LINQ to Object——立即执行的Enumerable类方法

前面说到LINQ to Object——延时执行的Enumerable类的方法，接下来说说LINQ to Object——立即执行的Enumerable类方法。

1.ToArray 序列转换成数组

            List<string> names_list = new List<string> { "张三", "范冰冰", "李冰冰", "迈克尔·杰克逊", "李四", "王五", "赵六", "田七" };
            string[] takenames_arry = names_list.ToArray();

            string[] takenames_arry2 = (from name in names_list
                                   select name).Take(4).ToArray();
            foreach (var name in takenames_arry2)
            {
                Console.WriteLine(name);
            }
            Console.WriteLine("----------------------------");
            Console.ReadKey();

运行结果：

2.ToList 序列转换成List

            string[] names_array = { "张三", "范冰冰", "李冰冰", "迈克尔·杰克逊", "李四", "王五", "赵六", "田七" };
            List<string> takenames_list = names_array.ToList();

            List<string> takenames_list2 = (from name in names_array select name).Skip(4).ToList();
            foreach (var name in takenames_list2)
            {
                Console.WriteLine(name);
            }
            Console.WriteLine("----------------------------");
            Console.ReadKey();

运行结果：

3.ToDictionary 把序列转换为泛型Dictionary

            List personList = new List()
            {
                new Person(){ Name = "张三", Age = 12, Phone = "555555"},
                new Person(){ Name = "李四", Age = 23, Phone = "666666"},
                new Person(){ Name = "王五", Age = 14, Phone = "777777"},
                new Person(){ Name = "赵六", Age = 25, Phone = "888888"},
                new Person(){ Name = "田七", Age = 16, Phone = "999999"},
            };

            Dictionary<string, Person> dictionary_person = personList.ToDictionary(guest => guest.Name);
            foreach (var person in dictionary_person)
            {
                Console.WriteLine("键：{0}——值：{1} {2} {3}", person.Key, person.Value.Name, person.Value.Age, person.Value.Phone);
            }
            Console.WriteLine("--------------------------------");
            Console.ReadKey();

            Dictionary<string, string> dictionary_person2 = personList.ToDictionary(person => person.Name, person => person.Phone);
            foreach (var person in dictionary_person2)
            {
                Console.WriteLine("键：{0}——值：{1}", person.Key, person.Value);
            }
            Console.WriteLine("--------------------------------");
            Console.ReadKey();

运行结果：

注意：Dictionary的Key和Value是一一对应关系。

4.ToLookup 用于将序列转换为泛型Lookup

和ToDictionary类似，不细写代码了。

注意：Lookup的Key和Value是一对多关系，Lookup没有公共构造函数，只能用ToLookup构建，创建后也不能删除Lookup中的元素。

5.SequenceEqual 比较两个序列是否相等

            bool sequence_equal = names_array.SequenceEqual(names_list);
            bool sequence_equal2 = names_array.Skip(1).Take(2).SequenceEqual(names_list.Take(3).SkipWhile(n => n.Length == 2));
            Console.WriteLine("{0},{1}", sequence_equal, sequence_equal2);
            Console.WriteLine("----------------------------");
            Console.ReadKey();

运行结果：

6.First 返回序列第一个满足条件元素

            var first_name = names_array.First();
            var first_name2 = names_array.First(n => n.Length == 3);
            Console.WriteLine("{0},{1}", first_name, first_name2);
            Console.WriteLine("----------------------------");
            Console.ReadKey();

运行结果：

7.FirstOrDefault 返回序列第一个满足条件元素，如果没有找到则返回默认值

            var first_or_default_name = names_array.FirstOrDefault();
            var first_or_default_name2 = names_array.FirstOrDefault(n => n == "123");
            Console.WriteLine("{0},{1}", first_or_default_name, first_or_default_name2);
            Console.WriteLine("----------------------------");
            Console.ReadKey();

运行结果：

8.Last 返回序列最后一个满足条件元素

            var last_name = names_array.Last();
            var last_name2 = names_array.LastOrDefault(n => n.Length == 3);
            Console.WriteLine("{0},{1}", last_name, last_name2);
            Console.WriteLine("----------------------------");
            Console.ReadKey();

运行结果：

9.LastOrDefault 返回序列最后一个满足条件元素，如果没有找到则返回默认值

            var last_or_default_name = names_array.LastOrDefault();
            var last_or_default_name2 = names_array.LastOrDefault(n => n == "123");
            Console.WriteLine("{0},{1}", last_or_default_name, last_or_default_name2);
            Console.WriteLine("----------------------------");
            Console.ReadKey();

运行结果：

10.Single 返回序列中唯一的元素，注意：如果序列中包含多个元素，会引发运行错误

            try
            {
                var single_name = names_array.Single();
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            Console.WriteLine("----------------------------");
            var single_name2 = names_array.Single(n => n == "张三");
            Console.WriteLine("{0}", single_name2);
            Console.WriteLine("----------------------------");
            Console.ReadKey();

