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Specification Pattern

Specification Pattern

In computer programming, the specification pattern is a particular software design pattern, whereby business rules can be recombined by chaining the business rules together using boolean logic. The pattern is frequently used in the context of domain-driven design.


using System;
using System.Collections.Generic;

namespace opencloseprinciple
{
    public enum Color { Red, Green, Blue }
    
    public enum Size { Small, Medium, Large }

    public class Product
    {
        public string Name { get; set; }
        public Color Color { get; set; }
        public Size Size { get; set; }

        public Product(string name, Color color, Size size)
        {
            this.Name = name;
            this.Color = color;
            this.Size = size;
        }
    }

    // Specification Pattern
    public interface ISpecification<T>
    {
        bool IsSatisfiedBy(T t);
    }

    public interface IFilter<T>
    {
        IEnumerable<T> Filter(IEnumerable<T> items, ISpecification<T> specification);
    }

    public class ColorSpecification : ISpecification<Product>
    {
        private readonly Color color;

        public ColorSpecification(Color color)
        {
            this.color = color;
        }

        public bool IsSatisfiedBy(Product product)
        {
            return product.Color == this.color;
        }
    }

    public class SizeSpecification : ISpecification<Product>
    {
        private readonly Size size;

        public SizeSpecification(Size size)
        {
            this.size = size;
        }

        public bool IsSatisfiedBy(Product product)
        {
            return product.Size == this.size;
        }
    }
    
    public class AndSpecification<T> : ISpecification<T>
    {
        private ISpecification<T> first, second;

        public AndSpecification(ISpecification<T> first, ISpecification<T> second)
        {
            if(first == null || second == null)
                throw new ArgumentNullException();

            this.first = first;
            this.second = second;
        }

        public bool IsSatisfiedBy(T t)
        {
            return first.IsSatisfiedBy(t) && second.IsSatisfiedBy(t);
        }
    }

    public class ProductFilter : IFilter<Product>
    {
        public IEnumerable<Product> Filter(IEnumerable<Product> products,
            ISpecification<Product> specification)
        {
            foreach (var product in products)
                if(specification.IsSatisfiedBy(product))
                    yield return product;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var apple = new Product("apple", Color.Green, Size.Small);
            var tree = new Product("tree", Color.Green, Size.Large);
            var house = new Product("house", Color.Blue, Size.Large);

            var products = new Product[] { apple, tree, house };

            var productFilter = new ProductFilter();

            foreach (var product in productFilter.Filter(
                products,
                new ColorSpecification(Color.Green)))
            {
                Console.WriteLine($" - {product.Name} is green");
            }

            foreach (var product in productFilter.Filter(
                products,
                new AndSpecification<Product>(
                    new ColorSpecification(Color.Green),
                    new SizeSpecification(Size.Large)
                )))
            {
                Console.WriteLine($" - {product.Name} is green and large");
            }
        }
    }
}

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