.net3.5下使用LINQ递归算法实现简洁代码

pre.hl { color:#ddbb00; background-color:#000000; font-size:10pt; font-family:'Verdana';}
.hl.line { color:#898989; }
.hl.kwa { color:#ffed8a; font-weight:bold; }
.hl.sym { color:#ababab; }
.hl.slc { color:#978345; font-style:italic; }
.hl.kwb { color:#ffed8a; }
.hl.kwd { color:#ddbb00; font-weight:bold; }
.hl.num { color:#ffffff; }
.parameter{font-family:Verdana,Arial,Helvetica,sans-serif;font-size:100%;font-style:italic;margin:0}

　　.net framework 3.5
有了Linq使得对委托有了更多的支持，下面让我们来看几个有趣的示例．通常情况下，我们实现一个递归算法要写一个函数，同时还有调用的几行代码．

　　现在来看使用Linq的如何实现简洁的代码，代码如下：

1 using System;
2 using System.Collections.Generic;
3 using System.Linq;
4 using System.Text;
5 using System.IO;
6 using NUnit.Framework;
7
8 namespace WindowsFormsApplication1
9 {
10     /// <summary>
11     /// TestRecursionWithLINQ
12     /// </summary>
13     /// <remark>Author : PetterLiu 2009-03-29 11:28  http://wintersun.cnblogs.com </remark>
14     [TestFixture]
15     public class TestRecursionWithLINQ
16     {
17
18         /// <summary>
19         /// Factorials this instance.
20         /// </summary>
21         /// <remark>Author : PetterLiu 2009-03-29 11:28  http://wintersun.cnblogs.com </remark>
22         [Test]
23         public void Factorial()
24         {
25             Func<int, int> fib = null;
26             fib = n => (n == 1) ? 1 : fib(n - 1) * n;
27             Console.WriteLine(fib(5));
28         }
29
30         /// <summary>
31         /// Fibonaccis this instance.
32         /// </summary>
33         /// <remark>Author : PetterLiu 2009-03-29 11:28  http://wintersun.cnblogs.com </remark>
34         [Test]
35         public void Fibonacci()
36         {
37             Func<int, int> fib = null;
38             fib = n => n > 1 ? fib(n - 1) + fib(n - 2) : n;
39             Console.WriteLine(fib(6));
40         }
41
42
43         /// <summary>
44         /// Recursions the get files.
45         /// </summary>
46         /// <remark>Author : PetterLiu 2009-03-29 11:27  http://wintersun.cnblogs.com </remark>
47         [Test]
48         public void RecursionGetFiles()
49         {
50             var RecGetFiles =
51                 Functional.Y<string, IEnumerable<string>>
52                 (f => d => Directory.GetFiles(d).Concat(Directory.GetDirectories(d).SelectMany(f)));
53
54             foreach (var f in RecGetFiles(Directory.GetCurrentDirectory()))
55                 Console.WriteLine(f);
56
57         }
58
59         /// <summary>
60         /// Factorial2s this instance.
61         /// </summary>
62         /// <remark>Author : PetterLiu 2009-03-29 11:28  http://wintersun.cnblogs.com </remark>
63         [Test]
64         public void Factorial2()
65         {
66             var dd = Functional.Y<int, int>(h => m => (m == 1) ? 1 : h(m - 1) * m);
67             Console.WriteLine(dd(5));
68         }
69     }
70
71     /// <summary>
72     /// Functional
73     /// </summary>
74     /// <remark>Author : Wes Dyer</remark>
75     public class Functional
76     {
77         /// <summary>
78         ///delegate  Func<A, R>
79         /// </summary>
80         private delegate Func<A, R> Recursive<A, R>(Recursive<A, R> r);
81         /// <summary>
82         /// Ys the specified f.
83         /// </summary>
84         /// <typeparam name="A"></typeparam>
85         /// <typeparam name="R"></typeparam>
86         /// <param name="f">The f.</param>
87         /// <returns></returns>
88         public static Func<A, R> Y<A, R>(Func<Func<A, R>, Func<A, R>> f)
89         {
90             Recursive<A, R> rec = r => a => f(r(r))(a);
91             return rec(rec);
92         }
93     }
94 }

Factorial是阶乘，接着是Fibonacci数列．之后把这个定义为一个名叫Funcional类，其中包含一个static方法．Factorial2使用这个类再实现阶乘，是不是简单的多．接着是RecursionGetFiles一个实际的应用，递归遍历文件夹取得文件名列表．像树型结构算法都可以用它来实现，是不是很有趣？

其中几个关键方法可以参考：

Func<(Of <(T,
TResult>)>)
委托

封装一个具有一个参数并返回 TResult 参数指定的类型值的方法。

Enumerable.SelectMany<(Of
<(TSource,
TResult>)>)
方法 (IEnumerable<(Of
<(TSource>)>),
Func<(Of <(TSource,
IEnumerable<(Of
<(TResult>)>)>)>))

将序列的每个元素投影到

IEnumerable<(Of
<(T>)>)
并将结果序列合并为一个序列。

Enumerable.Concat<(Of
<(TSource>)>)
方法

连接两个序列。

Author: Petter Liu http://wintersun.cnblogs.com