C# ExecutionContext 實現

網上關於ExecutionContext的說明比較少，咱們來看看微軟的描述吧，

	名稱	說明
	Capture()	捕獲從當前線程的執行上下文。
	CreateCopy()	建立當前執行上下文的副本。
	Dispose()	釋放 ExecutionContext 類的當前實例所使用的全部資源。
	Equals(Object)	肯定指定的對象是否等於當前對象。（繼承自 Object。）
	GetHashCode()	做爲默認哈希函數。（繼承自 Object。）
	GetObjectData(SerializationInfo, StreamingContext)	設置指定 SerializationInfo 從新建立當前執行上下文的實例所需的邏輯上下文信息的對象。
	GetType()	獲取當前實例的 Type。（繼承自 Object。）
	IsFlowSuppressed()	指示是否當前正在取消執行上下文的流動。
	RestoreFlow()	在異步線程間恢復執行上下文的流動。
	Run(ExecutionContext, ContextCallback, Object)	在當前線程上指定的執行上下文中運行的方法。
	SuppressFlow()	在異步線程間取消執行上下文的流動。
	ToString()	返回表示當前對象的字符串。（繼承自 Object。）

而實際開發中咱們用的比較多的應該是SuppressFlow，RestoreFlow，Capture，CreateCopy和Run方法，好比咱們在一個Barrier源碼中，調用回調方法就是採用ExecutionContext 的Run方法，可是該方法須要一個ExecutionContext 實例，因而咱們須要先捕獲一個ExecutionContext 實例，而後拷貝再傳遞給Run方法，而有些時候咱們又不想同步上下文，能夠用SuppressFlow來暫停同步。這裏須要藉助一個AsyncFlowControl結構：

public struct AsyncFlowControl: IDisposable
    {
        private bool useEC;
        private ExecutionContext _ec;
#if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK
        private SecurityContext _sc;
#endif // #if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK
        private Thread _thread;
#if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK
        [SecurityCritical]
        internal void Setup(SecurityContextDisableFlow flags)
        {
            useEC = false;
            Thread currentThread = Thread.CurrentThread;
            _sc = currentThread.GetMutableExecutionContext().SecurityContext;
            _sc._disableFlow = flags;
            _thread = currentThread;
        }
#endif // #if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK
        [SecurityCritical]
        internal void Setup()
        {
            useEC = true;
            Thread currentThread = Thread.CurrentThread;
            _ec = currentThread.GetMutableExecutionContext();
            _ec.isFlowSuppressed = true;
            _thread = currentThread;
        }
        
        public void Dispose()
        {
            Undo();
        }
        
        [SecuritySafeCritical]
        public void Undo()
        {
            if (_thread == null)
            {
                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_CannotUseAFCMultiple"));
            }  
            if (_thread != Thread.CurrentThread)
            {
                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_CannotUseAFCOtherThread"));
            }
            if (useEC) 
            {
                if (Thread.CurrentThread.GetMutableExecutionContext() != _ec)
                {
                    throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_AsyncFlowCtrlCtxMismatch"));
                }      
                ExecutionContext.RestoreFlow();
            }
#if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK            
            else
            {
                if (!Thread.CurrentThread.GetExecutionContextReader().SecurityContext.IsSame(_sc))
                {
                    throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_AsyncFlowCtrlCtxMismatch"));
                }      
                SecurityContext.RestoreFlow();
            }
#endif // #if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK            
            _thread = null;
        }
        
    }

AsyncFlowControl的code比較簡單，裏面有ExecutionContext和SecurityContext兩個上下文，SecurityContext是一個很重要的 與認證權限有關的，這裏咱們忽略它，咱們的核心主要關注ExecutionContext的實現方式和思路，注意Setup方法中【_ec = currentThread.GetMutableExecutionContext();_ec.isFlowSuppressed = true;】，如今咱們再來看看ExecutionContext的實現：

public sealed class ExecutionContext : IDisposable, ISerializable
    {
#if FEATURE_CAS_POLICY        
        private HostExecutionContext _hostExecutionContext;
#endif // FEATURE_CAS_POLICY
        private SynchronizationContext _syncContext;
        private SynchronizationContext _syncContextNoFlow;
#if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK
        private SecurityContext     _securityContext;
#endif // #if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK
#if FEATURE_REMOTING
        private LogicalCallContext  _logicalCallContext;
        private IllogicalCallContext _illogicalCallContext;  // this call context follows the physical thread
#endif // #if FEATURE_REMOTING
        private Flags _flags;
        private Dictionary<IAsyncLocal, object> _localValues;
        private List<IAsyncLocal> _localChangeNotifications;
        
        public static AsyncFlowControl SuppressFlow()
        {
            if (IsFlowSuppressed())
            {
                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_CannotSupressFlowMultipleTimes"));
            }
            Contract.EndContractBlock();
            AsyncFlowControl afc = new AsyncFlowControl();
            afc.Setup();
            return afc;
        }

        public static void RestoreFlow()
        {
            ExecutionContext ec = Thread.CurrentThread.GetMutableExecutionContext();
            if (!ec.isFlowSuppressed)
            {
                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_CannotRestoreUnsupressedFlow"));
            }
            ec.isFlowSuppressed = false;
        }

        public static bool IsFlowSuppressed()
        {
            return Thread.CurrentThread.GetExecutionContextReader().IsFlowSuppressed;
        }
        
        public static ExecutionContext Capture()
        {
            // set up a stack mark for finding the caller
            StackCrawlMark stackMark = StackCrawlMark.LookForMyCaller;
            return ExecutionContext.Capture(ref stackMark, CaptureOptions.None);            
        }
         static internal ExecutionContext Capture(ref StackCrawlMark stackMark, CaptureOptions options)
        {
            ExecutionContext.Reader ecCurrent = Thread.CurrentThread.GetExecutionContextReader();
            // check to see if Flow is suppressed
            if (ecCurrent.IsFlowSuppressed) 
                return null;

#if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK            
            // capture the security context
            SecurityContext secCtxNew = SecurityContext.Capture(ecCurrent, ref stackMark);
#endif // #if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK
#if FEATURE_CAS_POLICY
             // capture the host execution context
            HostExecutionContext hostCtxNew = HostExecutionContextManager.CaptureHostExecutionContext();             
#endif // FEATURE_CAS_POLICY
            SynchronizationContext syncCtxNew = null;

#if FEATURE_REMOTING
            LogicalCallContext logCtxNew = null;
#endif

            if (!ecCurrent.IsNull)
            {
                // capture the sync context
                if (0 == (options & CaptureOptions.IgnoreSyncCtx))
                    syncCtxNew = (ecCurrent.SynchronizationContext == null) ? null : ecCurrent.SynchronizationContext.CreateCopy();

#if FEATURE_REMOTING
                // copy over the Logical Call Context
                if (ecCurrent.LogicalCallContext.HasInfo)
                    logCtxNew = ecCurrent.LogicalCallContext.Clone();
#endif // #if FEATURE_REMOTING
            }

            Dictionary<IAsyncLocal, object> localValues = null;
            List<IAsyncLocal> localChangeNotifications = null;
            if (!ecCurrent.IsNull)
            {
                localValues = ecCurrent.DangerousGetRawExecutionContext()._localValues;
                localChangeNotifications = ecCurrent.DangerousGetRawExecutionContext()._localChangeNotifications;
            }

            //
            // If we didn't get anything but defaults, and we're allowed to return the 
            // dummy default EC, don't bother allocating a new context.
            //
            if (0 != (options & CaptureOptions.OptimizeDefaultCase) &&
#if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK            
                secCtxNew == null &&
#endif
#if FEATURE_CAS_POLICY
                hostCtxNew == null &&
#endif // FEATURE_CAS_POLICY
                syncCtxNew == null &&
#if FEATURE_REMOTING
                (logCtxNew == null || !logCtxNew.HasInfo) &&
#endif // #if FEATURE_REMOTING
                localValues == null &&
                localChangeNotifications == null
                )
            {
                return s_dummyDefaultEC;
            }

            //
            // Allocate the new context, and fill it in.
            //
            ExecutionContext ecNew = new ExecutionContext();
#if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK            
            ecNew.SecurityContext = secCtxNew;
            if (ecNew.SecurityContext != null)
                ecNew.SecurityContext.ExecutionContext = ecNew;
#endif
#if FEATURE_CAS_POLICY
            ecNew._hostExecutionContext = hostCtxNew;
#endif // FEATURE_CAS_POLICY
            ecNew._syncContext = syncCtxNew;
#if FEATURE_REMOTING
            ecNew.LogicalCallContext = logCtxNew;
#endif // #if FEATURE_REMOTING
            ecNew._localValues = localValues;
            ecNew._localChangeNotifications = localChangeNotifications;
            ecNew.isNewCapture = true;
            return ecNew;
        }
    
        public static void Run(ExecutionContext executionContext, ContextCallback callback, Object state)
        {
            if (executionContext == null)
                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_NullContext"));
            if (!executionContext.isNewCapture)
                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_NotNewCaptureContext"));
            
            Run(executionContext, callback, state, false);
       }
        
    
      internal static void Run(ExecutionContext executionContext, ContextCallback callback, Object state, bool preserveSyncCtx)
        {
            RunInternal(executionContext, callback, state, preserveSyncCtx);
        }
       internal static void RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, bool preserveSyncCtx)
        {
            Contract.Assert(executionContext != null);
            if (executionContext.IsPreAllocatedDefault)
            {
                Contract.Assert(executionContext.IsDefaultFTContext(preserveSyncCtx));
            }
            else
            {
                Contract.Assert(executionContext.isNewCapture);
                executionContext.isNewCapture = false;
            }

            Thread currentThread = Thread.CurrentThread;
            ExecutionContextSwitcher ecsw = default(ExecutionContextSwitcher);

            RuntimeHelpers.PrepareConstrainedRegions();
            try
            {
                ExecutionContext.Reader ec = currentThread.GetExecutionContextReader();
                if ( (ec.IsNull || ec.IsDefaultFTContext(preserveSyncCtx)) && 
    #if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK                
                    SecurityContext.CurrentlyInDefaultFTSecurityContext(ec) && 
    #endif // #if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK                
                    executionContext.IsDefaultFTContext(preserveSyncCtx) &&
                    ec.HasSameLocalValues(executionContext)
                    )
                {
                    // Neither context is interesting, so we don't need to set the context.
                    // We do need to reset any changes made by the user's callback,
                    // so here we establish a "copy-on-write scope".  Any changes will
                    // result in a copy of the context being made, preserving the original
                    // context.
                    EstablishCopyOnWriteScope(currentThread, true, ref ecsw);
                }
                else
                {
                    if (executionContext.IsPreAllocatedDefault)
                        executionContext = new ExecutionContext();
                    ecsw = SetExecutionContext(executionContext, preserveSyncCtx);
                }

                //
                // Call the user's callback
                //
                callback(state);
            }
            finally
            {
                ecsw.Undo();
            }
        }
        
        public ExecutionContext CreateCopy()
        {
            if (!isNewCapture)
            {
                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_CannotCopyUsedContext"));
            }
            ExecutionContext ec = new ExecutionContext();
            ec.isNewCapture = true;
            ec._syncContext = _syncContext == null ? null : _syncContext.CreateCopy();
            ec._localValues = _localValues;
            ec._localChangeNotifications = _localChangeNotifications;
#if FEATURE_CAS_POLICY
            // capture the host execution context
            ec._hostExecutionContext = _hostExecutionContext == null ? null : _hostExecutionContext.CreateCopy();
#endif // FEATURE_CAS_POLICY
#if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK
            if (_securityContext != null)
            {
                ec._securityContext = _securityContext.CreateCopy();
                ec._securityContext.ExecutionContext = ec;
            }
#endif // #if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK

#if FEATURE_REMOTING
            if (this._logicalCallContext != null)
                ec.LogicalCallContext = (LogicalCallContext)this.LogicalCallContext.Clone();

            Contract.Assert(this._illogicalCallContext == null);
#endif // #if FEATURE_REMOTING

            return ec;
        }
        
      internal  static ExecutionContextSwitcher SetExecutionContext(ExecutionContext executionContext, bool preserveSyncCtx)
        {
#if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK                        
            StackCrawlMark stackMark = StackCrawlMark.LookForMyCaller;
#endif // #if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK

            Contract.Assert(executionContext != null);
            Contract.Assert(executionContext != s_dummyDefaultEC);

            // Set up the switcher object to return;
            ExecutionContextSwitcher ecsw = new ExecutionContextSwitcher();
            
            Thread currentThread = Thread.CurrentThread;
            ExecutionContext.Reader outerEC = currentThread.GetExecutionContextReader();

            ecsw.thread = currentThread;
            ecsw.outerEC = outerEC;
            ecsw.outerECBelongsToScope = currentThread.ExecutionContextBelongsToCurrentScope;

            if (preserveSyncCtx)
                executionContext.SynchronizationContext = outerEC.SynchronizationContext;
            executionContext.SynchronizationContextNoFlow = outerEC.SynchronizationContextNoFlow;

            currentThread.SetExecutionContext(executionContext, belongsToCurrentScope: true);

