messengerapi/code/MessengerApi/Handlers/Endpoint/AckEndpointHandler.cs

using MessengerApi.Contracts.Models.Scoped;
using MessengerApi.Models.Scoped;

namespace MessengerApi.Handlers.Endpoint
{
    public class AckEndpointHandler
    {
        private readonly ILogger logger;
        private readonly IUnitOfWork unitOfWork;
        private readonly Identity identity;

        public AckEndpointHandler(
            ILogger logger,
            IUnitOfWork unitOfWork,
            Identity identity)
        {
            this.logger = logger;
            this.unitOfWork = unitOfWork;
            this.identity = identity;
        }

        public async Task AckMessage(Guid messageId)
        {
            var message = unitOfWork.Messages.GetById(messageId);
            
            // Authorize.
            if (message.ToId != this.identity.User.Id)
            {
                throw new InvalidOperationException("It's not your message to ack.");
            }
            else if(!message.IsDelivered)
            {
                throw new InvalidOperationException("Can't ack undelivered message.");
            }

            // Act.
            message.IsAcknowledged = true;
        }
    }
}