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LINQ排序操作符用法

2022-02-28 | 秩名 | 点击：次

Linq中的排序操作符包括OrderBy、OrderByDescending、ThenBy、ThenByDescending和Reverse，提供了升序或者降序排序。

一、OrderBy操作符

OrderBy操作符用于对输入序列中的元素进行排序，排序基于一个委托方法的返回值顺序。排序过程完成后，会返回一个类型为IOrderEnumerable<T>的集合对象。其中IOrderEnumerable<T>接口继承自IEnumerable<T>接口。下面来看看OrderBy的定义：

从上面的截图中可以看出，OrderBy是一个扩展方法，只要实现了IEnumerable<T>接口的就可以使用OrderBy进行排序。OrderBy共有两个重载方法：第一个重载的参数是一个委托类型和一个实现了IComparer<T>接口的类型。第二个重载的参数是一个委托类型。看看下面的示例：

定义产品类：

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using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

 

namespace OrderOperation

{

    public class Products

    {

        public int Id { get; set; }

        public int CategoryId { get; set; }

        public string Name { get; set; }

        public double Price { get; set; }

        public DateTime CreateTime { get; set; }

    }

}

在Main()方法里面调用：

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using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

 

namespace OrderOperation

{

    class Program

    {

        static void Main(string[] args)

        {

            // 初始化数据

            List<Products> listProduct = new List<Products>()

            {

               new Products(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now},

               new Products(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)},

               new Products(){Id=3,CategoryId=1, Name="ASP.NET Core", Price=57,CreateTime=DateTime.Now.AddMonths(-3)},

               new Products(){Id=4,CategoryId=1, Name="Entity Framework 6.x", Price=97,CreateTime=DateTime.Now.AddMonths(-1)}

            };

            Console.WriteLine("方法语法");

            // 1、查询方法，返回匿名类

            var list = listProduct.OrderBy(p => p.CreateTime).Select(p => new { id = p.Id, ProductName = p.Name,ProductPrice=p.Price,PublishTime=p.CreateTime }).ToList();

            foreach (var item in list)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.WriteLine("查询表达式");

            // 2、查询表达式,返回匿名类

            var listExpress = from p in listProduct orderby p.CreateTime select new { id = p.Id, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime };

            foreach (var item in listExpress)

            {

                Console.WriteLine($"item:{item}");

            }

 

            Console.ReadKey();

        }

    }

}

结果：

从截图中可以看出，集合按照CreateTime进行升序排序。

在来看看第一个重载方法的实现：

先定义PriceComparer类实现IComparer<T>接口，PriceComparer类定义如下：

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using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

 

namespace OrderOperation

{

    public class PriceComparer : IComparer<double>

    {

        public int Compare(double x, double y)

        {

            if (x > y)

            {

                return 1;     //表示x>y

            }

            else if (x < y)

            {

                return -1;    //表示x<y

            }

            else

            {

                return 0;     //表示x=y

            }

        }

    }

}

在Main()方法里面调用：

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using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

 

namespace OrderOperation

{

    class Program

    {

        static void Main(string[] args)

        {

            // 初始化数据

            List<Products> listProduct = new List<Products>()

            {

               new Products(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now},

               new Products(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)},

               new Products(){Id=3,CategoryId=1, Name="ASP.NET Core", Price=57,CreateTime=DateTime.Now.AddMonths(-3)},

               new Products(){Id=4,CategoryId=1, Name="Entity Framework 6.x", Price=97,CreateTime=DateTime.Now.AddMonths(-1)}

            };

            Console.WriteLine("方法语法");

            // 1、查询方法，按照价格升序排序,返回匿名类

            var list = listProduct.OrderBy(p => p.Price,new PriceComparer()).Select(p => new { id = p.Id, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }).ToList();

            foreach (var item in list)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.ReadKey();

        }

    }

}

结果：

注意：orderby必须在select之前出现,查询表达式最后只可能出现select或者groupby。

二、OrderByDescending

OrderByDescending操作符的功能与OrderBy操作符基本相同，二者只是排序的方式不同。OrderBy是升序排序，而OrderByDescending则是降序排列。下面看看OrderByDescending的定义：

从方法定义中可以看出，OrderByDescending的方法重载和OrderBy的方法重载一致。来看下面的例子：

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using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

 

namespace OrderOperation

{

    class Program

    {

        static void Main(string[] args)

        {

            // 初始化数据

            List<Products> listProduct = new List<Products>()

            {

               new Products(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now},

               new Products(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)},

               new Products(){Id=3,CategoryId=1, Name="ASP.NET Core", Price=57,CreateTime=DateTime.Now.AddMonths(-3)},

               new Products(){Id=4,CategoryId=1, Name="Entity Framework 6.x", Price=97,CreateTime=DateTime.Now.AddMonths(-1)}

            };

            // 注意：OrderByDescending的方法语法和查询表达式写法有些不同。

            Console.WriteLine("方法语法");

            // 1、查询方法，按照时间降序排序,返回匿名类

            var list = listProduct.OrderByDescending(p => p.CreateTime).Select(p => new { id = p.Id, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }).ToList();

            foreach (var item in list)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.WriteLine("查询表达式");

            var listExpress = from p in listProduct orderby p.CreateTime descending select new { id = p.Id, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime };

            foreach (var item in list)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.ReadKey();

        }

    }

}

结果：

从截图中可以看出：输出结果按照时间降序排序。在来看看另外一个重载方法的调用：

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using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

 

namespace OrderOperation

{

    class Program

    {

        static void Main(string[] args)

        {

            // 初始化数据

            List<Products> listProduct = new List<Products>()

