Ehsan Ghanbari

Experience, DotNet, Solutions

Simple command handler in .net

 

I want to introduce a simple command handler pattern in order to separate commands and queries of your applications if it’s needed. Sometimes especially in large-scale applications, you need to have two layers for reading and write separated and assign two teams to work on them.

You can use the pattern in this post based on the complexity and needs of your application. I recommend that don’t call this CQRS pattern as it’s something completely different and more complex.

 

Get started and Create an interface with no any member:

public interface ICommand

    {

    }

You will see later in this blog post that ICommand is just a flag to determine the commands for the handler. Every command which is trying to make a change in application’s state should return a result. so create another interface named ICommandResult

public interface ICommandResult

    {

        string Message { get; }

        bool Success { get; }

    }

implement the above interface as CommandResult and derive the FailureResult and SuccessResult classes from:

internal abstract class CommandResult : ICommandResult

    {

        public string Message { get; protected set; }

        public bool Success { get; protected set; }

    }



internal class FailureResult : CommandResult

    {

        public FailureResult(string message)

        {

            Success = false;

            Message = message;

        }

    }



    internal class SuccessResult : CommandResult

    {

        public SuccessResult(string message)

        {

            Success = true;

            Message = message;

        }

    }

Now create an interface for handling the validations in CommandBus class:

internal interface IValidationHandler<in TCommand> where TCommand : ICommand

    {

        IEnumerable<ValidationResult> Validate(TCommand command);

    }

 And create a class for validations:

public class ValidationResult

    {

        /// <summary>

        /// Initializes a new instance of the <see cref="ValidationResult"/> class.

        /// </summary>

        public ValidationResult()

        {

        }



        /// <summary>

        /// Initializes a new instance of the <see cref="ValidationResult"/> class.

        /// </summary>

        /// <param name="memeberName">Name of the memeber.</param>

        /// <param name="message">The message.</param>

        public ValidationResult(string memeberName, string message)

        {

            MemberName = memeberName;

            Message = message;

        }



        /// <summary>

        /// Initializes a new instance of the <see cref="ValidationResult"/> class.

        /// </summary>

        /// <param name="message">The message.</param>

        public ValidationResult(string message)

        {

            Message = message;

        }



        /// <summary>

        /// Gets or sets the name of the member.

        /// </summary>

        /// <value>

        /// The name of the member.  May be null for general validation issues.

        /// </value>

        public string MemberName { get; set; }



        /// <summary>

        /// Gets or sets the message.

        /// </summary>

        /// <value>

        /// The message.

        /// </value>

        public string Message { get; set; }

    }

And now create the most used interface in this pattern. ICommandBus will be called as bus handler in every command:

public interface ICommandBus

    {

        ICommandResult Send<TCommand>(TCommand command) where TCommand : ICommand;

        IEnumerable<ValidationResult> Validate<TCommand>(TCommand command) where TCommand : ICommand;

    }

To implementing this interface, create a class again with the following definition:

  internal class CommandBus : ICommandBus

    {

        public ICommandResult Send<TCommand>(TCommand command) where TCommand : ICommand

        {

            var handler = DependencyResolver.Current.GetService<ICommandHandler<TCommand>>();



            if (!((handler != null) && handler != null))

            {

                throw new CommandHandlerNotFoundException(typeof(TCommand));

            }

            return handler.Execute(command);

        }



        public IEnumerable<ValidationResult> Validate<TCommand>(TCommand command) where TCommand : ICommand

        {

            var handler = DependencyResolver.Current.GetService<IValidationHandler<TCommand>>();

         

            if (!((handler != null) && handler != null))

            {

                throw new ValidationHandlerNotFoundException(typeof(TCommand));

            }

            return handler.Validate(command);

        }

    }

The above class will handle every command received from the bus. And now for executing the handler or so-called for implementing the business, you need an interface:

internal interface ICommandHandler<in TCommand> where TCommand : ICommand

    {

        ICommandResult Execute(TCommand command);

    }

And finally as I’m personally interested in to have my own exception handler, so I create a customized handler:

public class CommandHandlerNotFoundException : Exception

    {

        public CommandHandlerNotFoundException(Type type)

            : base(string.Format("Command handler not found for command type: {0}", type))

        {

        }

    }



    internal class ValidationHandlerNotFoundException : Exception

    {

        public ValidationHandlerNotFoundException(Type type)

            : base(string.Format("Validation handler not found for command type: {0}", type))

        {

        }

    }

Finished! Now let’s use the above codes in a real work application. We are going to handle the products of an e-commerce application as its business is easier to comprehend for everybody:

    public interface IProductCommand : ICommand

    {

        string Name { get; set; }



        string OriginalName { get; set; }



        string Description { get; set; }



        decimal Price { get; set; }

     

