blocks|key|3856414|text|坏的|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3856415|下面的代码捕获了一个可以很容易地完全避免的异常。这使得代码更难理解，而且通常也会产生性能成本。|3856416|int+input1+=+GetInput1();
int+input2+=+GetInput2();

try
{
++++int+result+=+input1+/+input2;
++++Output("{0}+/+{1}+=+{2}",+input1,+input2,+result);
}
catch+(OverflowException)
{
++++Output("There+was+an+overflow+exception.+Make+sure+input2+is+not+zero.");
}|code-block|syntax|javascript|3856417|更好|3856418|此代码检查可能引发异常的条件，并在错误发生之前更正该情况。这样就不会有任何例外。代码更具可读性，性能很可能会更好。|3856419|int+input1+=+GetInput1();
int+input2+=+GetInput2();

while+(input2+==+0)
{
++++Output("input2+must+not+be+zero.+Enter+a+new+value.");
++++input2+=+GetInput2();
}

int+result+=+input1+/+input2;
Output("{0}+/+{1}+=+{2}",+input1,+input2,+result);|3856420|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|V|8|@]|9|@]|A|$G|H]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

<h2>Bad</h2>

The below code catches an exception that could easily be avoided altogether. This makes the code more difficult to follow and typically incurs a performance cost as well.

<pre><code>int input1 = GetInput1();
int input2 = GetInput2();

try
{
 int result = input1 / input2;
 Output("{0} / {1} = {2}", input1, input2, result);
}
catch (OverflowException)
{
 Output("There was an overflow exception. Make sure input2 is not zero.");
}
</code></pre>

<h2>Better</h2>

This code checks for a condition that would throw an exception, and corrects the situation before the error occurs. This way there is no exception at all. The code is more readable, and the performance is very likely to be better.

<pre><code>int input1 = GetInput1();
int input2 = GetInput2();

while (input2 == 0)
{
 Output("input2 must not be zero. Enter a new value.");
 input2 = GetInput2();
}

int result = input1 / input2;
Output("{0} / {1} = {2}", input1, input2, result);
</code></pre>

blocks|key|3856453|text|根据定义，异常是发生在软件正常流程之外的事件。我脑海中有一个简单的例子，就是使用FileNotFoundException来查看文件是否存在。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3856454|try
{
++++File.Open(@"c:\some+nonexistent+file.not+here");
}
catch(FileNotFoundException)
{
++++//+do+whatever+logic+is+needed+to+create+the+file.
++++...
}
//+proceed+with+the+rest+of+your+program.|code-block|syntax|javascript|3856455|在本例中，您没有使用File.Exists()方法，该方法实现了相同的结果，但没有异常的开销。|3856456|除了错误的使用，还有与异常、填充属性、创建堆栈跟踪等相关的开销。|3856457|entityMap^0|14|L|0|0|A|D|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|X|8|@]|D|@]|E|$]]|$1|O|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|P|$]]

By definition, an exception is an occurrence which happens outside the normal flow of your software. A quick example off the top of my head is using a <code>FileNotFoundException</code> to see if a file exists or not.

<pre><code>try
{
 File.Open(@"c:\some nonexistent file.not here");
}
catch(FileNotFoundException)
{
 // do whatever logic is needed to create the file.
 ...
}
// proceed with the rest of your program.
</code></pre>

In this case, you haven't used the <code>File.Exists()</code> method which achieves the same result but without the overhead of the exception.

Aside from the bad usage, there is overhead associated with an exception, populating the properties, creating the stack trace, etc.

blocks|key|3856466|text|下面是一个常见的例子：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3856467|public+bool+TryParseEnum<T>(string+value,+out+T+result)
{
++++result+=+default(T);

++++try
++++{
++++++++result+=+(T)Enum.Parse(typeof(T),+value,+true);
++++++++return+true;
++++}
++++catch
++++{
++++++++return+false;
++++}
}|code-block|syntax|javascript|3856468|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Here's a common one:

