关于C#之yield return 话题

          还是先来个具体的实例吧：

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;

namespace yeildTST
{
    public class List
    {
        public static IEnumerable Power(int number)
        {
            while (number< 100)
            {
                number = number + number;
                yield return number;
            }
        }

        static void Main()
        {
            foreach (int i in Power(1))
            {
                Console.Write("{0} ", i);
            }

            Console.ReadLine();
        }
}

我们来通过这个例子看看yield return的庐山真面目，我查了下原始编译代码：

using System;

using System.Collections.Generic;

using System.Text;

using System.Threading;


namespace yeildTST
{
    public class List

    {
        static void Main()

        {

            foreach (int i in Power(1))

            {

                Console.Write("{0} ", i);

            }

            Console.ReadLine();

        }

 

        public static IEnumerable Power(int number)

        {

            d__0 d__ = new d__0(-2);

            d__._3__number = number;

            return d__;

        }

 

        //// Nested Types

        //[CompilerGenerated]

        public class d__0 : IEnumerable<object>, IEnumerable, IEnumerator<object>, IEnumerator, IDisposable

        {

            // Fields

            private int _1__state;

            private object _2__current;

            public int _3__number;

            private int _l__initialThreadId;

            public int number;

 

            // Methods

            //[DebuggerHidden]

            public d__0(int _1__state)

            {

                this._1__state = _1__state;

                this._l__initialThreadId = Thread.CurrentThread.ManagedThreadId;

            }

 

            public  bool MoveNext()

            {

 

                switch (this._1__state)

                {

                    case 1:
                       // goto label_057;-----生成的源代码

                   //////////我修改后的代码，原来没有//////
                        _1__state = -1;
                        while (this.number < 100)
                        {
                            this.number += this.number;
                            this._2__current = this.number;
                            this._1__state = 1;
                            return true;
                        }
                        break;
                  ///////////////////////////////

                    case 0:
                        this._1__state = -1;
                        while (this.number < 100)
                        {
                            this.number += this.number;
                            this._2__current = this.number;
                            this._1__state = 1;

                            //label_****: -----生成的源代码
                            //this._1__state=-1;

                            return true;
                        }

                        break;

 

                }

                

                return false;

            }

 

            //[DebuggerHidden]

             IEnumerator<object> IEnumerable<object>.GetEnumerator()

            {

                     return null;////////不用到这个，所以代码去掉了。

            }

 

            //[DebuggerHidden]

            IEnumerator IEnumerable.GetEnumerator()

            {

                List.d__0 d__;

                if ((Thread.CurrentThread.ManagedThreadId == this._l__initialThreadId) && (this._1__state == -2))

                {

                    this._1__state = 0;

                    d__ = this;

                }

                else

                {

                    d__ = new List.d__0(0);

                }

                d__.number = this._3__number;

                return d__;

            } 

 

            //[DebuggerHidden]

            void IEnumerator.Reset()

            {

                throw new NotSupportedException();

            }

 

            void IDisposable.Dispose()

            {

            }

 

            // Properties

            object IEnumerator<object>.Current

            {

                // [DebuggerHidden]

                get

                {

                    return this._2__current;

                }

            }

 

            object IEnumerator.Current

            {

                //[DebuggerHidden]

                get

                {

                    return this._2__current;

                }

            }

        }

 

    }

}

仔细看这段代码，多了一个类，d_0,它实现了各种枚举接口，由于我们的例题没有用到泛型，所以新生成的代码中

泛型部分可省去，经过我的测试，它对类中执行方法的顺序是这样的：

开始

GetEnumerator() => MoveNext() => IEnumerator.Current

第二次，从MoveNext() => IEnumerator.Current 。。。。。。。直到循环完成。