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20 Exceptions and State Management
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performance . Some implementations compile exception handling constructs directly into a method, whereas other implementations store information related to exception handling in a data table associated with the method this table is accessed only if an exception is thrown . Some compilers can t inline methods that contain exception handlers, and some compilers won t enregister variables if the method contains exception handlers . The point is that you can t determine how much additional overhead is added to an application when using exception handling . In the managed world, it s even more difficult to tell because your assembly s code can run on any platform that supports the .NET Framework . So the code produced by the JIT compiler to manage exception handling when your assembly is running on an x86 machine will be very different from the code produced by the JIT compiler when your code is running on an x64 or IA64 processor . Also, JIT compilers associated with other CLR implementations (such as Microsoft s .NET Compact Framework or the open-source Mono project) are likely to produce different code . Actually, I ve been able to test some of my own code with a few different JIT compilers that Microsoft has internally, and the difference in performance that I ve observed has been quite dramatic and surprising . The point is that you must test your code on the various platforms that you expect your users to run on, and make changes accordingly . Again, I wouldn t worry about the performance of using exception handling; because the benefits typically far outweigh any negative performance impact . If you re interested in seeing how exception handling impacts the performance of your code, you can use the Performance Monitor tool that comes with Windows . The screen in Figure 20-8 shows the exception-related counters that are installed along with the .NET Framework .
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FIguRE 20-8 Performance Monitor showing the .NET CLR Exceptions counters
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Part IV
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Occasionally, you come across a method that you call frequently that has a high failure rate . In this situation, the performance hit of having exceptions thrown can be intolerable . For example, Microsoft heard back from several customers who were calling Int32 s Parse method, frequently passing in data entered from an end user that could not be parsed . Since Parse was called frequently, the performance hit of throwing and catching the exceptions was taking a large toll on the application s overall performance . To address customers concerns and to satisfy all the guidelines described in this chapter, Microsoft added a new method to the Int32 class . This new method is called TryParse, and it has two overloads that look like this:
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public static Boolean TryParse(String s, out Int32 result); public static Boolean TryParse(String s, NumberStyles styles, IFormatProvider, provider, out Int32 result);
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You ll notice that these methods return a Boolean that indicates whether the String passed in contains characters that can be parsed into an Int32 . These methods also return an output parameter named result . If the methods return true, result will contain the result of parsing the string into a 32-bit integer . If the methods return false, result will contain 0, but you really shouldn t execute any code that looks at it anyway . One thing I want to make absolutely clear: A TryXxx method s Boolean return value returns false to indicate one and only one type of failure . The method should still throw exceptions for any other type of failure . For example, Int32 s TryParse throws an ArgumentException if the style s argument is not valid, and it is certainly still possible to have an OutOfMemoryException thrown when calling TryParse . I also want to make it clear that object-oriented programming allows programmers to be productive . One way that it does this is by not exposing error codes in a type s members . In other words, constructors, methods, properties, etc . are all defined with the idea that calling them won t fail . And, if defined correctly, for most uses of a member, it will not fail, and there will be no performance hit because an exception will not be thrown . When defining types and their members, you should define the members so that it is unlikely that they will fail for the common scenarios in which you expect your types to be used . If you later hear from users that they are dissatisfied with the performance due to exceptions being thrown, then and only then should you consider adding TryXxx methods . In other words, you should produce the best object model first and then, if users push back, add some TryXxx methods to your type so that the users who experience performance trouble can benefit . Users who are not experiencing performance trouble should continue to use the non-TryXxx versions of the methods because this is the better object model .
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