How to Mock IMongoDatabase

c#, mongodb, moq, unit-testing

Solution

Things are a little tricky here.

Some background first.

According to documentation, `IMongoDatabase.RunCommandAsync<TResult>` is defined as

Task<TResult> RunCommandAsync<TResult>(
    Command<TResult> command,
    ReadPreference readPreference = null,
    CancellationToken cancellationToken = null
)

Note the `Command<TResult>`, while in your code you pass a `BsonDocument`.

Luckily `BsonDocument` has an implicit conversion operator from `BsonDocument` to `Command<TResult>`

When a setup does not get what was configured it tends to return null. So you need to make sure that it is setup properly so that it performs the expected behavior.

[TestClass]
public class UnitTest1 {
    [TestMethod]
    public async Task _IsConnectionOk_xxx_RunPing1Command() {
        var dbMock = new Mock<IMongoDatabase>();
        var resultCommand = new BsonDocument("ok", 1);
        dbMock
            .Setup(stub => stub.RunCommandAsync<BsonDocument>(It.IsAny<Command<BsonDocument>>(), It.IsAny<ReadPreference>(), It.IsAny<CancellationToken>()))
            .ReturnsAsync(resultCommand)
            .Verifiable();

        var mongoClientMock = new Mock<IMongoClient>();
        mongoClientMock
            .Setup(stub => stub.GetDatabase(It.IsAny<string>(), It.IsAny<MongoDatabaseSettings>()))
            .Returns(dbMock.Object);

        var client = new Client(mongoClientMock.Object);
        var pingCommand = new BsonDocument("ping", 1);

        //act
        var actual = await client.IsConectionOk();

        //assert
        Assert.IsTrue(actual);
        dbMock.Verify();
    }
}

Problem

I'm using Moq for mocking my objects in an ASP.NET Core project. I want to mock the following IsConnection() method: ``` public Client(IMongoClient client) { _client = client; } public async Task<bool> IsConectionOk() { var pingCommand = new BsonDocument("ping", 1); var mongoDb = _client.GetDatabase("Name"); var commandResult = await mongoDb.RunCommandAsync<BsonDocument>(pingCommand); return commandResult != null; } ``` As you see, there's only one injection, `IMongoClient`, so I need to mock this one. Now, I need to mock `IMongoDatabase` as well since the `_client.GetDatabase` returns me an `IMongoDatabase` which runs `RunCommandAsync` this is my unit test: ``` [Fact] public async Task IsConnectionOk_xxx_RunPing1Command() { var dbMock = new Mock<IMongoDatabase>(); var resultCommand = new BsonDocument("ok", 1); dbMock.Setup(stub => stub.RunCommandAsync<BsonDocument>(It.IsAny<BsonDocument>(), It.IsAny<ReadPreference>(), It.IsAny<CancellationToken>())).ReturnsAsync(resultCommand); var mongoClientMock = new Mock<IMongoClient>(); mongoClientMock.Setup(stub => stub.GetDatabase(It.IsAny<string>(), It.IsAny<MongoDatabaseSettings>())).Returns(dbMock.Object); var client = new Client(mongoClientMock.Object); var pingCommand = new BsonDocument("ping", 1); //act await client.IsConectionOk(); //assert dbMock.Verify(mock => mock.RunCommandAsync<BsonDocument>( It.Is<BsonDocument>(x => x.Equals(pingCommand)) , It.IsAny<ReadPreference>() ,It.IsAny<CancellationToken>())); } ``` You can see that I mocked a `IMongoDatabase` so my `mongoClientMock` can return it when the code is executing. When code is running, I have checked that `mongoClientMock.GetDatabase()` is returning a `MongoDatabase` (good until there), the problem is that when `MongoDatabaseMock` calls RunCommandAsync is not returning what I set up, it just returns null. I don't know what I could be missing here, any thoughts?

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