public async Task AppendAsync(string streamId, DomainEvent @event)
        {
            Argument.IsNotNull(@event, nameof(@event));

            if (!_streams.ContainsKey(streamId))
            {
                _streams[streamId] = new List<DomainEvent>();
            }
            _streams[streamId].Add(@event);
            await MockAsync();
        }
 public Task AppendAsync(string streamId, DomainEvent @event)
 {
     throw new NotImplementedException();
 }
 public InvalidEventException(string message, DomainEvent @event)
     : base(message)
 {
     Event = @event;
 }