LINQ操作符四:排序操作符
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:
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6 7 namespace OrderOperation 8 { 9 public class Products 10 { 11 public int Id { get; set; } 12 public int CategoryId { get; set; } 13 public string Name { get; set; } 14 public double Price { get; set; } 15 public DateTime CreateTime { get; set; } 16 } 17 }
在Main()方法里面调用:
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6 7 namespace OrderOperation 8 { 9 class Program 10 { 11 static void Main(string[] args) 12 { 13 // 初始化数据 14 List<Products> listProduct = new List<Products>() 15 { 16 new Products(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now}, 17 new Products(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)}, 18 new Products(){Id=3,CategoryId=1, Name="ASP.NET Core", Price=57,CreateTime=DateTime.Now.AddMonths(-3)}, 19 new Products(){Id=4,CategoryId=1, Name="Entity Framework 6.x", Price=97,CreateTime=DateTime.Now.AddMonths(-1)} 20 }; 21 Console.WriteLine("方法语法"); 22 // 1、查询方法,返回匿名类 23 var list = listProduct.OrderBy(p => p.CreateTime).Select(p => new { id = p.Id, ProductName = p.Name,ProductPrice=p.Price,PublishTime=p.CreateTime }).ToList(); 24 foreach (var item in list) 25 { 26 Console.WriteLine($"item:{item}"); 27 } 28 Console.WriteLine("查询表达式"); 29 // 2、查询表达式,返回匿名类 30 var listExpress = from p in listProduct orderby p.CreateTime select new { id = p.Id, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }; 31 foreach (var item in listExpress) 32 { 33 Console.WriteLine($"item:{item}"); 34 } 35 36 Console.ReadKey(); 37 } 38 } 39 }
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:
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6 7 namespace OrderOperation 8 { 9 public class PriceComparer : IComparer<double> 10 { 11 public int Compare(double x, double y) 12 { 13 if (x > y) 14 { 15 return 1; //表示x>y 16 } 17 else if (x < y) 18 { 19 return -1; //表示x<y 20 } 21 else 22 { 23 return 0; //表示x=y 24 } 25 } 26 } 27 }
在Main()方法里面调用:
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6 7 namespace OrderOperation 8 { 9 class Program 10 { 11 static void Main(string[] args) 12 { 13 // 初始化数据 14 List<Products> listProduct = new List<Products>() 15 { 16 new Products(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now}, 17 new Products(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)}, 18 new Products(){Id=3,CategoryId=1, Name="ASP.NET Core", Price=57,CreateTime=DateTime.Now.AddMonths(-3)}, 19 new Products(){Id=4,CategoryId=1, Name="Entity Framework 6.x", Price=97,CreateTime=DateTime.Now.AddMonths(-1)} 20 }; 21 Console.WriteLine("方法语法"); 22 // 1、查询方法,按照价格升序排序,返回匿名类 23 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(); 24 foreach (var item in list) 25 { 26 Console.WriteLine($"item:{item}"); 27 } 28 Console.ReadKey(); 29 } 30 } 31 }
结果:
注意:orderby必须在select之前出现,查询表达式最后只可能出现select或者groupby。
二、OrderByDescending
![](https://images2018.cnblogs.com/blog/1033738/201807/1033738-20180711212740062-1777318257.png)
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6 7 namespace OrderOperation 8 { 9 class Program 10 { 11 static void Main(string[] args) 12 { 13 // 初始化数据 14 List<Products> listProduct = new List<Products>() 15 { 16 new Products(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now}, 17 new Products(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)}, 18 new Products(){Id=3,CategoryId=1, Name="ASP.NET Core", Price=57,CreateTime=DateTime.Now.AddMonths(-3)}, 19 new Products(){Id=4,CategoryId=1, Name="Entity Framework 6.x", Price=97,CreateTime=DateTime.Now.AddMonths(-1)} 20 }; 21 // 注意:OrderByDescending的方法语法和查询表达式写法有些不同。 22 Console.WriteLine("方法语法"); 23 // 1、查询方法,按照时间降序排序,返回匿名类 24 var list = listProduct.OrderByDescending(p => p.CreateTime).Select(p => new { id = p.Id, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }).ToList(); 25 foreach (var item in list) 26 { 27 Console.WriteLine($"item:{item}"); 28 } 29 Console.WriteLine("查询表达式"); 30 var listExpress = from p in listProduct orderby p.CreateTime descending select new { id = p.Id, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }; 31 foreach (var item in list) 32 { 33 Console.WriteLine($"item:{item}"); 34 } 35 Console.ReadKey(); 36 } 37 } 38 }
结果:
从截图中可以看出:输出结果按照时间降序排序。在来看看另外一个重载方法的调用:
