Question

我使用.Net 4.0框架跟随C＃代码。这是在引用The Specification Pattern - by Jeff Perrin

后创建的

在GetProducts()中，要使用的条件在方法内定义（硬编码）。还有另一种名为GetProductsBasedOnInputFilters()的方法。在该方法中，规范列表作为方法的参数。

问题

在此方法中，在产品列表中应用这些过滤器的最佳方法是什么？

注意：我尝试在FindAll循环中应用foreach子句，并在list中添加结果。但是这种逻辑是错误的 - 只需要返回那些满足所有条件的项目。

注意：productSpeifications列表中的规格数量会因用户输入而异

注意：“Dynamically build LINQ filter for the Any() method?”中提到的方法似乎很有用。但是我不知道如何在这里使用这种方法，因为我正在处理specifications的列表;不是generic delegates。

过滤方法

public static class ProductFilterHelper
{
    public static List<Product> GetProducts(List<Product> list)
    {
        double priceLimit = 100;

        //FIRST::
        //List<Product> selectedList =  list.FindAll(new OnSaleSpecification().IsSatisfiedBy);

        //SECOND::
        //AndSpecification<Product> spec = new AndSpecification<Product>(new OnSaleSpecificationForProduct(), new PriceGreaterThanSpecificationForProduct(priceLimit));
        //List<Product> selectedList = list.FindAll(spec.IsSatisfiedBy); 


        //THIRD:
        List<Product> selectedList = list.FindAll(new OnSaleSpecificationForProduct()
                                                       .And(new PriceGreaterThanSpecificationForProduct(priceLimit))
                                                       .And(new PriceGreaterThan105())
                                                       .IsSatisfiedBy
                                                  );

        return selectedList;
    }

    public static List<Product> GetProductsBasedOnInputFilters(List<Product> productList, List<Specification<Product>> productSpeifications)
    {
        List<Product> selectedList = new List<Product>();

        foreach (Specification<Product> specification in productSpeifications)
        {
            List<Product> currentList = productList.FindAll(specification.IsSatisfiedBy);

            if (currentList != null && currentList.Count > 0)
            {
                foreach (Product p in currentList)
                {
                    if (!selectedList.Contains(p))
                    {
                        selectedList.Add(p);
                    }
                }
            }
        }

        return selectedList;
    }        

}

客户端

class Program
{

    static void Main(string[] args)
    {

        List<Product> list = new List<Product>();

        Product p1 = new Product(false, 99);
        Product p2 = new Product(true, 99);
        Product p3 = new Product(true, 101);
        Product p4 = new Product(true, 110);
        Product p5 = new Product(false, 110);

        list.Add(p1);
        list.Add(p2);
        list.Add(p3);
        list.Add(p4);
        list.Add(p5);

        double priceLimit = 100;

        List<Specification<Product>> specifications = new List<Specification<Product>>();
        specifications.Add(new OnSaleSpecificationForProduct());
        specifications.Add(new PriceGreaterThanSpecificationForProduct(priceLimit));
        specifications.Add(new PriceGreaterThan105());

        List<Product> selectedList = ProductFilterHelper.GetProductsBasedOnInputFilters(list, specifications);

        Console.ReadKey();
    }

}

抽象规格

public abstract class Specification<T>
{
    public abstract bool IsSatisfiedBy(T obj);

    public AndSpecification<T> And(Specification<T> specification)
    {
        return new AndSpecification<T>(this, specification);
    }

    public OrSpecification<T> Or(Specification<T> specification)
    {
        return new OrSpecification<T>(this, specification);
    }

    public NotSpecification<T> Not(Specification<T> specification)
    {
        return new NotSpecification<T>(this, specification);
    }
}

public abstract class CompositeSpecification<T> : Specification<T>
{
    protected readonly Specification<T> _leftSide;
    protected readonly Specification<T> _rightSide;

    public CompositeSpecification(Specification<T> leftSide, Specification<T> rightSide)
    {
        _leftSide = leftSide;
        _rightSide = rightSide;
    }
}

通用规格

public class AndSpecification<T> : CompositeSpecification<T>
{
    public AndSpecification(Specification<T> leftSide, Specification<T> rightSide)
        : base(leftSide, rightSide)
    {

    }

    public override bool IsSatisfiedBy(T obj)
    {
        return _leftSide.IsSatisfiedBy(obj) && _rightSide.IsSatisfiedBy(obj);
    }
}

public class OrSpecification<T> : CompositeSpecification<T>
{
    public OrSpecification(Specification<T> leftSide, Specification<T> rightSide)
        : base(leftSide, rightSide)
    {
    }

    public override bool IsSatisfiedBy(T obj)
    {
        return _leftSide.IsSatisfiedBy(obj) || _rightSide.IsSatisfiedBy(obj);
    }
}

public class NotSpecification<T> : CompositeSpecification<T>
{
    public NotSpecification(Specification<T> leftSide, Specification<T> rightSide)
        : base(leftSide, rightSide)
    {
    }

    public override bool IsSatisfiedBy(T obj)
    {
        return _leftSide.IsSatisfiedBy(obj) && !_rightSide.IsSatisfiedBy(obj);
    }
}

产品规格

public class OnSaleSpecificationForProduct : Specification<Product>
{
    public override bool IsSatisfiedBy(Product product)
    {
        return product.IsOnSale;
    }
}

public class PriceGreaterThanSpecificationForProduct : Specification<Product>
{
    private readonly double _price;
    public PriceGreaterThanSpecificationForProduct(double price)
    {
        _price = price;
    }

    public override bool IsSatisfiedBy(Product product)
    {
        return product.Price > _price;
    }
}

public class PriceGreaterThan105 : Specification<Product>
{

    public override bool IsSatisfiedBy(Product product)
    {
        return product.Price > 105;
    }
}

实体

public class Product
{
    private bool _isOnSale;
    private double _price = 0.0;

    public Product(bool isOnSale)
        : this(isOnSale, 0.0)
    {
        _isOnSale = isOnSale;
    }

    public Product(double price)
        : this(false, price)
    {
        _price = price;
    }

    public Product(bool isOnSale, double price)
    {
        _price = price;
        _isOnSale = isOnSale;
    }

    public bool IsOnSale
    {
        get { return _isOnSale; }
    }

    public double Price
    {
        get { return _price; }
    }
}

参考

Answer 1

您可以执行以下操作之一：

通过将Where个调用堆叠在一起来组合过滤器，就像@Lijo的回答一样

检查每个项目的所有规格：

return productList
  .Where(p => specifications.All(ps => ps.IsSatisfiedBy(p))
  .ToList()

创建一个复合'And'规范，接受多个子节点，而不只是两个：

public class AndSpecification<T> : ISpecification<T>
{
    private ISpecification<T>[] _components;

    public AndSpecification(ISpecification<T>[] components) 
    {
      _components = components;
    }

    public bool IsSatisfiedBy(T item) 
    {
      return components.All(c => c.IsSatisfiedBy(item));
    }
  }

然后你可以这样做：

var allFiltersSpecification = new AndSpecification(specifications)
return productList.Where(allFiltersSpecification.IsSatisfiedBy);

Answer 2

以下代码有效...欢迎提出建议。

 public static List<Product> GetProductsBasedOnInputFilters(List<Product> productList, List<Specification<Product>> productSpecifications)
 {
            IEnumerable<Product> selectedList = productList;
            foreach (Specification<Product> specification in productSpecifications)
            {
                selectedList = selectedList.Where(specification.IsSatisfiedBy);
            }
            return selectedList.ToList();
 }

值得看看以下内容......

如何在列表中应用多个过滤条件（同时）？

2 个答案: