.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 Framework 中的所有内容在这里下面。

// 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();
        }
    }
}

需要说明的是，.NET Core 中是没有这段内容的，你可以查看 System.Runtime.cs · dotnet/corefx 的大约 1166 行， Exception 类型是有运行时实现的。

参考资料

• exception.cs - Reference Source
• 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

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