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6 7 namespace OrderOperation 8 { 9 class Program 10 { 11 static void Main(string[] args) 12 { 13 // 初始化数据 14 List<Products> listProduct = new List<Products>() 15 { 16 new Products(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now}, 17 new Products(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)}, 18 new Products(){Id=3,CategoryId=1, Name="ASP.NET Core", Price=57,CreateTime=DateTime.Now.AddMonths(-3)}, 19 new Products(){Id=4,CategoryId=1, Name="Entity Framework 6.x", Price=97,CreateTime=DateTime.Now.AddMonths(-1)} 20 }; 21 Console.WriteLine("方法语法"); 22 // 1、查询方法,按照价格降序排序,返回匿名类 23 var list = listProduct.OrderByDescending(p => p.Price, new PriceComparer()).Select(p => new { id = p.Id, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }).ToList(); 24 foreach (var item in list) 25 { 26 Console.WriteLine($"item:{item}"); 27 } 28 Console.ReadKey(); 29 } 30 } 31 }
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:
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6 7 namespace OrderOperation 8 { 9 class Program 10 { 11 static void Main(string[] args) 12 { 13 // 初始化数据 14 List<Products> listProduct = new List<Products>() 15 { 16 new Products(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now}, 17 new Products(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)}, 18 new Products(){Id=3,CategoryId=2, Name="活着", Price=57,CreateTime=DateTime.Now.AddMonths(-3)}, 19 new Products(){Id=4,CategoryId=3, Name="高等数学", Price=97,CreateTime=DateTime.Now.AddMonths(-1)} 20 }; 21 // 注意:ThenBy()的方法语法和查询表达式写法有些不同。 22 Console.WriteLine("方法语法升序排序"); 23 // 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 24 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(); 25 foreach (var item in list) 26 { 27 Console.WriteLine($"item:{item}"); 28 } 29 Console.WriteLine("查询表达式升序排序"); 30 var listExpress = from p in listProduct orderby p.CategoryId,p.Price select new { id = p.CategoryId, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }; 31 foreach (var item in listExpress) 32 { 33 Console.WriteLine($"item:{item}"); 34 } 35 Console.WriteLine("方法语法降序排序"); 36 // 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 37 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(); 38 foreach (var item in listDesc) 39 { 40 Console.WriteLine($"item:{item}"); 41 } 42 Console.WriteLine("查询表达式降序排序"); 43 var listExpressDesc = from p in listProduct orderby p.CategoryId descending , p.Price select new { id = p.CategoryId, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }; 44 foreach (var item in listExpressDesc) 45 { 46 Console.WriteLine($"item:{item}"); 47 } 48 Console.ReadKey(); 49 } 50 } 51 }
结果:
四、ThenByDescending
![](https://images2018.cnblogs.com/blog/1033738/201807/1033738-20180711221344615-784273819.png)
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6 7 namespace OrderOperation 8 { 9 class Program 10 { 11 static void Main(string[] args) 12 { 13 // 初始化数据 14 List<Products> listProduct = new List<Products>() 15 { 16 new Products(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now}, 17 new Products(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)}, 18 new Products(){Id=3,CategoryId=2, Name="活着", Price=57,CreateTime=DateTime.Now.AddMonths(-3)}, 19 new Products(){Id=4,CategoryId=3, Name="高等数学", Price=97,CreateTime=DateTime.Now.AddMonths(-1)} 20 }; 21 // 注意:ThenByDescending()的方法语法和查询表达式写法有些不同。 22 Console.WriteLine("方法语法升序排序"); 23 // 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 24 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(); 25 foreach (var item in list) 26 { 27 Console.WriteLine($"item:{item}"); 28 } 29 Console.WriteLine("查询表达式升序排序"); 30 var listExpress = from p in listProduct orderby p.CategoryId, p.Price descending select new { id = p.CategoryId, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }; 31 foreach (var item in listExpress) 32 { 33 Console.WriteLine($"item:{item}"); 34 } 35 Console.WriteLine("方法语法降序排序"); 36 // 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 37 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(); 38 foreach (var item in listDesc) 39 { 40 Console.WriteLine($"item:{item}"); 41 } 42 Console.WriteLine("查询表达式降序排序"); 43 var listExpressDesc = from p in listProduct orderby p.CategoryId descending, p.Price descending select new { id = p.CategoryId, ProductName = p.Name, ProductPrice = p.Price, PublishTime = p.CreateTime }; 44 foreach (var item in listExpressDesc) 45 { 46 Console.WriteLine($"item:{item}"); 47 } 48 Console.ReadKey(); 49 } 50 } 51 }
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:
1 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:
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6 7 namespace ThenBy 8 { 9 class Program 10 { 11 static void Main(string[] args) 12 { 13 string[] str = { "A", "B", "C", "D", "E"}; 14 var query = str.Select(p => p).ToList(); 15 query.Reverse(); 16 foreach (var item in query) 17 { 18 Console.WriteLine(item); 19 } 20 21 Console.ReadKey(); 22 } 23 } 24 }
运行效果: