabaseContext
{
    /// <summary>
    /// Mongo database context.
    /// </summary>
    IMongoDatabase Database { get; set; }

    /// <summary>
    /// BomFamily database context.
    /// </summary>
    IMongoCollection<BomFamily> BomFamilies { get; set; }

    /// <summary>
    /// BomGroup database context.
    /// </summary>
    IMongoCollection<BomGroup> BomGroups { get; set; }

    /// <summary>
    /// BomItem database context.
    /// </summary>
    IMongoCollection<BomItem> BomItems { get; set; }

    /// <summary>
    /// Event database context.
    /// </summary>
    IMongoCollection<Event> Events { get; set; }

    /// <summary>
    /// EventAttribute database context.
    /// </summary>
    IMongoCollection<EventAttribute> EventAttributes { get; set; }

    /// <summary>
    /// EventType database context.
    /// </summary>
    IMongoCollection<EventType> EventTypes { get; set; }

    /// <summary>
    /// Job database context.
    /// </summary>
    IMongoCollection<Job> Jobs { get; set; }

    /// <summary>
    /// Product database context.
    /// </summary>
    IMongoCollection<Product> Products { get; set; }

    /// <summary>
    /// QualityEvent database context.
    /// </summary>
    IMongoCollection<QualityEvent> QualityEvents { get; set; }

    /// <summary>
    /// QualityTest database context.
    /// </summary>
    IMongoCollection<QualityTest> QualityTests { get; set; }

    /// <summary>
    /// QualityVariable database context.
    /// </summary>
    IMongoCollection<QualityVariable> QualityVariables { get; set; }

    /// <summary>
    /// Status database context.
    /// </summary>
    IMongoCollection<Status> Statuses { get; set; }

    /// <summary>
    /// StatusGroup database context.
    /// </summary>
    IMongoCollection<StatusGroup> StatusGroups { get; set; }

    /// <summary>
    /// User database context.
    /// </summary>
    IMongoCollection<User> Users { get; set; }

    /// <summary>
    /// WorkArea database context.
    /// </summary>
    IMongoCollection<WorkArea> WorkAreas { get; set; }

    /// <summary>
    /// WorkCell database context.
    /// </summary>
    IMongoCollection<WorkCell> WorkCells { get; set; }

    /// <summary>
    /// Workorder database context.
    /// </summary>
    IMongoCollection<Workorder> Workorders { get; set; }
}

New error when changing input type on DeleteOneAsync To It.IsAny<FilterDefinition<Workorder>>()

Test Name:  MESAPITests.RepositoryTests.WorkorderRepositoryTests.DeleteWorkorderById_ReturnsBooleanTrue
Test FullName:  MESAPITests.RepositoryTests.WorkorderRepositoryTests.DeleteWorkorderById_ReturnsBooleanTrue
Test Source: C:\Users\Zacke\Documents\Repositories\MES-API\MES-API     Tests\RepositoryTests\WorkorderRepositoryTests.cs : line 21
Test Outcome:   Failed
Test Duration:  0:00:00.283

Result StackTrace:  
at MESAPI.Repositories.WorkorderRepository.<DeleteWorkorderById>d__2.MoveNext() in C:\Users\Zacke\Documents\Repositories\MES-API\MES-API\Repositories\WorkRepositories\WorkorderRepository.cs:line 28
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at MESAPITests.RepositoryTests.WorkorderRepositoryTests.<DeleteWorkorderById_ReturnsBooleanTrue>d__1.MoveNext() in C:\Users\Zacke\Documents\Repositories\MES-API\MES-API Tests\RepositoryTests\WorkorderRepositoryTests.cs:line 33
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
Result Message: System.NullReferenceException : Object reference not set to an instance of an object.

Picture: Hovering over result in the line above tells me it's null for some reason..

Answer 1

Update based on comments. Credit to StuartLC

Moq is unable to mock extension methods and should be done directly on members of the interface being mocked.

IMongoCollection<TDocument>.DeleteOneAsync Method (FilterDefinition<TDocument>, CancellationToken) Method

mockCollection
    .Setup(_ => _.DeleteOneAsync(It.IsAny<FilterDefinition<Workorder>>(), It.IsAny<CancellationToken>()))
    .ReturnsAsync(new DeleteResult.Acknowledged(1));