How do I Moq IFindFluent so this call to ToListAsync works?

Question

I am unit testing a wrapper to the MongoDB C# driver. I have this line of code:

Collection.Find(predicate).ToListAsync();

Where Collection is of type IMongoCollection<T> and Find(predicate) returns an instance implementing IFindFluent<T, T>. ToListAsync() is an extension to turn the results into a list, I assume.

I am attempting to write unit tests, and I am stumped on handling this. I can't make a wrapper class because that's what I'm working on. I would prefer to either make it so ToListAsync() returns a created list OR to mock the Find() to return something that can be made a list of.

Answer 1

If anyone would struggle with getting it to work, what I did to mock Find() method was:

[TestFixture]
class QueryControllerTests
{
    private IOptions<MongoSettings> _mongoSettings;
    private QueryController _queryController;
    private Mock<IFakeMongoCollection> _fakeMongoCollection;
    private Mock<IFindFluent<BsonDocument, BsonDocument>> _fakeCollectionResult;

    [OneTimeSetUp]
    public void Setup()
    {
        _fakeMongoCollection = new Mock<IFakeMongoCollection>();
        _fakeCollectionResult = new Mock<IFindFluent<BsonDocument, BsonDocument>>();

    }
}

where IFakeMongoCollection is:

public interface IFakeMongoCollection : IMongoCollection<BsonDocument>
{
    IFindFluent<BsonDocument, BsonDocument> Find(FilterDefinition<BsonDocument> filter, FindOptions options);
}

Answer 2

I wound up making a little abstraction layer for this, since I couldn't find anything suitable and asking for help yielded no answers.

I created an interface/implementation pair called AppCollection specifically to handle the MongoDB interface. IAppCollection would have a method for IAppCollection.ToList(predicate), and the AppCollection would run the Collection.Find(predicate).ToListAsync(); call, returning the list. Later, it was a trivial matter to mock the IAppCollection to make sure the right calls were being made. While I couldn't test the predicates in native LINQ, I could at least compile the predicates and compare them to passing/failing objects.