Unit test of a mapping when members are hidden

I have to test a projection that handle a given events and persist it on a read only Mongo database (I'm using official c# Mongo Driver):

public class MyObjectProjection : IHandleMessages<RegisteredEvent>
{
    private MongoCollection<MyObjectView> _collection;

    public MyObjectProjection (MongoDatabase db)
    {
        _collection = db.GetCollection<MyObjectView>("my-object");
    }

    public void Handle(RegisteredEvent message)
    {
        var item = new MyObjectView();
        item.Id = message.Id;

        // some code omitted                

        _collection.Save(item);
    }
}

I need to unit test the Handle method, given that:

• I don't want an integration test. Database and collection are mocked, so the save is not real
• I just want to test the item-message mapping
• the members are hidden and I don't want to make them more visibile

Should I use alternative solution rather than reflection? What is the best practice in a case like this? Right now my test look like this:

    [TestMethod]
    public void TestMethod1()
    {
        // ARRANGE - some code omitted
        databaseMock
            .Setup(x => x.GetCollection<MyObjectView>(It.IsAny<string>()))
            .Returns(collection);

        collectionMock
            .Setup(x => x.Save(It.IsAny<MyObjectView>()))
            .Returns(It.IsAny<WriteConcernResult>);

        // ACT
        var handler = new MyObjectProjection(database);
        handler.Handle(evt);

        // ASSERT
        // nothin' to assert here!
    }

this works, but i have nothing to assert when Handle method is completed.