C# yield return. How It Work ?

2010-11-29

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Misc

摘要:C# yield return. How It Work ?

C# 常常拿來跟 Java 比較，在 .NET 1.1 時常常是不相上下，而 .NET 又因為較年輕 & 頂著 M$ 的名號，往往被當成玩具一樣，不過 M$ 的確是在 .NET 及 C# 下了很多功夫，作了很多 Sun 不願意在 Java 身上作的事，這次要探討的 yield return 及 IEnumerable<T> 這搭配的 Interface 就是一例...。

Java 在過去的版本，往往為了跨平台，把修改 VM 規格視為大忌，連帶的連語法修改都一樣，即使不影響編譯出來的 bytecode 相容性也是一樣不肯改。而 .NET 就為了語法簡潔，編譯器往往是讓步的一方，因此 C# 有相當多的 Syntax Sugar，讓你寫起 CODE 爽一點...。你只要寫簡單的 CODE，編譯器會幫你轉成較煩雜的 CODE，有點像是文言文或是成語那樣的味道。古代文人常常用簡單的四個字，就有一大票引申的意義跑出來...，寫作文時只要套上成語，就代表了成語背後的故事，寓意。

"yield return" 算是最甜的甜頭了，因為編譯器替你翻出來的 code 整整一大串。先來看個簡單的例子，如果我想實作 IEnumerator<T> Interface, 照順序輸出 1 ~ 100 的數字，正統的 C# code 看起來要像這樣:

用 IEnumerator 依序傳回 1 ~ 100 的數字[copy code]

	public class EnumSample1 : IEnumerator<int>
    {
        private int _start = 1;
        private int _end = 100;
        private int _current = 0;

        public EnumSample1(int start, int end)
        {
            this._start = start;
            this._end = end;
            this.Reset();
        }

        public int Current
        {
            get { return this._current; }
        }

        public void Dispose()
        {
        }

        object System.Collections.IEnumerator.Current
        {
            get { return this._current; }
        }

        public bool MoveNext()
        {
            this._current++;
            return !(this._current > this._end);
        }

        public void Reset()
        {
            this._current = 0;
        }
    }

			   1:  public class EnumSample1 : IEnumerator<int>

			   2:  {

			   3:      private int _start = 1;

			   4:      private int _end = 100;

			   5:      private int _current = 0;

			   6:      public EnumSample1(int start, int end)

			   7:      {

			   8:          this._start = start;

			   9:          this._end = end;

			  10:          this.Reset();

			  11:      }

			  12:      public int Current

			  13:      {

			  14:          get { return this._current; }

			  15:      }

			  16:      public void Dispose()

			  17:      {

			  18:      }

			  19:      object System.Collections.IEnumerator.Current

			  20:      {

			  21:          get { return this._current; }

			  22:      }

			  23:      public bool MoveNext()

			  24:      {

			  25:          this._current++;

			  26:          return !(this._current > this._end);

			  27:      }

			  28:      public void Reset()

			  29:      {

			  30:          this._current = 0;

			  31:      }

			  32:  }

好不容易寫好 IEnumerator 之後，再來是拿來用，一筆一筆印出來:

取得 IEnumerator 物件後，依序取出裡面的數字[copy code]

		EnumSample1 e = new EnumSample1(1, 100);



            while (e.MoveNext())
            {
                Console.WriteLine("Current Number: {0}", e.Current);
            }

			   1:  EnumSample1 e = new EnumSample1(1, 100);

			   2:  while (e.MoveNext())

			   3:  {

			   4:      Console.WriteLine("Current Number: {0}", e.Current);

			   5:  }

不過如果只是要列出 1 ~ 100，大部份的人都不會想這樣寫吧? 直接用計概第一堂教你的 loop 不就好了? 程式碼如下:

送分題: 用 LOOP 印出 1 ~ 100 的數字[copy code]

		for (int current = 1; current <= 100; current++)
            {
                Console.WriteLine("Current Number: {0}", current);
            }

			   1:  for (int current = 1; current <= 100; current++)

			   2:  {

			   3:      Console.WriteLine("Current Number: {0}", current);

			   4:  }

兩個範例都沒錯啊，那為什麼要用 IEnumerator ? 其實 IEnumerator 並不是 Microsoft 發明的，在四人幫寫的經典書籍 (Design Patterns) 裡就有這麼一個設計模式: Iterator，它的目的很明確:

"毋須知曉聚合物件的內部細節，即可依序存取內含的每一個元素。"

(摘自物件導向設計模式 Design Patterns 中文版，葉秉哲譯)

這裡指的 "聚合物件" 就是指 .NET 的 Collection, List, Array 等這類物件。意思是你不需要管 collection 裡每一個物件是怎麼擺的，用什麼結構處理的，用什麼邏輯或演算法處理的，我就只管照你安排好的順序一個一個拿出來就好。沒錯，這就是它主要的目的。換另一個說法，就是我們希望把物件巡訪的順序 (iteration) 跟依序拿到物件後要作什麼事 (process) 分開，那你就得參考 Iterator Pattern。不用? 那只好讓你的 iteration / process 混在一起吧。

差別在那? 我們再來看第二個例子。如果題目改一下，要列出 1 ~ 100 的數字，但如果不是 2 的倍數，也不是 3 的倍數，就跳過去。先來看看 Loop 的版本:

進階送分題，用LOOP印出 1~100 之中，2 或 3 的倍數[copy code]

		for (int current = 1; current <= 100; current++)
            {
                bool match = false;

                if (current % 2 == 0) match = true;
                if (current % 3 == 0) match = true;

                if (match == true)
                {
                    Console.WriteLine("Current Number: {0}", current);
                }
            }

			   1:  for (int current = 1; current <= 100; current++)

			   2:  {

			   3:      bool match = false;

			   4:      if (current % 2 == 0) match = true;

			   5:      if (current % 3 == 0) match = true;

			   6:      if (match == true)

			   7:      {

			   8:          Console.WriteLine("Current Number: {0}", current);

			   9:      }

			  10:  }

再來看看 IEnumerator 的版本:

用 IEnumerator 列出 1 ~ 100 中 2 或 3 的倍數[copy code]

		public class EnumSample2 : IEnumerator<int>
    {
        private int _start = 1;
        private int _end = 100;
        private int _current = 0;

        public EnumSample2(int start, int end)
        {
            this._start = start;
            this._end = end;
            this.Reset();
        }

        public int Current
        {
            get { return this._current; }
        }

        public void Dispose()
        {
        }

        object System.Collections.IEnumerator.Current
        {
            get { return this._current; }
        }

        public bool MoveNext()
        {
            do {
                this._current++;
            } while(this._current %2 > 0 && this._current %3 > 0);
            return !(this._current > this._end);
        }

        public void Reset()
        {
            this._current = 0;
        }
    }

			   1:  public class EnumSample2 : IEnumerator<int>

			   2:   {

			   3:       private int _start = 1;

			   4:       private int _end = 100;

			   5:       private int _current = 0;

			   6:       public EnumSample2(int start, int end)

			   7:       {

			   8:           this._start = start;

			   9:           this._end = end;

			  10:           this.Reset();

			  11:       }

			  12:       public int Current

			  13:       {

			  14:           get { return this._current; }

			  15:       }

			  16:       public void Dispose()

			  17:       {

			  18:       }

			  19:       object System.Collections.IEnumerator.Current

			  20:       {

			  21:           get { return this._current; }

			  22:       }

			  23:       public bool MoveNext()

			  24:       {

			  25:           do {

			  26:               this._current++;

			  27:           } while(this._current %2 > 0 && this._current %3 > 0);

			  28:           return !(this._current > this._end);

			  29:       }

			  30:       public void Reset()

			  31:       {

			  32:           this._current = 0;

			  33:       }

			  34:   }

而扣掉 IEnumerator 的部份，要把數字印出來的程式碼則完全沒有改變:

取出 IEnumerator 的每個數字，印到畫面上[copy code]

		EnumSample2 e = new EnumSample2(1, 100);



            while (e.MoveNext())
            {
                Console.WriteLine("Current Number: {0}", e.Current);
            }

			   1:  EnumSample2 e = new EnumSample2(1, 100);

			   2:  while (e.MoveNext())

			   3:  {

			   4:      Console.WriteLine("Current Number: {0}", e.Current);

			   5:  }

可以看的到，Loop 版本的確是把 iteration 跟 process 的 code 完全混在一起了，未來任何一方的邏輯要抽換都很麻煩，而 IEnumerator 則不會，分的很清楚，不過... 這 Code 會不會太 "髒" 了一點啊...? 試問一下，有誰會這麼勤勞，都用 IEnumerator 來寫 Code? 有的話請留個言，讓我崇拜一下...。

屁話講了一堆，最後就是要帶出來 "的確有魚與熊掌得兼的方法"，怎麼作? 來看看用 C# 的 yield return 版本的程式碼:

傳回 IEnumerable 的 METHOD (不用再寫 CLASS，實作 IEnumerator 了)[copy code]

		public static IEnumerable<int> YieldReturnSample3(int start, int end)
        {
            for (int current = 1; current <= 100; current++)
            {
                bool match = false;

                if (current % 2 == 0) match = true;
                if (current % 3 == 0) match = true;

                if (match == true)
                {
                    yield return current;
                }
            }
        }

			   1:  public static IEnumerable<int> YieldReturnSample3(int start, int end)

			   2:  {

			   3:      for (int current = 1; current <= 100; current++)

			   4:      {

			   5:          bool match = false;

			   6:          if (current % 2 == 0) match = true;

			   7:          if (current % 3 == 0) match = true;

			   8:          if (match == true)

			   9:          {

			  10:              yield return current;

			  11:          }

			  12:      }

			  13:  }

用 foreach 搭配 IEnumerable 印出每一筆數字[copy code]

		foreach (int current in YieldReturnSample3(1, 100))
            {
                Console.WriteLine("Current Number: {0}", current);
            }

			   1:  foreach (int current in YieldReturnSample3(1, 100))

			   2:  {

			   3:      Console.WriteLine("Current Number: {0}", current);

			   4:  }

真是太神奇了，安德魯。如何? 完美的結合兩者的優點，這種 code 實在是令人挑不出什麼缺點... 真是優雅... 不過念過系統程式的人一定都會吶悶... 這樣的程式執行方式，不就完全的違背了一般結構化程式典型的 function call / return 的鐵律了? 程式呼叫某個 function 就應該完全執行完才能 return 啊，怎麼能 "yield" return 後，跑完一圈又回到剛才執行到一半的 function 繼續跑，然後再 "yield" return ? 好像同實有兩段獨立的邏輯在運作... 還可以在兩者之間跳來跳去?

這就是 C# compiler 猛的地方了。搬出 reflector 來看看編譯出來的 code, 再被反組譯回來變成什麼樣子:

反組譯 YieldReturnSample3[copy code]

		public static IEnumerable<int> YieldReturnSample3(int start, int end)
{
    <YieldReturnSample3>d__0 d__ = new <YieldReturnSample3>d__0(-2);
    d__.<>3__start = start;
    d__.<>3__end = end;
    return d__;
}

			   1:  public static IEnumerable<int> YieldReturnSample3(int start, int end)

			   2:  {

			   3:      <YieldReturnSample3>d__0 d__ = new <YieldReturnSample3>d__0(-2);

			   4:      d__.<>3__start = start;

			   5:      d__.<>3__end = end;

			   6:      return d__;

			   7:  }

8:

9:

耶? 看到一個多出來的 class: <YieldReturnSample3>d__0 ... 再看看它的 class 長啥樣:

編譯器自動產生的 IEnumerator 衍生類別[copy code]

		[CompilerGenerated]
private sealed class <YieldReturnSample3>d__0 : IEnumerable<int>, IEnumerable, IEnumerator<int>, IEnumerator, IDisposable
{
    // Fields
    private int <>1__state;
    private int <>2__current;
    public int <>3__end;
    public int <>3__start;
    private int <>l__initialThreadId;
    public int <current>5__1;
    public bool <match>5__2;
    public int end;
    public int start;

    // Methods
    [DebuggerHidden]
    public <YieldReturnSample3>d__0(int <>1__state)
    {
        this.<>1__state = <>1__state;
        this.<>l__initialThreadId = Thread.CurrentThread.ManagedThreadId;
    }

    private bool MoveNext()
    {
        switch (this.<>1__state)
        {
            case 0:
                this.<>1__state = -1;
                this.<current>5__1 = 1;
                while (this.<current>5__1 <= 100)
                {
                    this.<match>5__2 = false;
                    if ((this.<current>5__1 % 2) == 0)
                    {
                        this.<match>5__2 = true;
                    }
                    if ((this.<current>5__1 % 3) == 0)
                    {
                        this.<match>5__2 = true;
                    }
                    if (!this.<match>5__2)
                    {
                        goto Label_0098;
                    }
                    this.<>2__current = this.<current>5__1;
                    this.<>1__state = 1;
                    return true;
                Label_0090:
                    this.<>1__state = -1;
                Label_0098:
                    this.<current>5__1++;
                }
                break;

            case 1:
                goto Label_0090;
        }
        return false;
    }

    [DebuggerHidden]
    IEnumerator<int> IEnumerable<int>.GetEnumerator()
    {
        Program.<YieldReturnSample3>d__0 d__;
        if ((Thread.CurrentThread.ManagedThreadId == this.<>l__initialThreadId) && (this.<>1__state == -2))
        {
            this.<>1__state = 0;
            d__ = this;
        }
        else
        {
            d__ = new Program.<YieldReturnSample3>d__0(0);
        }
        d__.start = this.<>3__start;
        d__.end = this.<>3__end;
        return d__;
    }

    [DebuggerHidden]
    IEnumerator IEnumerable.GetEnumerator()
    {
        return this.System.Collections.Generic.IEnumerable<System.Int32>.GetEnumerator();
    }

    [DebuggerHidden]
    void IEnumerator.Reset()
    {
        throw new NotSupportedException();
    }

    void IDisposable.Dispose()
    {
    }

    // Properties
    int IEnumerator<int>.Current
    {
        [DebuggerHidden]
        get
        {
            return this.<>2__current;
        }
    }

    object IEnumerator.Current
    {
        [DebuggerHidden]
        get
        {
            return this.<>2__current;
        }
    }
}

			   1:  [CompilerGenerated]

			   2:  private sealed class <YieldReturnSample3>d__0 : IEnumerable<int>, IEnumerable, IEnumerator<int>, IEnumerator, IDisposable

			   3:  {

			   4:      // Fields

			   5:      private int <>1__state;

			   6:      private int <>2__current;

			   7:      public int <>3__end;

			   8:      public int <>3__start;

			   9:      private int <>l__initialThreadId;

			  10:      public int <current>5__1;

			  11:      public bool <match>5__2;

			  12:      public int end;

			  13:      public int start;

14:

			  15:      // Methods

			  16:      [DebuggerHidden]

			  17:      public <YieldReturnSample3>d__0(int <>1__state)

			  18:      {

			  19:          this.<>1__state = <>1__state;

			  20:          this.<>l__initialThreadId = Thread.CurrentThread.ManagedThreadId;

			  21:      }

22:

			  23:      private bool MoveNext()

			  24:      {

			  25:          switch (this.<>1__state)

			  26:          {

			  27:              case 0:

