LINQ的技术演进

以一个简单的例子来说明：

var developersUsingCSharp =

   from d in developers
   where d.Language == "C#"
   select d.Name;

1）提供对IEnumerable<T>的Extension Method使得它可以将where, select应用到任意实现了IEnumerable<T>接口的对象上；

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

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

     由此得到演进的No.1形式：

      var filteredDevelopers = developers.Where(delegate (Developer d) {
                return (d.Language == "C#");
      });

2）采用lambda expression实现anonymous delegate;

     由此得到演进的No.2形式：

     IEnumerable<string> developersUsingCSharp =
           developers
           .Where(d => d.Language == "C#")
           .Select(d => d.Name);

3）C# Compiler将Linq Query Expression翻译成lambda expression的形式。由此得到了对用户最友好的方式。

     在有些特殊情况下依然需要使用扩展方法来实现。

Note-1: Group and Into Clause(可以使用LINQPad进行练习)

class Developer
	{
	  public string Name {get;  set;}
	  public string Language {get;set;}
	  public int Age {get;set;}
	}
void Main()
{	
	Developer[] developers = new Developer[] {
	new Developer { Name = "Paolo", Language = "C#", Age = 32 },
	new Developer { Name = "Marco", Language = "C#", Age = 37},
	new Developer { Name = "Frank", Language = "VB.NET", Age = 48 },
	};
	
	var developersGroupedByLanguage =
	   from d in developers
	   //group d by d.Language;
	   group d by new { d.Language, AgeCluster = (d.Age / 10) * 10 };
	foreach (var group in developersGroupedByLanguage) {
		Console.WriteLine("Language: {0}", group.Key);
		foreach (var item in group) {
			Console.WriteLine("\t{0}-{1}", item.Name,item.Age);
		}
	}
	
	var developersGroupedByLanguage2 =
		from d in developers
		group d by d.Language into developersGrouped
		select new {
			Language = developersGrouped.Key,
			DevelopersCount = developersGrouped.Count()
		};
		foreach (var group in developersGroupedByLanguage2) {
			Console.WriteLine ("Language {0} contains {1} developers",
			group.Language, group.DevelopersCount);
		}
}

输出结果如下：