Linq

Linq

所有常用用法汇总介绍整理，来自于 LINQ之路系列博客导航 - Life a Poem - 博客园 (cnblogs.com)

通用示例为

     string[] names = { "Tom", "Dick", "Harry", "Mary", "Jay" };

• Where：返回符合给定条件的elements子集（Where支持第二个可选参数为index位置）

    var query = names.Where((n, i) => i % 2 == 0);
  

• Take & Skip：返回开始的N个元素，忽略剩下的元素；忽略开始的N个元素，返回之后的元素

      // 假设用户在一个图书数据库中查找包含"mercury"的所有图书
      // 如果查找结果包含100条记录，下面的查询会返回前面20条
      IQueryable<Book> query = dataContext.Books
          .Where(b => b.Title.Contains("mercury"))
          .OrderBy(b => b.Title)
          .Take(20);
  
      // 下面的查询则返回第21到40行数据（第2页）
      IQueryable<Book> query = dataContext.Books
          .Where(b => b.Title.Contains("mercury"))
          .OrderBy(b => b.Title)
          .Skip(20).Take(20);
      }
  

• TakeWhile & SkipWhile：遍历输入，返回每一个元素，直到遇到一个不符合的，忽略剩下的；遍历输入，忽略每一个元素，直到遇到一个不符合的，返回剩下的

      int[] numbers = { 3, 5, 2, 234, 4, 1 };
      var takeWhileSmall = numbers.TakeWhile(n => n < 100); // { 3, 5, 2 }
      var skipWhileSmall = numbers.SkipWhile(n => n < 100); // { 234, 4, 1 }
  

• Distinct：去除重复

      char[] distinctLetters = "HelloWorld".Distinct().ToArray();
      string s = new string(distinctLetters); // HeloWrd
  

• Select： 转换每一个元素，个数一样

      string[] names = { "Tom", "Dick", "Harry", "Mary", "Jay" };
      IEnumerable<string> query = names
          .Select((s, i) => i + "=" + s); // { "0=Tom", "1=Dick", ... }
  

• SelectMany：转换元素，个数不一样，多层Select

      string[] fullNames = { "Anne Williams", "John Fred Smith", "Sue Green" };
      IEnumerable<string> query = fullNames.SelectMany(name => name.Split());
      foreach (string name in query)
          Console.Write(name + "|"); // Anne|Williams|John|Fred|Smith|Sue|Green|
  

• OrderBy & OrderByDescending：升序排列；降序排列

• ThenBy & ThenByDescending：前一次排序相同键值元素重新升序排列；降序排列

     string[] names = { "Tom", "Dick", "Harry", "Mary", "Jay" };
     
     //按姓名长度进行排序
     IEnumerable<string> query = names.OrderBy(s => s.Length);
     // Result: { "Jay", "Tom", "Mary", "Dick", "Harry" };
     
   //Jay和Tom、Mary和Dick，除非我们添加额外的ThenBy运算符
     IEnumerable<string> query = names.OrderBy(s => s.Length).ThenBy(s => s);
     // Result: { "Jay", "Tom", "Dick", "Mary", "Harry" };
  

• GroupBy：读取每一个元素，放入一个临时列表中，key相同的元素会被放入同一个子列表中

      string[] files = Directory.GetFiles("c:\\temp");
      IEnumerable<IGrouping<string, string>> query =
          files.GroupBy(file => Path.GetExtension(file));
  
      foreach (IGrouping<string, string> grouping in query)
      {
          Console.WriteLine("Extension: " + grouping.Key);
          foreach (string filename in grouping)
              Console.WriteLine(" - " + filename);
      }
      
      // Result:
      Extension: .pdf
       - chapter03.pdf
       - chapter04.pdf
      Extension: .doc
       - todo.doc
       - menu.doc
       - Copy of menu.doc
      ...
  

• First & FirstOrDefault：第一个元素

• Last & LastOrDefault：最后一个元素

      int[] numbers = { 1, 2, 3, 4, 5 };
  
      int firstEven = numbers.First(n => n % 2 == 0);     // 2
      int lastEven = numbers.Last(n => n % 2 == 0);       // 4
      
      int firstBigError = numbers.First(n => n > 10); // Exception
      int firstBigNumber = numbers.FirstOrDefault(n => n > 10); // 0
  

• ElementAt & ElementOrDefault：特定位置元素

      int[] numbers = { 1, 2, 3, 4, 5 };
      int third = numbers.ElementAt(2);       // 3
      
      int tenthError = numbers.ElementAt(9);  // Exception
      int tenth = numbers.ElementAtOrDefault(9);  // 0
  

