为了篇幅关系，这里删除了部分复杂的实现，取而代之的是介绍方法作用的注释性文字，对于简单的方法或是重要的方法保留了代码。

namespace System.Runtime.CompilerServices {     public struct AsyncTaskMethodBuilder<TResult>     {         internal static readonly Task<TResult> s_defaultResultTask = AsyncTaskCache.CreateCacheableTask<TResult>(default(TResult));         //这也是一个很重要的类，AsyncTaskMethodBuilder将一些操作进一步交给AsynchronousMethodBuilderCore来完成         private AsyncMethodBuilderCore m_coreState;         private Task<TResult> m_task;         [__DynamicallyInvokable]         public Task<TResult> Task         {             [__DynamicallyInvokable]             get             {                 Task<TResult> task = this.m_task;                 if (task == null)                 {                     task = (this.m_task = new Task<TResult>());                 }                 return task;             }         }         private object ObjectIdForDebugger         {             get             {                 return this.Task;             }         }         [__DynamicallyInvokable]         public static AsyncTaskMethodBuilder<TResult> Create()         {             return default(AsyncTaskMethodBuilder<TResult>);         }         //开始状态机的执行         [__DynamicallyInvokable, DebuggerStepThrough, SecuritySafeCritical]         public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine         {             if (stateMachine == null)             {                 throw new ArgumentNullException("stateMachine");             }             //保存当前ExecutionContext，这是很重要的一步，后文会具体介绍             ExecutionContextSwitcher executionContextSwitcher = default(ExecutionContextSwitcher);             RuntimeHelpers.PrepareConstrainedRegions();             try             {                 ExecutionContext.EstablishCopyOnWriteScope(ref executionContextSwitcher);                 stateMachine.MoveNext();             }             finally             {                 executionContextSwitcher.Undo();             }         }         [__DynamicallyInvokable]         public void SetStateMachine(IAsyncStateMachine stateMachine)         {             this.m_coreState.SetStateMachine(stateMachine);         }         [__DynamicallyInvokable]         public void AwaitOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine)                  where TAwaiter : INotifyCompletion                  where TStateMachine : IAsyncStateMachine         {             try             {                 AsyncMethodBuilderCore.MoveNextRunner runner = null;                 Action completionAction = this.m_coreState.GetCompletionAction(AsyncCausalityTracer.LoggingOn ? this.Task : null, ref runner);                 if (this.m_coreState.m_stateMachine == null)                 {                     Task<TResult> task = this.Task;                     this.m_coreState.PostBoxInitialization(stateMachine, runner, task);                 }                 awaiter.OnCompleted(completionAction);             }             catch (Exception arg_5C_0)             {                 AsyncMethodBuilderCore.ThrowAsync(arg_5C_0, null);             }         }         [__DynamicallyInvokable, SecuritySafeCritical]         public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine)                  where TAwaiter : ICriticalNotifyCompletion                  where TStateMachine : IAsyncStateMachine         {             try             {                 AsyncMethodBuilderCore.MoveNextRunner runner = null;                 //这是整个方法乃至类中最重要的一部分                 //获取当前状态执行完毕后下一步的操作                 Action completionAction = this.m_coreState.GetCompletionAction(AsyncCausalityTracer.LoggingOn ? this.Task : null, ref runner);                 if (this.m_coreState.m_stateMachine == null)                 {                     Task<TResult> task = this.Task;                     this.m_coreState.PostBoxInitialization(stateMachine, runner, task);                 }                 //将下一步操作传递给awaiter对象，实际进入下一步还是通过awaiter来进行的。                 awaiter.UnsafeOnCompleted(completionAction);             }             catch (Exception arg_5C_0)             {                 AsyncMethodBuilderCore.ThrowAsync(arg_5C_0, null);             }         }         [__DynamicallyInvokable]         public void SetResult(TResult result)         {             //设置结果             //通过Task上的方法来完成         }         internal void SetResult(Task<TResult> completedTask)         {             //设置结果，调用上面的方法来完成                     }         public void SetException(Exception exception)         {             //设置异常             //通过Task上的方法来实现         }         internal void SetNotificationForWaitCompletion(bool enabled)         {             this.Task.SetNotificationForWaitCompletion(enabled);         }         private Task<TResult> GetTaskForResult(TResult result)         {             //获取Task包装的结果         }     } }

状态机的几种状态如下：

-1：表示还未开始执行

-2：执行结束，可能是正常完成，也可能遇到异常处理异常后结束

0~：下一个状态。如0表示初始的-1之后的下一个状态，1表示0后的下一状态，以此类推。