            RuntimeHelpers.PrepareConstrainedRegions();
            try
            {
                OnAsyncLocalContextChanged(outerEC.DangerousGetRawExecutionContext(), executionContext);

#if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK                    
                //set the security context
                SecurityContext sc = executionContext.SecurityContext;
                if (sc != null)
                {
                    // non-null SC: needs to be set
                    SecurityContext.Reader prevSeC = outerEC.SecurityContext;
                    ecsw.scsw = SecurityContext.SetSecurityContext(sc, prevSeC, false, ref stackMark);
                }
                else if (!SecurityContext.CurrentlyInDefaultFTSecurityContext(ecsw.outerEC))
                {
                    // null incoming SC, but we're currently not in FT: use static FTSC to set
                    SecurityContext.Reader prevSeC = outerEC.SecurityContext;
                    ecsw.scsw = SecurityContext.SetSecurityContext(SecurityContext.FullTrustSecurityContext, prevSeC, false, ref stackMark);
                }
#endif // #if FEATURE_IMPERSONATION || FEATURE_COMPRESSEDSTACK
#if FEATURE_CAS_POLICY                
                // set the Host Context
                HostExecutionContext hostContext = executionContext.HostExecutionContext;
                if (hostContext != null)
                {
                    ecsw.hecsw = HostExecutionContextManager.SetHostExecutionContextInternal(hostContext);
                } 
#endif // FEATURE_CAS_POLICY
            }
            catch
            {
                ecsw.UndoNoThrow();
                throw;
            }
            return ecsw;    
        }
    }

從ExecutionContext的成員變量來看，ExecutionContext包含不少上下文的，HostExecutionContext，SynchronizationContext，SecurityContext，LogicalCallContext和IllogicalCallContext。

SuppressFlow實例化一個AsyncFlowControl而後調用SetUP方法【_ec = currentThread.GetMutableExecutionContext();_ec.isFlowSuppressed = true;】，RestoreFlow獲取執行上下文【 ExecutionContext ec = Thread.CurrentThread.GetMutableExecutionContext()】，這2個方法的執行上細文是相同的。

接下來咱們來看看Capture方法，首先獲取ExecutionContext.Reader【線程上下文的一個包裝】

internal ExecutionContext.Reader GetExecutionContextReader()
{
return new ExecutionContext.Reader(m_ExecutionContext);
}

而後檢查IsFlowSuppressed，最後依次捕獲上下文

1.  SecurityContext secCtxNew = SecurityContext.Capture(ecCurrent, ref stackMark)

2. HostExecutionContext hostCtxNew = HostExecutionContextManager.CaptureHostExecutionContext()

3.  syncCtxNew = (ecCurrent.SynchronizationContext == null) ? null : ecCurrent.SynchronizationContext.CreateCopy()

4.  logCtxNew = ecCurrent.LogicalCallContext.Clone()

5. localValues = ecCurrent.DangerousGetRawExecutionContext()._localValues;localChangeNotifications = ecCurrent.DangerousGetRawExecutionContext()._localChangeNotifications;

這個咱們捕獲這些上下文，那麼後面的CreateCopy其實也須要拷貝這些上下文的。

這裏的Run方法，使用上比較好理解【調用ContextCallback傳入特定線程的上下文】，可是代碼層面就不是那麼好理解了，裏面還藉助了ExecutionContextSwitcher對象，可是和興實現是ecsw = SetExecutionContext(executionContext, preserveSyncCtx)【還原線程上下文】

如：

ExecutionContext ec = new ExecutionContext();
ec.isNewCapture = true;
ec._syncContext = _syncContext == null ? null : _syncContext.CreateCopy();
ec._localValues = _localValues;
ec._localChangeNotifications = _localChangeNotifications;
ec._hostExecutionContext = _hostExecutionContext == null ? null : _hostExecutionContext.CreateCopy();
if (_securityContext != null)
{
ec._securityContext = _securityContext.CreateCopy();
ec._securityContext.ExecutionContext = ec;
}
if (this._logicalCallContext != null)
ec.LogicalCallContext = (LogicalCallContext)this.LogicalCallContext.Clone();

因此從使用ExecutionContext 的角度來說，仍是很好理解的，先用Capture方法捕獲線程上這些上下文保存到ExecutionContext 實例裏面，最後在調用Run方法時須要還原線程的這些上下文【來源先前保存到ExecutionContext 實例】。