LINQ基础(一)

　　LINQ(Language Integrated Query,语言集成查询)，在C#语言中集成了查询语法，可以用相同的语法访问不同的数据源。
　　LINQ提供了不同数据源的抽象层，所以可以使用相同的语法。
　　这里主要介绍LINQ的核心原理和C#中支持C# LINQ查询的语言扩展。

1.语法
　　使用LINQ查询出来自巴西的所以世界冠军。这里可以使用List<T>类的FindAll()方法，但使用LINQ查询语法更简单　　

　　　　static void LINQQuery()
        {
          //
          var query = from r in Formula1.GetChampions()
                      where r.Country == "Brazil"
                      orderby r.Wins descending
                      select r;

          foreach (var r in query)
          {
            Console.WriteLine("{0:A}", r);
          }

        }    

　　变量query只指定了LINQ查询。该查询不是通过这个赋值语句执行的，而是使用foreach循环访问查询时执行的。

2.扩展方法
　　编译器会转换LINQ查询，以调用方法而不是LINQ查询。LINQ为IEnumerable<T>接口提供了各种扩展方法（扩展方法在http://www.cnblogs.com/afei-24/p/6703843.html介绍到），以便用户在实现了该接口的任意集合上使用LINQ查询。
　　定义LINQ扩展方法的一个类是System.Linq名称空间中的Enumerable。只需要导入这个名称空间，就打开了这个类的扩展方法的作用域。下面是Where()扩展方法的实现代码：

　　　　public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source,Func<TSource,bool> predicate)
        {
            foreach(TSource item in source)
            {
                if(predicate(item))
                {
                    yield return item;
                }

            }
        }

　　因为Where()作为一个泛型方法，所以它可以用于包含在集合中的任意类型。实现了IEnumerable<T>接口的任意集合都支持它。

3.推迟查询的执行
　　前面提到，在运行期间定义LINQ查询表达式时，查询不会运行。查询在迭代数据项时才会运行。
　　举个例子：　　

　　　　static void DeferredQuery()
        {
          var names = new List<string> { "Nino", "Alberto", "Juan", "Mike", "Phil" };

          var namesWithJ = from n in names
                           where n.StartsWith("J")
                           orderby n
                           select n;

          Console.WriteLine("First iteration");
          foreach (string name in namesWithJ)
          {
            Console.WriteLine(name);
          }
          Console.WriteLine();

          names.Add("John");
          names.Add("Jim");
          names.Add("Jack");
          names.Add("Denny");

          Console.WriteLine("Second iteration");
          foreach (string name in namesWithJ)
          {
            Console.WriteLine(name);
          }

        }

　　输出：
　　
　　因为查询在迭代时才执行，所以在第一次输出后有添加项再输出，会显示又添加的项。

　　但在调用方法ToArray(),ToList等方法时，不会延迟执行：　　

　　　　static void NotDeferredQuery()
        {
            var names = new List<string> { "Nino", "Alberto", "Juan", "Mike", "Phil" };

            var namesWithJ = (from n in names
                             where n.StartsWith("J")
                             orderby n
                             select n).ToList();

            Console.WriteLine("First iteration");
            foreach (string name in namesWithJ)
            {
                Console.WriteLine(name);
            }
            Console.WriteLine();

            names.Add("John");
            names.Add("Jim");
            names.Add("Jack");
            names.Add("Denny");

            Console.WriteLine("Second iteration");
            foreach (string name in namesWithJ)
            {
                Console.WriteLine(name);
            }

        }

　　输出：
　　

下面是用到的类，后续的也需要用到这些代码。

//这个类创建需要的列表
        public static class Formula1
          {
            private static List<Racer> racers;
            //返回一组赛车手
            public static IList<Racer> GetChampions()
            {
              if (racers == null)
              {
                racers = new List<Racer>();
                racers.Add(new Racer("Nino", "Farina", "Italy", , ,
　　　　　　　　　　 new int[] {  }, new string[] { "Alfa Romeo" }));
                racers.Add(new Racer("Alberto", "Ascari", "Italy", , , 
　　　　　　　　　　new int[] { ,  }, new string[] { "Ferrari" }));
                racers.Add(new Racer("Juan Manuel", "Fangio", "Argentina", , ,
　　　　　　　　　　 new int[] { , , , ,  }, new string[] { "Alfa Romeo", "Maserati", "Mercedes", "Ferrari" }));
                racers.Add(new Racer("Mike", "Hawthorn", "UK", , ,
　　　　　　　　　　 new int[] {  }, new string[] { "Ferrari" }));
                racers.Add(new Racer("Phil", "Hill", "USA", , ,
　　　　　　　　　　 new int[] {  }, new string[] { "Ferrari" }));
                racers.Add(new Racer("John", "Surtees", "UK", , ,
　　　　　　　　　　 new int[] {  }, new string[] { "Ferrari" }));
                racers.Add(new Racer("Jim", "Clark", "UK", , , 
　　　　　　　　　　new int[] { ,  }, new string[] { "Lotus" }));
                racers.Add(new Racer("Jack", "Brabham", "Australia", , ,
　　　　　　　　　　 new int[] { , ,  }, new string[] { "Cooper", "Brabham" }));
                racers.Add(new Racer("Denny", "Hulme", "New Zealand", , , 
　　　　　　　　　　new int[] {  }, new string[] { "Brabham" }));
              }

