C#知识点集锦（四）LINQ入门

先看2个例子

例子1.选择整数数组中的偶数，并且顺序排列。

            int[] numbers = { 5, 10, 8, 3, 6, 12 };
            IEnumerable<int> numQuery = numbers.Where(num => num % 2 == 0).OrderBy(n => n).Select(o=>o);
            foreach (int i in numQuery)
                Console.Write(i + " ");

输出 6 8 10 12

例子2.从一组对象中，选择Name 以J为起始，Rank 降序排列，结果为对象中Name的集合。

    public class person
    {
        public string Name { get; set; }
        public int Rank { get; set; }
    }

            person[] ps = new person[]{
                new person(){Name="Jerry",Rank=3},
                new person(){Name="Tom",Rank=4},
                new person(){Name="Larry",Rank=2},
                new person(){Name="John",Rank=23}
            };
            var result = ps.Where(e => e.Name.StartsWith("J")).OrderByDescending(n => n.Rank).Select(o => o.Name);
            foreach(var v in result)
            {
                Console.WriteLine(v);
            }

输出

John

Jerry

说明：

Where OrderBy Select都是IEnumerable<TSource>的扩展方法

public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate);


public static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector);


其中public delegate TResult Func<T1, T2, TResult>(T1 arg1, T2 arg2);


public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector);


其中
public delegate TResult Func<T, TResult>(T arg);

 例子3. 词典中根据Value查找Key的集合。

            Dictionary<int, string> c = new Dictionary<int, string>();
            c.Add(1, "11");
            c.Add(2, "8");
            c.Add(3,"12");
            c.Add(4,"12");
            c.Add(5,"13");
            var result = c.Where(e => e.Value.Equals("12")).Select(e => e.Key);

因为Dictionary本质是IEnumerable<KeyValuePair<TKey, TValue>>

 例子4.对字符串数组进行分组。

List<string> list = new List<string>() { "1_32", "2_10", "1_8", "1_25", "2_3", "3_5", "5_15", "3_16" };

            var groupList2 = list.GroupBy(s => s.Split('_')[0]).Select(g => g);

            foreach(var group in groupList2)
            {
                foreach(var item in group)
                {
                    Console.WriteLine("With Key " + group.Key + " " + item.ToString());
                }
            }

用_前的数字做key分组，打印每组的值

输出：

With Key 1 1_32
With Key 1 1_8
With Key 1 1_25
With Key 2 2_10
With Key 2 2_3
With Key 3 3_5
With Key 3 3_16
With Key 5 5_15

或者输出每组 的个数

            var groupList3=list.GroupBy(s=>s.Split('_')[0]).Select(g=>new {mykey=g.Key,count=g.Count()});
            foreach(var item in groupList3)
            {
                Console.WriteLine("With Key " + item.mykey + " has " + item.count);
            }

输出：

With Key 1 has 3
With Key 2 has 2
With Key 3 has 2
With Key 5 has 1

参阅：

https://www.cnblogs.com/heyuquan/p/Linq-to-Objects.html

https://www.cnblogs.com/willick/p/13586024.html