C#中ToDictionary,ToLookup

学习标签： 集合

本文导读：C#中ToDictionary用于将集合转换为Dictionary，C#中ToLookup操作符将创建一个Lookup<TKey, TElement>对象，这是一个one-to-many集合，一个Key可以对应多个Value。Lookup，不像Dictionary, 是不可改变的。这意味着一旦你创建一个lookup, 你不能添加或删除元素。

一、C#中ToDictionary

ToDictionary中并没有给我们做key的重复值判断，那也就侧面说明ToDictionary在kv中只能是 “一对一”的关系，也就是v中永远只会有一条记录

 

定义

 

C# 代码   复制

public static Dictionary<TKey, TSource> ToDictionary<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector
)

 

实例

 

C# 代码   复制

      class Package
      {
         public string Company { get; set; }
         public double Weight { get; set; }
         public long TrackingNumber { get; set; }
      }
      public static void ToDictionaryEx1()
      {
         List<Package> packages =
             new List<Package>
                     { new Package { Company = "Coho Vineyard", Weight = 25.2, TrackingNumber = 89453312L },
                       new Package { Company = "Lucerne Publishing", Weight = 18.7, TrackingNumber = 89112755L },
                       new Package { Company = "Wingtip Toys", Weight = 6.0, TrackingNumber = 299456122L },
                       new Package { Company = "Adventure Works", Weight = 33.8, TrackingNumber = 4665518773L } };
         // Create a Dictionary of Package objects, 
         // using TrackingNumber as the key.
         Dictionary<long, Package> dictionary =
             packages.ToDictionary(p => p.TrackingNumber);
         foreach (KeyValuePair<long, Package> kvp in dictionary)
         {
            outputBlock.Text += String.Format(
                "Key {0}: {1}, {2} pounds",
                kvp.Key,
                kvp.Value.Company,
                kvp.Value.Weight) + "\\n";
         }
      }

 

二、C#中ToLookup

ToLookup（） 方法创建一个类似 字典（Dictionary ） 的列表，这是一个one-to-many集合，一个Key可以对应多个Value。

Lookup，不像Dictionary, 是不可改变的。 这意味着一旦你创建一个lookup, 你不能添加或删除元素。

 

定义

 

C# 代码   复制

public static ILookup<TKey, TSource> ToLookup<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector
)

 

实例1

 

C# 代码   复制

            var dic = ticketlist.ToLookup(i => i.OrderID);
            foreach (var item in dic)
            {
                Console.WriteLine("订单号:" + item.Key);
                foreach (var item1 in item)
                {
                    Console.WriteLine("\\t\\t" + item1.TicketNo + "  " + item1.Description);
                }
            }

 

实例2、得到某个类别的所有产品

 

C# 代码   复制

        private static void PrintCategory(ILookup<string, Product> productsByCategory,string categoryName)
        {
            foreach (var item in productsByCategory[categoryName])
            {
                Console.WriteLine(item);
            }
        }