Question

感谢那些回答我last question的人，我得到了下面的代码，它允许开发人员将多个where子句发送到一个方法，该方法包含LINQ语句中的每个子句。 但是，我怎样才能将where子句包含在内？而不是：

return customers
       .Where(whereClauses[0])
       .Where(whereClauses[1])
       .ToList();

像这样的东西（伪代码）：

List<Customer> customers = new List<Customer>();
foreach (var whereClause in whereClauses)
{
    customers
    .Where(whereClause...???)
    .ToList();
}
return customers;

以下是有效的代码：

using System;
using System.Collections.Generic;
using System.Linq;

namespace TestDynamicLinq2343
{
    public class Program
    {
        static void Main(string[] args)
        {
            List<Customer> customers = Customer.GetCustomers();

            List<Func<Customer, bool>> whereClauses = new List<Func<Customer, bool>>();
            whereClauses.Add(c => c.LastName.ToUpper().Contains("A"));
            whereClauses.Add(c => c.FirstName.ToUpper().Contains("J"));

            foreach (var customer in Customer.GetFilteredCustomers(customers, whereClauses))
            {
                Console.WriteLine(customer.LastName);
            }

            Console.ReadLine();
        }
    }

    public class Customer
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Street { get; set; }
        public string Location { get; set; }
        public string ZipCode { get; set; }

        public static List<Customer> GetCustomers()
        {
            List<Customer> customers = new List<Customer>();
            customers.Add(new Customer { FirstName = "Jim", LastName = "Jones" });
            customers.Add(new Customer { FirstName = "Joe", LastName = "Adams" });
            customers.Add(new Customer { FirstName = "Jake", LastName = "Johnson" });
            customers.Add(new Customer { FirstName = "Angie", LastName = "Reckar" });
            customers.Add(new Customer { FirstName = "Jean", LastName = "Anderson" });
            return customers;
        }

        public static List<Customer> GetFilteredCustomers(List<Customer> customers, List<Func<Customer, bool>> whereClauses)
        {
            return customers
                   .Where(whereClauses[0])
                   .Where(whereClauses[1])
                   .ToList();
        }
    }
}

Answer 1

IEnumerable<Customer> dbCustomers = customers;
foreach (var whereClause in whereClauses)
{
    dbCustomers = dbCustomers.Where(whereClause);
}
return dbCustomers.ToList();

也许有趣的extensionMethod：

public static class IEnumerableExtension
{
    public static void AttachWhereClauses<T>(this IEnumerable<T> source, IEnumerable<Func<T, bool>> whereClauses)
    {
        foreach (var whereClause in whereClauses)
        {
            source = source.Where(whereClause);
        }
    }
}
var listedCustomers = customers.AttachWhereClauses(whereClauses).ToList();

但是：没有经过测试 - 我不确定，如果将whereClauses附加到同一个对象有效！

Answer 2

这样的事情：

public IEnumerable<Customer> ApplyConditions(IEnumerable<Func<Customer, bool>> conditions)
{
    IEnumerable<Customer> customers = ...;
    foreach(var condition in conditions)
        customers = customers.Where(condition);
    return customers;
}

如何在LINQ语句中处理多个where子句？

2 个答案: