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不知你是否见过 try { } finally { } 代码中，try 块留空，而只往 finally 中写代码的情况呢？这种写法有其特殊的目的。

本文就来说说这种不一样的写法。

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你可以点开这个链接查看 Exception 类，在里面你可以看到一段异常处理的代码非常奇怪：

// 代码已经过简化。 internal void RestoreExceptionDispatchInfo(ExceptionDispatchInfo exceptionDispatchInfo) { // 省略代码。 try{} finally { // 省略代码。 } // 省略代码。 } 

神奇之处就在于，其 try 块是空的，重要代码都放在 finally 中。那为什么会这么写呢？

在代码注释中的解释为：

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.

翻译过来是：

在 finally 子句中执行此操作以确保在获取锁时无法注入 ThreadAbort。这是为了防止不相关的异常恢复因 TAE 而被阻止。

也就是说，此方法是为了与 Thread.Abort 对抗，防止 Thread.Abort 中断此处代码的执行。Thread.Abort 的执行交给 CLR 管理，finally 的执行也是交给 CLR 管理。CLR 确保 finally块执行的时候不会被 Thread.Abort 阻止。

代码在 .NET Core 和 .NET Framework 中的实现完全一样：

// 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 这里 查看 .NET Framework 版本，在这里 查看 .NET Core 的版本。

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参考资料

• 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：吕毅