21.C#序列过虑、排序、let子句和连接(十一章11.3-11.5)

 1 class Jewellery
 2 {
 3     /// <summary>
 4     /// 珠宝类型
 5     /// <list type="Ring">戒指</list>
 6     /// <list type="Necklace">项链</list>
 7     /// <list type="Bracelet">手链</list>
 8     /// </summary>
 9     public enum JewelleryType
10     {
11         Ring,
12         Necklace,
13         Bracelet
14     }
15     /// <summary>
16     /// 当前珠宝状态
17     /// <list type="Stock">存货</list>
18     /// <list type="Repair">修理中</list>
19     /// <list type="Sold">已售出</list>
20     /// </summary>
21     public enum JewelleryState
22     {
23         Stock,
24         Repair,
25         Sold
26     }
27     public JewelleryType Type { get; set; }
28     public double Price { get; set; }
29     public JewelleryState State { get; set; }
30 }

 1 List<Jewellery> list = new List<Jewellery>()
 2 {
 3     new Jewellery() {Type=Jewellery.JewelleryType.Bracelet,State=Jewellery.JewelleryState.Repair,Price=100 },
 4     new Jewellery() {Type=Jewellery.JewelleryType.Necklace,State=Jewellery.JewelleryState.Sold,Price=200 },
 5     new Jewellery() {Type=Jewellery.JewelleryType.Ring,State=Jewellery.JewelleryState.Stock,Price=300 },
 6     new Jewellery() {Type=Jewellery.JewelleryType.Necklace,State=Jewellery.JewelleryState.Sold,Price=400 },
 7     new Jewellery() {Type=Jewellery.JewelleryType.Bracelet,State=Jewellery.JewelleryState.Stock,Price=500 },
 8     new Jewellery() {Type=Jewellery.JewelleryType.Ring,State=Jewellery.JewelleryState.Repair,Price=600 }
 9 };
10 
11 //查找价格大于300的手链
12 var result = from item in list
13              where item.Price > 300 && item.Type == Jewellery.JewelleryType.Bracelet
14              select item;
15 //转换为list.Where(item => item.Price > 300).Where(item => item.Type == Jewellery.JewelleryType.Bracelet);
16 Console.WriteLine(result.Count());
17 
18 //记录当前正在修理中的珠宝总值
19 var result1 = (from item in list
20                where item.State == Jewellery.JewelleryState.Repair
21                select item).Sum(item => item.Price);
22 //转换为list.Where(item => item.State == Jewellery.JewelleryState.Repair).Sum(item => item.Price);
23 Console.WriteLine(result1);
24 
25 Console.ReadKey();

1 var resutl2 = from item in list
2               select item;
3 //这个就是所谓的退化表达式，编译器会故意生成一个对Select方法的调用，即使它什么都没有做，但result2和list有很大的不同，
4 //两个序列返回的数据是相同的，但是Select方法的结果只是数据项的序列，而不是数据源本身。查询表达式的结果和源数据永远不会是同一个对象
5 //当有其它的操作的时候，就不用为编译器保留一个"空操作"Select子句

1 //按价格排序，从大到小
2 var resutl3 = from item in list
3               orderby item.Price descending
4               select item;
5 //转换为list.OrderByDescending(item => item.Price);

1 //先用价格再按状态排序
2 var result4 = from item in list
3               orderby item.Price, item.State
4               select item;

1 //货品是用RMB的，现在使用HK来记录，使用投影返回匿名对象的序列
2 var result5 = from item in list
3               let hk = item.Price / 0.8
4               select new { hk = hk, item = item };

 1 /// <summary>
 2 /// 用于存放珠宝的盒子
 3 /// </summary>
 4 class Box
 5 {
 6     /// <summary>
 7     /// 盒子有一个珠宝类型的属性，与要放的珠宝类型一致
 8     /// </summary>
 9     public Jewellery.JewelleryType jewelleryType { get; set; }
10     public string BoxName { get; set; }
11 }

1 //打印每个珠宝存放的盒子,珠宝的状态要是存货
2 var result6 = from box in boxList
3               join item in list
4               on box.jewelleryType equals item.Type
5               where item.State == Jewellery.JewelleryState.Stock
6               select new { BoxName = box.BoxName, Price = item.Price };
7 
8 result6.ToList().ForEach(item => Console.WriteLine("位置:" + item.BoxName + ";" + "价格:" + item.Price));