MongoDB C# driver - Serialize collection to interface

Question

Due to environment restrictions at work I'm implementing a rough Event Sourcing store in MongoDB. I'm trying to get a list of IClientEvents from Mongo like so:

 var events = await _db.GetCollection<IClientEvent>("ClientEvents").FindAsync(c => c.ClientId == clientId);

I get the following exception when I run the above mentioned repository method:

Message: System.InvalidOperationException : {document}.ClientId is not supported.

The IClientEvent interface is defined as:

public interface IClientEvent
{
    Guid Id { get; set; }
    long TimeStamp { get; set; }
    Guid ClientId { get; set; }
}

public class ClientChangedEvent : IClientEvent
{
    public Guid Id { get; set; }
    public long TimeStamp { get; set; }
    public Guid ClientId { get; set; }

    public IEnumerable<Change> Changes;
    // ... other properties for the event
}

There will be many different event types stored into a single collection, all of which will implement IClientEvent. I want to just get, in a single call, all events that have occurred to a Client by clientId.

I have registered all of the concrete implementations of IClientEvent and even added a custom discriminator:

        var clientEventsDiscriminator = new ClientEventsMongoDiscriminatorConvention();
        BsonSerializer.RegisterDiscriminatorConvention(typeof(IClientEvent),clientEventsDiscriminator);
        BsonClassMap.RegisterClassMap<ClientChangedEvent>();
        BsonSerializer.RegisterDiscriminatorConvention(typeof(ClientChangedEvent), clientEventsDiscriminator);

I have even tried registering an ImpliedImplementationInterfaceSerializer as mentioned in this SO post but it throws an exception when I register the 2nd concrete implementation that I have already registered a serializer for IClientEvent.

Not sure where to go from here. Any help is greatly appreciated!

-- EDIT for more code:

Here is the full registration code:

        var clientEventsDiscriminator = new ClientEventsMongoDiscriminatorConvention();
        BsonSerializer.RegisterDiscriminatorConvention(typeof(IClientEvent),clientEventsDiscriminator);

        clientEventsDiscriminator.AddEventType<ClientChangedEvent>();
        BsonClassMap.RegisterClassMap<ClientChangedEvent>();
        BsonSerializer.RegisterDiscriminatorConvention(typeof(ClientChangedEvent), clientEventsDiscriminator);

        clientEventsDiscriminator.AddEventType<ClientAddedEvent>();
        BsonClassMap.RegisterClassMap<ClientAddedEvent>();
        BsonSerializer.RegisterDiscriminatorConvention(typeof(ClientAddedEvent), clientEventsDiscriminator);

Here is the Discriminator:

    public class ClientEventsMongoDiscriminatorConvention : IDiscriminatorConvention
{
    private Dictionary<string, Type> _eventTypes = new Dictionary<string, Type>();

    public string ElementName => "_eventType";

    public BsonValue GetDiscriminator(Type nominalType, Type actualType)
    {
        return GetDiscriminatorValueForEventType(actualType);
    }

    public Type GetActualType(IBsonReader bsonReader, Type nominalType)
    {
        var bookmark = bsonReader.GetBookmark();
        bsonReader.ReadStartDocument();
        if (!bsonReader.FindElement(ElementName))
        {
            throw new InvalidCastException($"Unable to find property '{ElementName}' in document. Cannot map to an EventType.");
        }

        var value = bsonReader.ReadString();
        bsonReader.ReturnToBookmark(bookmark);

        if (_eventTypes.TryGetValue(value, out var type))
        {
            return type;
        }

        throw new InvalidCastException($"The type '{value}' has not been registered with the '{nameof(ClientEventsMongoDiscriminatorConvention)}'.");
    }

    private string GetDiscriminatorValueForEventType(Type type)
    {
        var indexOfEventWord = type.Name.IndexOf("Event");
        if (indexOfEventWord == -1)
        {
            return type.Name;
        }
        return type.Name.Substring(0, indexOfEventWord);
    }

    public void AddEventType<T>()
    {
        var discriminatorName = GetDiscriminatorValueForEventType(typeof(T));
        _eventTypes.TryAdd(discriminatorName, typeof(T));
    }
}

When running the code it doesn't appear to ever hit the GetActualType method of the discriminator.

Answer 1

I managed to get it to work by simply changing IClientEvent from an interface to an abstract class.