IEnumerable/IEnumerator 的兩三事

以下是一個傳統且結實的 Enumerable & Enumerator 範例:

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Collections;

namespace ConsoleApplication1 {

    class Program {

        class MyEnumerable : IEnumerable <int >, IEnumerable {

            private int[] values;

            private int offset;

            public MyEnumerable( int[] values, int offset) {

                this.values = values;

                this.offset = offset;

            }

            public IEnumerator<int > GetEnumerator() {

                return new MyIterator(this );

            }

            IEnumerator IEnumerable .GetEnumerator() {

                return this.GetEnumerator();

            }

            private class MyIterator : IEnumerator <int >, IEnumerator {

                private MyEnumerable target;

                private int pos;

                public MyIterator( MyEnumerable target) {

                    this.target = target;

                    this.pos = -1;

                }

                public bool MoveNext() {

                    if( this.pos != this.target.values.Length) this.pos++;

                    return this.pos < this.target.values.Length;

                }

                public int Current {

                    get {

                        if( this.pos == -1 || this.pos == this.target.values.Length) {

                            throw new InvalidOperationException();

                        }

                        int index = this.pos + this.target.offset;

                        index = index % this.target.values.Length;

                        return this.target.values[index];

                    }

                }

                object IEnumerator.Current {

                    get {

                        return this.Current;

                    }

                }

                public void Reset() { this.pos = -1; }

                void IEnumerator.Reset() { this .Reset(); }

                public void Dispose() { }

            }

        }

        static void Main( string[] args) {

            MyEnumerable list = new MyEnumerable (new int [] {1,2,3,4,5,6}, 2);

            foreach( int i in list) Console.WriteLine(i);

            Console.WriteLine( "-------------------------");

            foreach( object o in (( IEnumerable)list)) Console .WriteLine(o);

            Console.WriteLine( "-------------------------");

            IEnumerator<int > iterator = list.GetEnumerator();

            while(iterator.MoveNext()) Console.WriteLine(iterator.Current.ToString());

            (iterator as IDisposable).Dispose();

            Console.ReadKey();

        }

    }

}

自從 C# 2.0 之後, 上面的程式可以被簡化成下面這樣

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Collections;

namespace ConsoleApplication1 {

    class Program {

        class MyEnumerable : IEnumerable <int >, IEnumerable {

            private int[] values;

            private int offset;

            public MyEnumerable( int[] values, int offset) {

                this.values = values;

                this.offset = offset;

            }

            public IEnumerator<int > GetEnumerator() {

                for( int index = 0; index < values.Length; index++) {

                    yield return values[(index + this.offset) % this.values.Length];

                }

            }

            IEnumerator IEnumerable .GetEnumerator() {

                return this.GetEnumerator();

            }

        }

        static void Main( string[] args) {

            MyEnumerable list = new MyEnumerable (new int [] {1,2,3,4,5,6}, 2);

            foreach( int i in list) Console.WriteLine(i);

            Console.WriteLine( "-------------------------");

            foreach( object o in (( IEnumerable)list)) Console .WriteLine(o);

            Console.WriteLine( "-------------------------");

            IEnumerator<int > iterator = list.GetEnumerator();

            while(iterator.MoveNext()) Console.WriteLine(iterator.Current.ToString());

            (iterator as IDisposable).Dispose();

            Console.ReadKey();

        }

    }

}

實際上發生的事情是: 

當 Compiler 看到 yield return 時,

它會自動地幫我們產生一個 Enumerator class,

就像我們之前自己寫的那樣!!!

不過, 有幾個注意事項, 首先觀察下面程式碼的輸出:

using System;

using System.Collections.Generic;

namespace ConsoleApplication1 {

    class Program {

        private static readonly String padding = new String( ' ', 30);

        private static IEnumerable<int > CreateEnumerable() {

            Console.WriteLine( "{0}Start of CreateEnumerable()" , padding);

            for( int i = 0; i < 3; i++) {

                Console.WriteLine( "{0}About to yield {1}", padding, i);

                yield return i;

                Console.WriteLine( "{0}After yield", padding);

            }

            Console.WriteLine( "{0}End of CreateEnumerable()", padding);

        }

        static void Main( string[] args) {

            IEnumerable<int > iterable = CreateEnumerable();

            IEnumerator<int > iterator = iterable.GetEnumerator();

            Console.WriteLine( "Starting to iterate");

            while( true) {

                Console.WriteLine( "Calling MoveNext()...");

                bool result = iterator.MoveNext();

                Console.WriteLine( "... MoveNext result={0}", result);

                if(!result)

                    break;

                Console.WriteLine( "Fetching Current...");

                Console.WriteLine( "... Current result={0}", iterator.Current);

            }

            Console.ReadKey();

        }

    }

}

因為 Compiler 動了很大的手腳, 

所以上面那隻程式的輸出會讓人很意外!!!

有興趣的話可以使用 Reflector 或 ILDasm 來觀察 Compiler 究竟做了甚麼事.

使用 yield return 來產生 Enumerator/Enumerable 時,

有幾個特別的事項要提醒:

1. code 會在第一次 MoveNext() 時才被執行

2. code 裏頭可以有 try 以及 finally 區塊, 但不可以有 catch 區塊

3. finally 區塊實際上會被移至 Enumerator 的 Dispose() 裡

4. 在第一次 MoveNext() 被呼叫前, Current 會是 default(T)

5. 當 MoveNext() 回傳 false 後, Current 永遠會是最後值

6. 呼叫 Reset() 會丟例外

出處: C# In Depth 2Ed Chapter 7