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.NET/C# 异常处理：写一个空的 try 块代码，而把重要代码写到 finally 中（Constrained Execution Regions）...

發(fā)布時(shí)間：2024/7/5 C# 48 豆豆

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不知你是否見(jiàn)過(guò) try { } finally { } 代碼中，try 塊留空，而只往 finally 中寫代碼的情況呢？這種寫法有其特殊的目的。

本文就來(lái)說(shuō)說(shuō)這種不一樣的寫法。

本文內(nèi)容

- - 空的 try 塊
  - 受約束的執(zhí)行區(qū)域（Constrained Execution Regions）
    - 參考資料

空的 try 塊

你可以點(diǎn)開(kāi)這個(gè)鏈接查看 Exception 類，在里面你可以看到一段異常處理的代碼非常奇怪：

// 代碼已經(jīng)過(guò)簡(jiǎn)化。 internal void RestoreExceptionDispatchInfo(ExceptionDispatchInfo exceptionDispatchInfo) {// 省略代碼。try{}finally{// 省略代碼。}// 省略代碼。 }

神奇之處就在于，其 try 塊是空的，重要代碼都放在 finally 中。那為什么會(huì)這么寫呢？

在代碼注釋中的解釋為：

We do this inside a finally clause to ensure ThreadAbort cannot be injected while we have taken the lock. This is to prevent unrelated exception restorations from getting blocked due to TAE.

翻譯過(guò)來(lái)是：

在 finally 子句中執(zhí)行此操作以確保在獲取鎖時(shí)無(wú)法注入 ThreadAbort。這是為了防止不相關(guān)的異常恢復(fù)因 TAE 而被阻止。

也就是說(shuō)，此方法是為了與 Thread.Abort 對(duì)抗，防止 Thread.Abort 中斷此處代碼的執(zhí)行。 Thread.Abort 的執(zhí)行交給 CLR 管理，finally 的執(zhí)行也是交給 CLR 管理。CLR 確保 finally 塊執(zhí)行的時(shí)候不會(huì)被 Thread.Abort 阻止。

代碼在 .NET Core 和 .NET Framework 中的實(shí)現(xiàn)完全一樣：

// This is invoked by ExceptionDispatchInfo.Throw to restore the exception stack trace, corresponding to the original throw of the // exception, just before the exception is "rethrown". [SecuritySafeCritical] internal void RestoreExceptionDispatchInfo(System.Runtime.ExceptionServices.ExceptionDispatchInfo exceptionDispatchInfo) {bool fCanProcessException = !(IsImmutableAgileException(this));// Restore only for non-preallocated exceptionsif (fCanProcessException){// Take a lock to ensure only one thread can restore the details// at a time against this exception object that could have// multiple ExceptionDispatchInfo instances associated with it.//// We do this inside a finally clause to ensure ThreadAbort cannot// be injected while we have taken the lock. This is to prevent// unrelated exception restorations from getting blocked due to TAE.try{}finally{// When restoring back the fields, we again create a copy and set reference to them// in the exception object. This will ensure that when this exception is thrown and these// fields are modified, then EDI's references remain intact.//// Since deep copying can throw on OOM, try to get the copies// outside the lock.object _stackTraceCopy = (exceptionDispatchInfo.BinaryStackTraceArray == null)?null:DeepCopyStackTrace(exceptionDispatchInfo.BinaryStackTraceArray);object _dynamicMethodsCopy = (exceptionDispatchInfo.DynamicMethodArray == null)?null:DeepCopyDynamicMethods(exceptionDispatchInfo.DynamicMethodArray);// Finally, restore the information. //// Since EDI can be created at various points during exception dispatch (e.g. at various frames on the stack) for the same exception instance,// they can have different data to be restored. Thus, to ensure atomicity of restoration from each EDI, perform the restore under a lock.lock(Exception.s_EDILock){_watsonBuckets = exceptionDispatchInfo.WatsonBuckets;_ipForWatsonBuckets = exceptionDispatchInfo.IPForWatsonBuckets;_remoteStackTraceString = exceptionDispatchInfo.RemoteStackTrace;SaveStackTracesFromDeepCopy(this, _stackTraceCopy, _dynamicMethodsCopy);}_stackTraceString = null;// Marks the TES state to indicate we have restored foreign exception// dispatch information.Exception.PrepareForForeignExceptionRaise();}} }

你可以在這里查看 .NET Framework 版本，在這里查看 .NET Core 的版本。

受約束的執(zhí)行區(qū)域（Constrained Execution Regions）

這種現(xiàn)象在微軟官方文檔可靠性最佳做法中有介紹。

Doing so instructs the just-in-time compiler to prepare all the code in the finally block before running the try block. This guarantees that the code in the finally block is built and will run in all cases. It is not uncommon in a CER to have an empty try block. Using a CER protects against asynchronous thread aborts and out-of-memory exceptions. See ExecuteCodeWithGuaranteedCleanup for a form of a CER that additionally handles stack overflows for exceedingly deep code.

使用 try-finally 形成一個(gè)受約束的執(zhí)行區(qū)域，使得 finally 中的代碼被可靠地執(zhí)行。