.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

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。

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。

参考资料

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

作者：吕毅