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Linq起源

2009-12-08 686

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简介： 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; se...

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