Linq起源

书籍，书籍集合，从书籍集合中查找符合条件的书籍 

（一）书籍

public class Book
{
   public int Unid { get; set; }
   public string Name { get; set; }
   public int Price { get; set; }
   public string Publisher { get; set; }
}

（二）书籍集合

public static class BookHelper
{
    public static IList<Book> SearchBook(IList<Book> list)
    {
        IList<Book> _list = new List<Book>(); 
        foreach (Book book in list)
        {
           if (book.Price < 100)
              _list.Add(book);
        }
        return _list;
    }
}

从书籍集合中查找价格在100以下的书籍。这里采用了扩展方法。所以可以以

BookHelper.SearchBook的方式来调用方法

（三）测试

（1）准备

IList<Book> _list = new List<Book>();

[SetUp]
public void Init()
{
   _list.Add(new Book { Price = 20, Name = "book1" });
   _list.Add(new Book { Price = 320, Name = "book6" });
   _list.Add(new Book { Price = 200, Name = "book2" });
   _list.Add(new Book { Price = 80, Name = "book3" });
   _list.Add(new Book { Price = 870, Name = "book4" });
   _list.Add(new Book { Price = 250, Name = "book5" });
}

（2）测试

[Test(Description="原始方法")]
public void TestOrigin()
{
    List<Book> _newList = BookHelper.SearchBook(_list) as List<Book>;

   foreach (Book book in _newList)
      Console.WriteLine(book.Name);
}

（四）现在把价格判断分离到独立的方法

public static IList<Book> SearchBook1(IList<Book> list)
{
   IList<Book> _list = new List<Book>();

   foreach (Book book in list)
   {
      if (CheckCondition(book))
                _list.Add(book);
   }
   return _list;
}

public static bool CheckCondition(Book book)
{
    if (book.Price < 100)
        return true;
    return false;
 }

 

（五）测试

[Test(Description = "分离判断方法")]
public void TestOrigin1()
{
    List<Book> _newList = BookHelper.SearchBook1(_list) as List<Book>; 

    foreach (Book book in _newList)
        Console.WriteLine(book.Name);
}

（六）通过委托来传递判断方法

public delegate bool DCheckCondition(Book book);
/// <summary>
/// 传递分离的判断方法
/// </summary>
/// <param name="list"></param>
/// <returns></returns>
public static IList<Book> SearchBook2(IList<Book> list,DCheckCondition check)
{
    IList<Book> _list = new List<Book>(); 

    foreach (Book book in list)
    {
        if (check(book))
           _list.Add(book);
    }

    return _list;
}

（七）测试

[Test(Description = "传递分离的判断方法")]
public void TestOrigin2()
{
    List<Book> _newList = BookHelper.SearchBook2(_list,BookHelper.CheckCondition) as List<Book>;

    foreach (Book book in _newList)
        Console.WriteLine(book.Name);
}

（八）自定义判断方法

[Test(Description = "代理传递分离的判断方法")]
public void TestOrigin3()
 {
      List<Book> _newList = BookHelper.SearchBook2(_list, 
      new LOrigin.BookHelper.DCheckCondition(CheckCondition)) as List<Book>;

      foreach (Book book in _newList)
            Console.WriteLine(book.Name);
} 

public bool CheckCondition(Book book)
{
   if (book.Price < 100)
        return true;
   return false;
}

（九）使用匿名方法来替换

[Test(Description = "匿名方法")]
public void TestOrigin4()
{
    IList<Book> _newList = BookHelper.SearchBook2(_list, 
               delegate(Book book) { return book.Price < 100; });
    foreach (Book book in _newList)
        Console.WriteLine(book.Name);
}

（十）匿名方法符号化

[Test(Description = "匿名方法的符号化")]
public void TestOrigin5()
{
    IList<Book> _newList = BookHelper.SearchBook2(_list,book => book.Price <100);
    foreach (Book book in _newList)
       Console.WriteLine(book.Name);
}

（十一）

[Test(Description = "IList的静态方法Where，并采用goes to")]
public void TestOrigin6()
{
    IList<Book> _newList = _list.Where(book => book.Price < 100).ToArray<Book>();

    foreach (Book book in _newList)
        Console.WriteLine(book.Name);
}

 

不管是IList还是IList都可以使用这个Where方法了但是做集合操作的时候我们不仅仅需要Where，还需要OrderBy，Group等等，我想把所有的SQL能干的都移植过来。当然微软也意识到了这点，如是在.net 3.5里，微软发布了我们梦寐以求的Linq，将查询集成到语言里面来。它给IEnumerable添加了很多扩展方法，这样你可以很自然的去调用。你可以使用Reflector打开System.Core.dll，打开System.Linq命名空间，在这个命名空间里有一个Enumerable类，这里面就是微软为我们添加的扩展方法，看看，是不是SQL里所有的东西都可以在这里找到了

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