			  28:                  this.<>1__state = -1;

			  29:                  this.<current>5__1 = 1;

			  30:                  while (this.<current>5__1 <= 100)

			  31:                  {

			  32:                      this.<match>5__2 = false;

			  33:                      if ((this.<current>5__1 % 2) == 0)

			  34:                      {

			  35:                          this.<match>5__2 = true;

			  36:                      }

			  37:                      if ((this.<current>5__1 % 3) == 0)

			  38:                      {

			  39:                          this.<match>5__2 = true;

			  40:                      }

			  41:                      if (!this.<match>5__2)

			  42:                      {

			  43:                          goto Label_0098;

			  44:                      }

			  45:                      this.<>2__current = this.<current>5__1;

			  46:                      this.<>1__state = 1;

			  47:                      return true;

			  48:                  Label_0090:

			  49:                      this.<>1__state = -1;

			  50:                  Label_0098:

			  51:                      this.<current>5__1++;

			  52:                  }

			  53:                  break;

54:

			  55:              case 1:

			  56:                  goto Label_0090;

			  57:          }

			  58:          return false;

			  59:      }

60:

			  61:      [DebuggerHidden]

			  62:      IEnumerator<int> IEnumerable<int>.GetEnumerator()

			  63:      {

			  64:          Program.<YieldReturnSample3>d__0 d__;

			  65:          if ((Thread.CurrentThread.ManagedThreadId == this.<>l__initialThreadId) && (this.<>1__state == -2))

			  66:          {

			  67:              this.<>1__state = 0;

			  68:              d__ = this;

			  69:          }

			  70:          else

			  71:          {

			  72:              d__ = new Program.<YieldReturnSample3>d__0(0);

			  73:          }

			  74:          d__.start = this.<>3__start;

			  75:          d__.end = this.<>3__end;

			  76:          return d__;

			  77:      }

78:

			  79:      [DebuggerHidden]

			  80:      IEnumerator IEnumerable.GetEnumerator()

			  81:      {

			  82:          return this.System.Collections.Generic.IEnumerable<System.Int32>.GetEnumerator();

			  83:      }

84:

			  85:      [DebuggerHidden]

			  86:      void IEnumerator.Reset()

			  87:      {

			  88:          throw new NotSupportedException();

			  89:      }

90:

			  91:      void IDisposable.Dispose()

			  92:      {

			  93:      }

94:

			  95:      // Properties

			  96:      int IEnumerator<int>.Current

			  97:      {

			  98:          [DebuggerHidden]

			  99:          get

			 100:          {

			 101:              return this.<>2__current;

			 102:          }

			 103:      }

			 104:

			 105:      object IEnumerator.Current

			 106:      {

			 107:          [DebuggerHidden]

			 108:          get

			 109:          {

			 110:              return this.<>2__current;

			 111:          }

			 112:      }

			 113:  }

耶? 不就完全跟之前手工寫的 IEnumerator 一樣嘛? 只不過這個 IEnumerator 是自動產生出來的，不是手寫的...。畢竟是機器產生的 CODE，總是沒那麼精簡。想到了嗎? 沒錯，這就是 C# compiler 送給你的 syntax sugar ...，你可以腦袋裡想像著計概課入門時教你的 LOOP 那樣簡單的想法，compiler 就幫你換成 IEnumerator 的實作方式，讓你隨隨便便就可以跟別人宣稱:

"看! 我的程式有用到 Iterator 這個設計模式喔..."

聽起來好像很臭屁的樣子... 哈哈! 如果是在真的用的到 Iterator Patterns 的情況下，真的是可以很臭屁的拿出來炫耀一下。不過，我幹嘛突然講起 yield return ? 各位看的過程中有沒有聯想到前幾篇 POST 講的 Thread Sync 那兩篇文章 ( #1, #2 ) ? IEnumerator 跟 Thread Sync 又有什麼關係? 賣個關子，下篇繼續!

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