    }

  

    public class ProductCommand : IProductCommand

    {

        public string Name { get; set; }



        public string OriginalName { get; set; }



        public string Description { get; set; }



        public decimal Price { get; set; }     

    }



    public class CreateProductCommand : ProductCommand

    {

    }



    public class EditProductCommand : ProductCommand

    {

        public Guid ProductId { get; set; }



        public DateTime CreationDate { get; set; }



        public DateTime LastUpdateDate { get; set; }

    }



public class DeleteProductCommand : ICommand

    {

        public Guid[] ProductIds { get; set; }

    }

You can see that I created an interface derived from ICommand named IProductCommand and finally created the target classes. And at the end we should create the Product handler class:

internal partial class ProductCommandHandler :

        ICommandHandler<CreateProductCommand>,

        ICommandHandler<EditProductCommand>,   

        ICommandHandler<DeleteProductCommand>       

    {

        private readonly IProductRepository _productRepository;

        private readonly IMembershipRepository _membershipRepository;     

        private readonly IUnitOfWork _unitOfWork;

        private readonly ICacheManager _cacheManager;

        private readonly ILogger _logger;



        public ProductCommandHandler(

            IProductRepository productRepository,        

            IUnitOfWork unitOfWork,

            ILogger logger,

            ICacheManager cacheManager)

        {

            _productRepository = productRepository;          

            _unitOfWork = unitOfWork;

            _logger = logger;

            _cacheManager = cacheManager;        

        }

       

        public ICommandResult Execute(CreateProductCommand command)

        {

            try

            {

                if (command == null)

                {

                    throw new ArgumentNullException();

                }



                var product = new Product();

                AddProductAppurtenance(command, product);

                _productRepository.Add(product);

                _unitOfWork.Commit();

                return new SuccessResult(ProductCommandMessage.ProductCreatedSuccessfully);

            }

            catch (Exception exception)

            {

                _logger.Error(exception.Message);

                return new FailureResult(ProductCommandMessage.ProductCreationFailed);

            }

        }



        public ICommandResult Execute(EditProductCommand command)

        {

            try

            {

                if (command == null)

                {

                    throw new ArgumentNullException();

                }



                var product = _productRepository.GetProductDetailById(command.ProductId);

                ClearProductAppurtenance(product);

                AddProductAppurtenance(command, product);

                _productRepository.Edit(product);

                _unitOfWork.Commit();



                return new SuccessResult(ProductCommandMessage.ProductEditedSuccessfully);

            }

            catch (Exception exception)

            {

                _logger.Error(exception.Message);

                return new FailureResult(ProductCommandMessage.ProductEditionFailed);

            }

        }



        public ICommandResult Execute(DeleteProductCommand command)

        {

            if (command.ProductIds == null)

            {

                throw new ArgumentNullException();

            }



            var exceptions = new List<Exception>();

            foreach (var productId in command.ProductIds)

            {

                try

                {

                    var product = DeleteProduct(productId);

                    _productRepository.Edit(product);

                }

                catch (Exception exception)

                {

                    exceptions.Add(exception);

                    return new FailureResult(ProductCommandMessage.ProductDeletionFailed);

                }

            }



            if (exceptions.Any())

            {

                throw new AggregateException(exceptions);

            }



            _unitOfWork.Commit();

            return new SuccessResult(ProductCommandMessage.ProductsDeletedSuccessufully);

        }  

 

We used all of the classes introduced in infrastructure.

Note 1: so you need to have an IOC container tool, to the best of my Knowledge, AsClosedTypesOf in Autofac resolves the ICommandHandler. This a piece of code I wrote for IOC handler:

  public class CommandModule : Module

    {

        protected override void Load(ContainerBuilder builder)

        {

            builder.RegisterModule<RepositoryModule>();

            builder.RegisterType<CommandBus>().As<ICommandBus>().InstancePerRequest();

            builder.RegisterAssemblyTypes(ThisAssembly).AsClosedTypesOf(typeof(ICommandHandler<>)).InstancePerRequest();

        }

    }

Note 2: a penny for your thoughts! how to use ICommandBus!? See the below piece of code as an example:

[HttpPost]

        public JsonResult DeleteProduct(DeleteProductCommand command)

        {

            var result = _commandBus.Send(command);

            return JsonMessage(result);

        }

have fun.



Implementing thread safe Singleton pattern in C#

A singleton is a class which allows one instance of itself to be created and allow you to easily access to that instance. A singleton just gives you a single instance of a class. When you need a class that has only one instance. Singleton ensures you that a class has only one instance and also you have a global point to access that instance. This a simple example of a singleton pattern.

 

    public class ThreadSafeSingleton
    {
        private static volatile ThreadSafeSingleton _instance;
        private static readonly object Sync= new Object();

        private ThreadSafeSingleton()
        {
        }

        public static ThreadSafeSingleton Instance
        {
            get
            {
                if (_instance == null)
                {
                    lock (Sync)
                    {
                        if (_instance == null)
                        {
                            _instance = new ThreadSafeSingleton();
                        }
                    }
                }

                return _instance;
            }
        }
    }

 

The benefit of implementing Singleton:

  1. Ensuring that all objects access the single instance.
  2. The class has the flexibility to change the instantiation process.
  3. By changing in one instance you can change all instances of a class.

