Liqn基础

Linq：语言集成查询 (LINQ) 是 Visual Studio 2008 中引入的一组功能，可为 C# 和 Visual Basic 语言语法提供强大的查询功能。 LINQ 引入了标准易学的数据查询和更新模式，可以扩展该方法来支持任何类型的数据存储。 Visual Studio 包括 LINQ 提供程序集，后者支持将 LINQ 与 .NET Framework 集合、SQL Server 数据库、ADO.NET 数据集和 XML 文档结合使用。

语法形式：

  var 临时数据=  from 临时变量 in 集合对象或数据库对象 　
　           where 条件表达式 　　
               order by条件　　
               select 临时变量中被查询的值 　
               group by 条件

linq 查询与 Lamda表达式查询比对

  public static IList<Racer> GetChampions()
        {
            if (racers == null)
            {
                racers = new List<Racer>(40);
                racers.Add(new Racer("Nino", "Farina", "Italy", 33, 5, new int[] { 1950 }, new string[] { "Alfa Romeo" }));
                racers.Add(new Racer("Alberto", "Ascari", "Italy", 32, 10, new int[] { 1952, 1953 }, new string[] { "Ferrari" }));
                racers.Add(new Racer("Juan Manuel", "Fangio", "Argentina", 51, 24, new int[] { 1951, 1954, 1955, 1956, 1957 }, new string[] { "Alfa Romeo", "Maserati", "Mercedes", "Ferrari" }));
                racers.Add(new Racer("Mike", "Hawthorn", "UK", 45, 3, new int[] { 1958 }, new string[] { "Ferrari" }));
                racers.Add(new Racer("Phil", "Hill", "USA", 48, 3, new int[] { 1961 }, new string[] { "Ferrari" }));
                racers.Add(new Racer("John", "Surtees", "UK", 111, 6, new int[] { 1964 }, new string[] { "Ferrari" }));
                racers.Add(new Racer("Jim", "Clark", "UK", 72, 25, new int[] { 1963, 1965 }, new string[] { "Lotus" }));
                racers.Add(new Racer("Jack", "Brabham", "Australia", 125, 14, new int[] { 1959, 1960, 1966 }, new string[] { "Cooper", "Brabham" }));
                racers.Add(new Racer("Denny", "Hulme", "New Zealand", 112, 8, new int[] { 1967 }, new string[] { "Brabham" }));
                racers.Add(new Racer("Graham", "Hill", "UK", 176, 14, new int[] { 1962, 1968 }, new string[] { "BRM", "Lotus" }));
                racers.Add(new Racer("Jochen", "Rindt", "Austria", 60, 6, new int[] { 1970 }, new string[] { "Lotus" }));
                racers.Add(new Racer("Jackie", "Stewart", "UK", 99, 27, new int[] { 1969, 1971, 1973 }, new string[] { "Matra", "Tyrrell" }));
                racers.Add(new Racer("Emerson", "Fittipaldi", "Brazil", 143, 14, new int[] { 1972, 1974 }, new string[] { "Lotus", "McLaren" }));
                racers.Add(new Racer("James", "Hunt", "UK", 91, 10, new int[] { 1976 }, new string[] { "McLaren" }));
                racers.Add(new Racer("Mario", "Andretti", "USA", 128, 12, new int[] { 1978 }, new string[] { "Lotus" }));
                racers.Add(new Racer("Jody", "Scheckter", "South Africa", 112, 10, new int[] { 1979 }, new string[] { "Ferrari" }));
                racers.Add(new Racer("Alan", "Jones", "Australia", 115, 12, new int[] { 1980 }, new string[] { "Williams" }));
                racers.Add(new Racer("Keke", "Rosberg", "Finland", 114, 5, new int[] { 1982 }, new string[] { "Williams" }));
                racers.Add(new Racer("Niki", "Lauda", "Austria", 173, 25, new int[] { 1975, 1977, 1984 }, new string[] { "Ferrari", "McLaren" }));
                racers.Add(new Racer("Nelson", "Piquet", "Brazil", 204, 23, new int[] { 1981, 1983, 1987 }, new string[] { "Brabham", "Williams" }));
                racers.Add(new Racer("Ayrton", "Senna", "Brazil", 161, 41, new int[] { 1988, 1990, 1991 }, new string[] { "McLaren" }));
                racers.Add(new Racer("Nigel", "Mansell", "UK", 187, 31, new int[] { 1992 }, new string[] { "Williams" }));
                racers.Add(new Racer("Alain", "Prost", "France", 197, 51, new int[] { 1985, 1986, 1989, 1993 }, new string[] { "McLaren", "Williams" }));
                racers.Add(new Racer("Damon", "Hill", "UK", 114, 22, new int[] { 1996 }, new string[] { "Williams" }));
                racers.Add(new Racer("Jacques", "Villeneuve", "Canada", 165, 11, new int[] { 1997 }, new string[] { "Williams" }));
                racers.Add(new Racer("Mika", "Hakkinen", "Finland", 160, 20, new int[] { 1998, 1999 }, new string[] { "McLaren" }));
                racers.Add(new Racer("Michael", "Schumacher", "Germany", 287, 91, new int[] { 1994, 1995, 2000, 2001, 2002, 2003, 2004 }, new string[] { "Benetton", "Ferrari" }));
                racers.Add(new Racer("Fernando", "Alonso", "Spain", 177, 27, new int[] { 2005, 2006 }, new string[] { "Renault" }));
                racers.Add(new Racer("Kimi", "Räikkönen", "Finland", 148, 17, new int[] { 2007 }, new string[] { "Ferrari" }));
                racers.Add(new Racer("Lewis", "Hamilton", "UK", 90, 17, new int[] { 2008 }, new string[] { "McLaren" }));
                racers.Add(new Racer("Jenson", "Button", "UK", 208, 12, new int[] { 2009 }, new string[] { "Brawn GP" }));
                racers.Add(new Racer("Sebastian", "Vettel", "Germany", 81, 21, new int[] { 2010, 2011 }, new string[] { "Red Bull Racing" }));
            }
            return racers;
        }

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

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

            // Lamda表达式查询
            IList<Racer> list = Formula1.GetChampions().Where(r => r.Country == "Brazil").OrderBy(a => a.Wins).ToList();

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

Formula1.GetChampions()是个集合对象。我们这边用了lamda表达式也可以很快的查询 。from、where、orderby、 descending和select都是这个查询中的预定义关键字，查询语句必须以form开头 以select 或者 group 子句结尾。变量query 指定了linq 查询，该语句不是通过赋值语句查询的，只要使用foreach循环方法var可以将查询结果的类型显式声明为 IEnumerable<string>。 但是，在第二个表达式中必须使用 var，因为结果是一个匿名类型集合，而该类型的名称只有编译器本身可以访问。

编译器会转换linq查询，但是调用方法而不是linq查询。这里的Where是个扩展方法。