            {

               new Products(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now},

               new Products(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)},

               new Products(){Id=3,CategoryId=1, Name="ASP.NET Core", Price=57,CreateTime=DateTime.Now.AddMonths(-3)},

               new Products(){Id=4,CategoryId=1, Name="Entity Framework 6.x", Price=97,CreateTime=DateTime.Now.AddMonths(-1)}

            };

            Console.WriteLine("方法语法");

            // 1、查询方法，按照价格降序排序,返回匿名类

            var list = listProduct.OrderByDescending(p => p.Price, new PriceComparer()).Select(p => new { id = p.Id, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }).ToList();

            foreach (var item in list)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.ReadKey();

        }

    }

}

结果：

输出结果也是按照时间降序排序。

三、ThenBy排序

ThenBy操作符可以对一个类型为IOrderedEnumerable<T>,(OrderBy和OrderByDesceding操作符的返回值类型)的序列再次按照特定的条件顺序排序。ThenBy操作符实现按照次关键字对序列进行升序排列。下面来看看ThenBy的定义：

从截图中可以看出：ThenBy()方法扩展的是IOrderedEnumerable<T>，因此ThenBy操作符长常常跟在OrderBy和OrderByDesceding之后。看下面的示例：

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using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

 

namespace OrderOperation

{

    class Program

    {

        static void Main(string[] args)

        {

            // 初始化数据

            List<Products> listProduct = new List<Products>()

            {

               new Products(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now},

               new Products(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)},

               new Products(){Id=3,CategoryId=2, Name="活着", Price=57,CreateTime=DateTime.Now.AddMonths(-3)},

               new Products(){Id=4,CategoryId=3, Name="高等数学", Price=97,CreateTime=DateTime.Now.AddMonths(-1)}

            };

            // 注意：ThenBy()的方法语法和查询表达式写法有些不同。

            Console.WriteLine("方法语法升序排序");

            // 1、查询方法，按照商品分类升序排序,如果商品分类相同在按照价格升序排序 返回匿名类

            var list = listProduct.OrderBy(p => p.CategoryId).ThenBy(p=>p.Price).Select(p => new { id = p.CategoryId, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }).ToList();

            foreach (var item in list)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.WriteLine("查询表达式升序排序");

            var listExpress = from p in listProduct orderby p.CategoryId,p.Price select new { id = p.CategoryId, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime };

            foreach (var item in listExpress)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.WriteLine("方法语法降序排序");

            // 1、查询方法，按照商品分类降序排序,如果商品分类相同在按照价格升序排序 返回匿名类

            var listDesc = listProduct.OrderByDescending(p => p.CategoryId).ThenBy(p => p.Price).Select(p => new { id = p.CategoryId, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }).ToList();

            foreach (var item in listDesc)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.WriteLine("查询表达式降序排序");

            var listExpressDesc = from p in listProduct orderby p.CategoryId descending , p.Price select new { id = p.CategoryId, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime };

            foreach (var item in listExpressDesc)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.ReadKey();

        }

    }

}

结果：

四、ThenByDescending

ThenByDescending操作符于ThenBy操作符非常类似，只是是按照降序排序，实现按照次关键字对序列进行降序排列。来看看ThenByDescending的定义：

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using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

 

namespace OrderOperation

{

    class Program

    {

        static void Main(string[] args)

        {

            // 初始化数据

            List<Products> listProduct = new List<Products>()

            {

               new Products(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now},

               new Products(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)},

               new Products(){Id=3,CategoryId=2, Name="活着", Price=57,CreateTime=DateTime.Now.AddMonths(-3)},

               new Products(){Id=4,CategoryId=3, Name="高等数学", Price=97,CreateTime=DateTime.Now.AddMonths(-1)}

            };

            // 注意：ThenByDescending()的方法语法和查询表达式写法有些不同。

            Console.WriteLine("方法语法升序排序");

            // 1、查询方法，按照商品分类升序排序,如果商品分类相同在按照价格降序排序 返回匿名类

            var list = listProduct.OrderBy(p => p.CategoryId).ThenByDescending(p => p.Price).Select(p => new { id = p.CategoryId, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }).ToList();

            foreach (var item in list)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.WriteLine("查询表达式升序排序");

            var listExpress = from p in listProduct orderby p.CategoryId, p.Price descending select new { id = p.CategoryId, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime };

            foreach (var item in listExpress)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.WriteLine("方法语法降序排序");

            // 1、查询方法，按照商品分类降序排序,如果商品分类相同在按照价格降序排序 返回匿名类

            var listDesc = listProduct.OrderByDescending(p => p.CategoryId).ThenByDescending(p => p.Price).Select(p => new { id = p.CategoryId, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }).ToList();

            foreach (var item in listDesc)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.WriteLine("查询表达式降序排序");

            var listExpressDesc = from p in listProduct orderby p.CategoryId descending, p.Price descending select new { id = p.CategoryId, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime };

            foreach (var item in listExpressDesc)

            {

                Console.WriteLine($"item:{item}");

            }

            Console.ReadKey();

        }

    }

}

结果：

五、Reverse

Reverse操作符用于生成一个与输入序列中元素相同，但元素排列顺序相反的新序列。下面来看看Reverse()方法的定义：

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public static IEnumerable<TSource> Reverse<TSource>(this IEnumerable<TSource> source)

从方法定义中可以看到，这个扩展方法，不需要输入参数，返回一个新集合。需要注意的是，Reverse方法的返回值是void。看下面的例子：

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using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

 

namespace ThenBy

{

    class Program

    {

        static void Main(string[] args)

        {

            string[] str = { "A", "B", "C", "D", "E"};

            var query = str.Select(p => p).ToList();

            query.Reverse();

            foreach (var item in query)

            {

                Console.WriteLine(item);

            }

 

            Console.ReadKey();

        }

    }

}

运行效果：

原文链接：https://www.cnblogs.com/dotnet261010/p/6850936.html
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