              return racers;
            }

            private static List<Team> teams;
            //返回一组冠军车队
            public static IList<Team> GetContructorChampions()
            {
              if (teams == null)
              {
                teams = new List<Team>()
                        {
                            new Team("Vanwall", ),
                            new Team("Cooper", , ),
                            new Team("Ferrari", , , , , , , , , , 
　　　　　　　　　　　　　　　　　　　　2000, , , , , , ),
                            new Team("BRM", ),
                            new Team("Lotus", , , , , , , ),
                            new Team("Brabham", , ),
                            new Team("Matra", ),
                            new Team("Tyrrell", ),
                            new Team("McLaren", , , , , , , , ),
                            new Team("Williams", , , , , , , , , ),
                            new Team("Benetton", ),
                            new Team("Renault", ,  ),
                            new Team("Brawn GP", ),
                            new Team("Red Bull Racing", , )
                        };
              }
              return teams;
            }

            private static List<Championship> championships;
            //返回GetChampionships类型的集合
            public static IEnumerable<Championship> GetChampionships()
            {
              if (championships == null)
              {
                championships = new List<Championship>();
                championships.Add(new Championship
                {
                  Year = ,
                  First = "Nino Farina",
                  Second = "Juan Manuel Fangio",
                  Third = "Luigi Fagioli"
                });
                championships.Add(new Championship
                {
                  Year = ,
                  First = "Juan Manuel Fangio",
                  Second = "Alberto Ascari",
                  Third = "Froilan Gonzalez"
                });
                championships.Add(new Championship
                {
                  Year = ,
                  First = "Alberto Ascari",
                  Second = "Nino Farina",
                  Third = "Piero Taruffi"
                });
                championships.Add(new Championship
                {
                  Year = ,
                  First = "Alberto Ascari",
                  Second = "Juan Manuel Fangio",
                  Third = "Nino Farina"
                });
                championships.Add(new Championship
                {
                  Year = ,
                  First = "Juan Manuel Fangio",
                  Second = "Froilan Gonzalez",
                  Third = "Mike Hawthorn"
                });
                championships.Add(new Championship
                {
                  Year = ,
                  First = "Juan Manuel Fangio",
                  Second = "Stirling Moss",
                  Third = "Eugenio Castellotti"
                });
                championships.Add(new Championship
                {
                  Year = ,
                  First = "Juan Manuel Fangio",
                  Second = "Stirling Moss",
                  Third = "Peter Collins"
                });

              }
              return championships;
            }
        }

//车手类

    [Serializable]
      public class Racer : IComparable<Racer>, IFormattable
      {
        public Racer(string firstName, string lastName, string country, int starts, int wins)
          : this(firstName, lastName, country, starts, wins, null, null)
        {
        }
        public Racer(string firstName, string lastName, string country, int starts,
　　　　　　 int wins, IEnumerable<int> years, IEnumerable<string> cars)
        {
          this.FirstName = firstName;
          this.LastName = lastName;
          this.Country = country;
          this.Starts = starts;
          this.Wins = wins;
          this.Years = new List<int>(years);
          this.Cars = new List<string>(cars);
        }
        //单值属性
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Country { get; set; }
        public int Wins { get; set; }
        public int Starts { get; set; }
        //多值属性，车手可能多次获得冠军，所在的车队也可能不同
        public IEnumerable<string> Cars { get; private set; }
        public IEnumerable<int> Years { get; private set; }

        public override string ToString()
        {
          return String.Format("{0} {1}", FirstName, LastName);
        }

        public int CompareTo(Racer other)
        {
          if (other == null) return -;
          return string.Compare(this.LastName, other.LastName);
        }

        public string ToString(string format)
        {
          return ToString(format, null);
        }

        public string ToString(string format,
              IFormatProvider formatProvider)
        {
          switch (format)
          {
            case null:
            case "N":
              return ToString();
            case "F":
              return FirstName;
            case "L":
              return LastName;
            case "C":
              return Country;
            case "S":
              return Starts.ToString();
            case "W":
              return Wins.ToString();
            case "A":
              return String.Format("{0} {1}, {2}; starts: {3}, wins: {4}",
                    FirstName, LastName, Country, Starts, Wins);
            default:
              throw new FormatException(String.Format("Format {0} not supported", format));
          }
        }
      }

//获得冠军的车队
        [Serializable]
        public class Team
        {
            public Team(string name, params int[] years)
            {
                this.Name = name;
                this.Years = new List<int>(years);
            }
            public string Name { get; private set; }
            public IEnumerable<int> Years { get; private set; }
        }

        //获奖选手和年份
          public class Championship
          {
            public int Year { get; set; }
            public string First { get; set; }
            public string Second { get; set; }
            public string Third { get; set; }
          }