不知你是否見過 try { } finally { } 程式碼中，try 塊留空，而只往 finally 中寫程式碼的情況呢？這種寫法有其特殊的目的。

本文就來說說這種不一樣的寫法。

你可以點開這個連結檢視 Exception 類，在裡面你可以看到一段異常處理的程式碼非常奇怪：

// 程式碼已經過簡化。
internal void RestoreExceptionDispatchInfo(ExceptionDispatchInfo exceptionDispatchInfo) { // 省略程式碼。 try{} finally { // 省略程式碼。 } // 省略程式碼。 } 
 

神奇之處就在於，其 try 塊是空的，重要程式碼都放在 finally 中。那為什麼會這麼寫呢？

在程式碼註釋中的解釋為：

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.

翻譯過來是：

在 finally 子句中執行此操作以確保在獲取鎖時無法注入 ThreadAbort。這是為了防止不相關的異常恢復因 TAE 而被阻止。

也就是說，此方法是為了與 Thread.Abort 對抗，防止 Thread.Abort 中斷此處程式碼的執行。Thread.Abort 的執行交給 CLR 管理，finally 的執行也是交給 CLR 管理。CLR 確保 finally塊執行的時候不會被 Thread.Abort 阻止。

程式碼在 .NET Core 和 .NET Framework 中的實現完全一樣：

// 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 exceptions if (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 這裡 檢視 .NET Framework 版本，在這裡 檢視 .NET Core 的版本。

參考資料

• exception.cs - Reference Source
• RestoreExceptionDispatchInfo
• The empty try block mystery - Some Creativity
• c# - Why use try {} finally {} with an empty try block? - Stack Overflow
• corefx/System.Runtime.cs at master · dotnet/corefx

原文地址： https://walterlv.com/post/empty-try-block.html

作者：呂毅