运行结果：

11.SingleOrDefault 找出序列中满足一定条件的元素，注意：如果序列为空则返回默认值， 如果序列中包含多个多个元素会引发运行错误

            try
            {
                var single_or_default_name = Enumerable.Empty<int>().SingleOrDefault();
                Console.WriteLine("{0}", single_or_default_name);//不报错，如果序列为空就返回默认值 
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            Console.WriteLine("----------------------------");

            try
            {
                var single_or_default_name2 = names_array.SingleOrDefault();
                Console.WriteLine("{0}", single_or_default_name2);//报错，序列包含多行错误
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            Console.WriteLine("----------------------------");

            var single_or_default_name3 = Enumerable.Empty<string>().DefaultIfEmpty("默认值").SingleOrDefault();
            Console.WriteLine("{0}", single_or_default_name3);
            Console.WriteLine("----------------------------");

            var single_or_default_name4 = names_array.SingleOrDefault(n => n == "123");
            Console.WriteLine("{0}", single_or_default_name4);
            Console.WriteLine("----------------------------");
            Console.ReadKey();

运行结果：

12.ElementAt 获得指定索引处的元素

            var element_at_name = names_array.ElementAt(3);
            Console.WriteLine("{0}", element_at_name);
            Console.WriteLine("----------------------------");
            Console.ReadKey();

运行结果：

13.ElementAtOrDefault 获得指定索引处的元素，如果超出索引，则返回元素类型的默认值

            var element_at_or_default_name = names_array.ElementAtOrDefault(5);
            var element_at_or_default_name2 = names_array.ElementAtOrDefault(10);
            Console.WriteLine("{0},{1}", element_at_or_default_name, element_at_or_default_name2);
            Console.WriteLine("----------------------------");
            Console.ReadKey();

运行结果：

14.All 序列中的所有元素是否都满足条件

            bool all_names = names_array.All(s => s.GetTypeCode() == TypeCode.String);
            bool all_names2 = names_array.All(s => s.Length < 10);
            Console.WriteLine("{0},{1}", all_names, all_names2);
            Console.WriteLine("----------------------");
            Console.ReadKey();

运行结果：

15.Any 序列中的元素是否存在或满足条件

            bool any_names = names_array.Any();
            bool any_names2 = names_array.Any(s => s.Length > 7);
            Console.WriteLine("{0},{1}", any_names, any_names2);
            Console.WriteLine("----------------------");
            Console.ReadKey();

运行结果：

16.Contains 确定元素是否在序列中

            bool contains_name = names_array.Contains("田");
            Console.WriteLine("{0}", contains_name);
            Console.WriteLine("----------------------");
            Console.ReadKey();

运行结果：

17.Count 序列包含元素的数量

            int count_names = names_array.Count();
            int count_names2 = names_array.Count(n => n.Length == 7);
            Console.WriteLine("{0},{1}", count_names, count_names2);
            Console.WriteLine("----------------------");
            Console.ReadKey();

运行结果：

18.LongCount 获取一个Int64类型的元素数量

            long longcount_names = names_array.LongCount();
            long longcount_names2 = names_array.LongCount(n => n.Length == 3);
            Console.WriteLine("{0},{1}", longcount_names, longcount_names2);
            Console.WriteLine("----------------------");
            Console.ReadKey();

运行结果：

19.Aggregate 将序列元素进行累加

            int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 21, 22, 232, 8, 64, 9, 46, 7 };
            int aggregate_numbers = numbers.Aggregate((n1, n2) => n1 + n2);
            int aggregate_numbers2 = numbers.Aggregate(99, (n1, n2) => n1 + n2);            
            Console.WriteLine("{0},{1}", aggregate_numbers, aggregate_numbers2);
            Console.WriteLine("----------------------");
            Console.ReadKey();

            string aggregate_names = names_array.Aggregate((name1, name2) => string.Format("{0}、{1}", name1, name2));//相当于name1 += name2
            string aggregate_names2 = names_array.Aggregate("累加结果：", (name1, name2) => string.Format("{0}、{1}", name1, name2));
            Console.WriteLine("{0}", aggregate_names);
            Console.WriteLine("{0}", aggregate_names2);
            Console.WriteLine("----------------------");
            Console.ReadKey();

运行结果：

20.Sum 序列之和

21.Average 序列平均值

22.Min 序列的最小值

23.Max 序列的最大值

            //20、Sum序列之和
            int sum_numbers = numbers.Sum();
            Console.WriteLine(sum_numbers);
            Console.WriteLine("----------------------");

            //21、Average序列平均值
            double avg_numbers = numbers.Average();
            Console.WriteLine(avg_numbers);
            Console.WriteLine("----------------------");

            //22、Min序列的最小值
            int min_numbers = numbers.Min();
            Console.WriteLine(min_numbers);
            Console.WriteLine("----------------------");

            //23、Max序列的最大值
            int max_numbers = numbers.Max();
            Console.WriteLine(max_numbers);
            Console.WriteLine("----------------------");
            Console.ReadKey();

运行结果：

常用立即执行的Enumerable类方法学习和整理完毕！

转载于:https://www.cnblogs.com/jiechou/p/9226494.html