Pipeline.cs

public class Pipeline<TRequest, TResponse> 
    : ICancellableAsyncRequestHandler<TRequest, TResponse>
        where TRequest : ICancellableAsyncRequest<TResponse>
{
    private readonly ICancellableAsyncRequestHandler<TRequest, TResponse> _inner;
    private readonly IPostRequestHandler<TRequest, TResponse>[] _postHandlers;

    public Pipeline(
        ICancellableAsyncRequestHandler<TRequest, TResponse> inner,
        IPostRequestHandler<TRequest, TResponse>[] postHandlers
        )
    {
        _inner = inner;
        _postHandlers = postHandlers;
    }

    public async Task<TResponse> Handle(TRequest message, CancellationToken cancellationToken)
    {
        var response = await _inner.Handle(message, cancellationToken);

        foreach (var postHandler in _postHandlers)
        {
             postHandler.Handle(message, response);
        }
        return response;
    }
}