<pre><code>public bool TryParseEnum&lt;T&gt;(string value, out T result)
{
 result = default(T);

 try
 {
 result = (T)Enum.Parse(typeof(T), value, true);
 return true;
 }
 catch
 {
 return false;
 }
}
</code></pre>

blocks|key|1488885|text|它大体上等同于goto，除了在单词Exception方面更糟糕，而且开销更大。您告诉代码跳转到catch块：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1488886|bool+worked;
try
{
++++foreach+(Item+someItem+in+SomeItems)
++++{
++++++++if+(someItem.SomeTestFailed())+throw+new+TestFailedException();
++++}
++++worked+=+true;
}
catch(TestFailedException+testFailedEx)
{
++++worked+=+false;
}
if+(worked)+//+...+logic+continues|code-block|syntax|javascript|1488887|正如您所看到的，它正在运行一些(虚构的)测试；如果测试失败，就会抛出一个异常，并且worked将被设置为false。|offset|length|style|CODE|1488888|当然，直接更新bool+worked要容易得多！|1488889|希望这能有所帮助！|1488890|entityMap^0|0|0|15|6|1G|5|0|7|B|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@$I|V|J|W|K|L]|$I|X|J|Y|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|Z|8|@$I|10|J|11|K|L]]|9|@]|A|$]]|$1|O|3|P|5|6|7|12|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|13|8|@]|9|@]|A|$]]]|R|$]]

It's roughly equivalent to a goto, except worse in terms of the word Exception, and with more overhead. You're telling the code to jump to the catch block:

<pre><code>bool worked;
try
{
 foreach (Item someItem in SomeItems)
 {
 if (someItem.SomeTestFailed()) throw new TestFailedException();
 }
 worked = true;
}
catch(TestFailedException testFailedEx)
{
 worked = false;
}
if (worked) // ... logic continues
</code></pre>

As you can see, it's running some (made-up) tests; if they fail, an exception is thrown, and <code>worked</code> will be set to <code>false</code>.

Much easier to just update the <code>bool worked</code> directly, of course!

Hope that helps!

blocks|key|2448333|text|我不喜欢C#，但是您可以看到try-catch-finally语句和普通控制流语句if-then-else之间的一些相似之处。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2448334|想想看，每当您throw一个异常时，您就强制将您的控制传递给catch子句。所以如果你有|offset|length|style|CODE|2448335|if+(doSomething()+==+BAD)+
{
++//recover+or+whatever
}|code-block|syntax|javascript|2448336|你可以很容易地把它想象成try-catch：|2448337|try
{
++doSomething();
}
catch+(Exception+e)
{
++//recover+or+do+whatever
}|2448338|exception的强大之处在于，你不必在同一个主体中改变程序的流程，只要你想要，你就可以抛出一个异常，并保证控制流会突然发散并到达catch子句。这是强大的，但同时也是危险的，因为您可以在结束时执行需要一些备份的操作，这就是finally语句存在的原因。|2448339|此外，您还可以在不有效使用while语句的条件的情况下对其进行建模：|2448340|while+(!finished)
{
++//do+whatever
}|2448341|可能会变成|2448342|try
{
++while+(true)
++{
+++++doSomethingThatEventuallyWillThrowAnException();
++}
}
catch+(Exception+e)
{
++//loop+finished
}|2448343|entityMap^0|0|7|5|U|5|0|0|0|0|35|7|0|D|5|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|13|8|@$D|14|E|15|F|G]|$D|16|E|17|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|18|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|19|8|@]|9|@]|A|$]]|$1|O|3|P|5|J|7|1A|8|@]|9|@]|A|$K|L]]|$1|Q|3|R|5|6|7|1B|8|@$D|1C|E|1D|F|G]]|9|@]|A|$]]|$1|S|3|T|5|6|7|1E|8|@$D|1F|E|1G|F|G]]|9|@]|A|$]]|$1|U|3|V|5|J|7|1H|8|@]|9|@]|A|$K|L]]|$1|W|3|X|5|6|7|1I|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|J|7|1J|8|@]|9|@]|A|$K|L]]|$1|10|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]]|11|$]]

I'm not fond of C# but you can see some similarities between try-catch-finally statements and normal control flow statements if-then-else.

Think about that whenever you <code>throw</code> an exception you force your control to be passed to the <code>catch</code> clause. So if you have

<pre><code>if (doSomething() == BAD) 
{
 //recover or whatever
}
</code></pre>

You can easily think of it in terms of try-catch:

<pre><code>try
{
 doSomething();
}
catch (Exception e)
{
 //recover or do whatever
}
</code></pre>

The powerful thing about exception is that you don't have to be in the same body to alter the flow of the program, you can throw an exception whenever you want with the guarantee that control flow will suddently diverge and reach the catch clause. This is powerful but dangerous at the same time since you could have done actions that need some backup at the end, that's why the <code>finally</code> statement exists.

In addition you can model also a <code>while</code> statement without effectively using the condition of it:

<pre><code>while (!finished)
{
 //do whatever
}
</code></pre>

can become

<pre><code>try
{
 while (true)
 {
 doSomethingThatEventuallyWillThrowAnException();
 }
}
catch (Exception e)
{
 //loop finished
}
</code></pre>

blocks|key|1485932|text|可能是我见过的最严重的违规行为：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1485933|//+I+haz+an+array...
public+int+ArrayCount(object[]+array)
{
++++int+count+=+0;
++++try
++++{
++++++++while+(true)
++++++++{
++++++++++++var+temp+=+array[count];
++++++++++++count%2B%2B;
++++++++}
++++}
++++catch+(IndexOutOfRangeException)
++++{
++++++++return+count;
++++}
}|code-block|syntax|javascript|1485934|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Probably the grossest violation I've ever seen:

<pre><code>// I haz an array...
public int ArrayCount(object[] array)
{
 int count = 0;
 try
 {
 while (true)
 {
 var temp = array[count];
 count++;
 }
 }
 catch (IndexOutOfRangeException)
 {
 return count;
 }
}
</code></pre>

blocks|key|742925|text|我目前正在使用一个第三方程序来做这件事。它们有一个“光标”接口(基本上是一个IEnumerable替代接口)，在那里告诉程序你已经完成的唯一方法就是引发一个异常。代码基本上如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|742926|//+Just+showing+the+relevant+section
bool+finished+=+false;

public+bool+IsFinished()
{
++++return+finished;
}

//+Using+something+like:
//+int+index+=+0;
//+int+count+=+42;

public+void+NextRecord()
{
++++if+(finished)
++++++++return;

++++if+(index+>=+count)
++++++++throw+new+APIProgramSpecificException("End+of+cursor",+WEIRD_CONSTANT);
++++else
++++++++%2B%2Bindex;
}

//+Other+methods+to+retrieve+the+current+value|code-block|syntax|javascript|742927|不用说，我讨厌+API+--但它是流控制异常的一个很好的例子(也是一种疯狂的工作方式)。|offset|length|style|BOLD|742928|entityMap^0|0|0|7|11|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I'm currently working with a 3rd party program that does this. They have a "cursor" interface (basically an IEnumerable alternative), where the only way to tell the program you're finished is to raise an exception. The code basically looks like:

<pre><code>// Just showing the relevant section
bool finished = false;

public bool IsFinished()
{
 return finished;
}

// Using something like:
// int index = 0;
// int count = 42;

public void NextRecord()
{
 if (finished)
 return;

 if (index &gt;= count)
 throw new APIProgramSpecificException("End of cursor", WEIRD_CONSTANT);
 else
 ++index;
}

// Other methods to retrieve the current value
</code></pre>

Needless to say, I hate the API - but its a good example of exceptions for flow control (and an insane way of working).

blocks|key|3856638|text|由合作伙伴开发的一个模块导致我们的应用程序需要很长时间才能加载。仔细检查后，该模块在应用程序启动时查找配置文件。这本身并不是很令人反感，但它做这件事的方式是非常糟糕的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3856639|对于app目录中的每个文件，它都会打开该文件并尝试将其解析为XML。如果一个文件抛出了一个异常(因为它不是XML)，它就会捕获这个异常，抑制它，并尝试下一个文件！|3856640|当合作伙伴测试此模块时，他们的app目录中只有3个文件。bonehead配置文件搜索对测试应用程序启动没有明显影响。当我们将它添加到我们的应用程序中时，应用程序目录中有100个文件，并且应用程序在启动时冻结了近一分钟。|3856641|为了增加安全性，模块正在搜索的配置文件的名称是预先确定的，并且是恒定的。不需要任何类型的文件搜索。|3856642|天才是有限度的。愚蠢是无限的。|3856643|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|N|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|O|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|P|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|K|$]]

A module developed by a partner caused our application to take a very long time to load. On closer examination, the module was looking for a config file at app startup. This by itself was not too objectionable, but the way in which it was doing it was outrageously bad:

For every file in the app directory, it opened the file and tried to parse it as XML. If a file threw an exception (because it wasn't XML), it caught the exception, squelched it, and tried the next file!

When the partner tested this module, they only had 3 files in the app directory. The bonehead config file search didn't have a noticeable effect on the test app startup. When we added it to our application, there were 100's of files in the app directory, and the app froze for nearly a minute at startup.

To add salt to the wound, the name of the config file the module was searching for was predetermined and constant. There was no need for a file search of any kind.

Genius has its limits. Stupidity is unbounded.

What would a piece of code which "uses exceptions to control flow" look like? I've tried to find a direct C# example, but cannot. Why is it bad?

Thanks

Example of "using exceptions to control flow"

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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一段“使用异常控制流”的代码会是什么样子的？我试图找到一个直接的C#示例，但是找不到。为什么它不好呢？谢谢

问“使用异常控制流”的示例
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