• Count & LongCount：返回元素个数；返回元素个数（64-bit整数）

• Min & Max：返回最小元素；返回最大元素

• Sum & Average：求和；求平均数

• Contains：包含

• Any：任意元素满足条件返回true

• All：所有元素满足条件返回true

• SequenceEqual：比较两个sequence，若他们拥有相同元素、相同顺序，返回true

• Concat & Union：两个sequence所有元素；两个sequence所有元素，去重

      int[] seq1 = { 1, 2, 3 }, seq2 = { 3, 4, 5 };
      IEnumerable<int> concat = seq1.Concat(seq2); // { 1, 2, 3, 3, 4, 5 }
      IEnumerable<int> union = seq1.Union(seq2); // { 1, 2, 3, 4, 5 }
  

• Intersect & Except：两个sequence都存在元素；第一个sequence有，第二个sequence没有的所有元素

      int[] seq1 = { 1, 2, 3 }, seq2 = { 3, 4, 5 };
      IEnumerable<int>
      commonality = seq1.Intersect(seq2), // { 3 }
      difference1 = seq1.Except(seq2), // { 1, 2 }
      difference2 = seq2.Except(seq1); // { 4, 5 }
  

• Zip： 同步遍历两个sequence， 返回的sequence基于在每一个元素对上应用lambda表达式

      int[] numbers = { 3, 5, 7 };
      string[] words = { "three", "five", "seven", "ignored" };
      IEnumerable<string> zip = numbers.Zip (words, (n, w) => n + "=" + w);
   
      // 产生的sequence包含如下元素
      // 3=three
      // 5=five
      // 7=seven
  

• OfType & Cast：接收一个非泛型集合，返回一个泛型集合，前者遇到不能转换的会忽略，后者会抛异常

      DateTime offender = DateTime.Now;
      ArrayList classicList = new ArrayList(); // in System.Collections
      classicList.Add(offender);
      IEnumerable<int>
      sequence2 = classicList.OfType<int>(), // OK - ignores offending DateTime
      sequence3 = classicList.Cast<int>(); // Throws exception
  

• ToArray & ToList & ToDictionary：

• Repeat & Range：只能操作int整数，前者重复某个数值，后者是索引和元素个数

      foreach (int i in Enumerable.Repeat(5, 3))
          Console.Write(i + ""); // 5 5 5
      foreach (int i in Enumerable.Range(5, 5))
          Console.Write(i + ""); // 5 6 7 8 9
  

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隐式类型变量 var

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自动属性

简化定义字段和属性

注意：定义自动属性时，必须同时提供get和set关键字，所以不能定义只读或者只写的自动属性

    class Person
    {
        private string firstName = string.Empty;
        public string FirstName
        {
            get { return firstName; }
            set { firstName = value; }
        }
    }
//-------------------------------------------------------
    class Person
    {
        public string FirstName { get; set; }
    }

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匿名类型

相当于临时的自定义类，来封装一些数据

    var data = new { Name = "Hello", Age = 1 };
    Debug.LogFormat("{0},{1}", data.Name,data.Age);

匿名类型的相等语义，Equals重写时基于值，==重写时基于引用

    static void AnonymousTypeEqualityTest()
    {
        // 构建两个匿名类型，拥有相同的名称/值对
        var worker1 = new { FirstName = "Harry", SecondName = "Folwer", Level = 2 };
        var worker2 = new { FirstName = "Harry", SecondName = "Folwer", Level = 2 };

        // Equals测试
        if (worker1.Equals(worker2))
            Console.WriteLine("worker1 equals worker2");
        else
            Console.WriteLine("worker1 not equals worker2");

        // ==测试
        if (worker1 == worker2)
            Console.WriteLine("worker1 == worker2");
        else
            Console.WriteLine("worker1 != worker2");

        // Type Name测试
        if (worker1.GetType().Name == worker2.GetType().Name)
            Console.WriteLine("we are both the same type");
        else
            Console.WriteLine("we are different types");
    }

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拓展方法

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Lambda表达式

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对象初始化器

    public class Point
    {
        public int X { get; set; }
        public int Y { get; set; }
    }
 
    static void ObjectInitSyntax()
    {
        // 手动初始化各属性
        Point aPoint = new Point();
        aPoint.X = 10;
        aPoint.Y = 20;
 
        // 使用新的对象初始化语法进行初始化
        Point bPoint = new Point { X = 10, Y = 20 };
    }

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查询运算符

    string[] names = { "Tom", "Dick", "Harry", "Mary", "Jay" };
        
    var list = names
        .Where(n => n.Contains("a"))
        .OrderBy(n => n.Length)
        .Select(n => n.ToUpper());

    foreach (string name in list) Debug.Log(name);

---

子查询

    string[] names = { "Tom", "Dick", "Harry", "Mary", "Jay" };
    // 获取所有长度最短的名字（注意：可能有多个）
    IEnumerable<string> outQuery = names
        .Where(n => n.Length == names     

            .OrderBy(n2 => n2.Length)
            .Select(n2 => n2.Length).First());      // Tom, Jay"