 

Read these articles for complete information

  1. http://www.yoda.arachsys.com/csharp/singleton.html
  2. http://csharpindepth.com/articles/general/singleton.aspx



When to use dependency injection?

In enterprise applications, when it comes to a large amount of data and different components and you are aiming for code reuse; there is a pattern named Dependency Injection to avoid tight coupling of object implementations and make the objects loosely coupled. It can be applied in any situation across the layers, but it doesn't mean that you should apply for DI everywhere!

 

If you don't need different implementation and different configuration Do not use Dependency injection. Although there are some related and of course older replacement patterns for DI such as abstract factory or container patterns, nut dependency injection makes the code testable as well, and it is one the biggest advantage of DI in compare to other similar patterns.

 



Dependency Inversion of SOLID principle

As you know, Dependency Inversion Principle is the last principle of SOLID. I talked about SOLID principle and fundamental in my previous post that was about SRP. Now I'm gonna to consider the dependency inversion of SOLID. Based on definition "High level modules should not depend upon low level module, both should be depend upon abstraction". Or in other definition "abstraction should not depend upon details, details should depend upon abstraction". When you have relationship between you classes in OPP, that relationship should be stated on terms of abstraction rather than concrete implementation. "The benefits of using loose coupling aren’t always instantly evident, but they become visible over time, as the complexity of a code grows." in concrete implementaion changes to one class will break the other class.

Take a look at this implementation:

  1.    public class OrderRepository
  2.     {
  3.         public void Add()
  4.         {
  5.             //Add Order
  6.         }
  7.      }
  8.  
  9.     public class OrderService
  10.     {
  11.         private readonly  OrderRepository _orderRepository;
  12.     
  13.         public OrderService()
  14.         {
  15.             _orderRepository = new OrderRepository();
  16.         }
  17.  
  18.         public void CreateOrder()
  19.         {
  20.             _orderRepository.Add();
  21.         }
  22.      }

 

I just create an instance of the OrderRepository inside the OrderService to use the add method. As you know every changes in OrderRepository will destroy the OrderService class and we should modify the OrderService class every times. Now notice about this implementation:

 

  1.     public interface  IOrderRepository
  2.     {
  3.         void Add();
  4.     }
  5.  
  6.     public class OrderRepository :IOrderRepository
  7.     {
  8.         public void Add()
  9.         {
  10.             throw new NotImplementedException();
  11.         }
  12.     }
  13.  
  14.    public class OrderService
  15.     {
  16.         private readonly IOrderRepository _orderRepository;
  17.  
  18.         public OrderService(IOrderRepository orderRepository)
  19.         {
  20.             _orderRepository = orderRepository;
  21.         }
  22.     
  23.         public void CreateOrder()
  24.         {
  25.             _orderRepository.Add();
  26.         }
  27.     }

 

I just added an interface, but if you take a deeper look at OrderService,it doesn't have any concrete instantiating of OrderRepository and every change in IOrderRepository should be implemented in OrderRepository and OrderService just talks to IOrderService. (Don't call use we'll call you!). I also used constructor injection that you can read more about that here.

 

You can read more about DIP

  1. http://www.codeproject.com/Articles/495019/Dependency-Inversion-Principle-and-the-Dependency
  2. http://lostechies.com/derickbailey/2011/09/22/dependency-injection-is-not-the-same-as-the-dependency-inversion-principle/



Does it cause any kind of performance issue using a base class in Layer SuperType pattern?

I had a long discussion with one of my colleague about using a base class in Data access layer or repository instead of implementing all common scenario like CRUD operation for every aggregate. it's Layer SuperType pattern that I'm talking about. I've posted about Layer SuperType pattern before, but now I'm going to know that what will happen in an application with large amounts of requests and transactions to performance in the case of using a base class for Common methods of DAL.

This this the answer of Scott Millet on Twitter for my question, here is the discussion 

 

          LayerSupertype

As you can see in the StackOverflow link suggested by Scott here, when you inherit from a base class and construct an object, the runtime never separate instances of the base class from inherited class. In other words, it's not like that you have two objects in memory when you have the variables of a base class in inherited class. When you inherit from a base class you have all variable of the base class and runs the base class constructor and also Both abstract and normal base classes act the same.  So it's not like that when you have an Insert method for a large application with lots request, you are using just the same insert to cause a performance problem!

 



About Me

Ehsan Ghanbari

Hi! my name is Ehsan. I'm a developer, passionate technologist, and fan of clean code. I'm interested in enterprise and large-scale applications architecture and design patterns. I'm spending a lot of my time on software architecture. Since 2008, I've been as a developer for companies and organizations and I've been focusing on Microsoft ecosystem all the time. During the Read More

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