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20 Exceptions and State Management
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The catch Block
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A catch block contains code to execute in response to an exception . A try block can have zero or more catch blocks associated with it . If the code in a try block doesn t cause an exception to be thrown, the CLR will never execute the code contained within any of its catch blocks . The thread will simply skip over all of the catch blocks and execute the code in the finally block (if one exists) . After the code in the finally block executes, execution continues with the statement following the finally block . The parenthetical expression appearing after the catch keyword is called the catch type . In C#, you must specify a catch type of System.Exception or a type derived from System.Exception . For example, the previous code contains catch blocks for handling an InvalidOperationException (or any exception derived from it) and an IOException (or any exception derived from it) . The last catch block (which doesn t specify a catch type) handles any exception at all except for the exception type specified by earlier catch blocks; this is equivalent to having a catch block that specifies a catch type of System.Exception except that you cannot access the exception information via code inside the catch block s braces . Note When debugging through a catch block by using Microsoft Visual Studio, you can see
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the currently thrown exception object by adding the special $exception variable name to a watch window .
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The CLR searches from top to bottom for a matching catch type, and therefore you should place the more specific exception types at the top . The most-derived exception types should appear first, followed by their base types (if any), down to System.Exception (or an exception block that doesn t specify a catch type) . In fact, the C# compiler generates an error if more specific catch blocks appear closer to the bottom because the catch block would be unreachable . If an exception is thrown by code executing within the try block (or any method called from within the try block), the CLR starts searching for catch blocks whose catch type is the same type as or a base type of the thrown exception . If none of the catch types matches the exception, the CLR continues searching up the call stack looking for a catch type that matches the exception . If after reaching the top of the call stack, no catch block is found with a matching catch type, an unhandled exception occurs . I ll talk more about unhandled exceptions later in this chapter . Once the CLR locates a catch block with a matching catch type, it executes the code in all inner finally blocks, starting from within the try block whose code threw the exception and stopping with the catch block that matched the exception . Note that any finally block associated with the catch block that matched the exception is not executed yet . The code in this finally block won t execute until after the code in the handling catch block has executed .
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Part IV
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After all the code in the inner finally blocks has executed, the code in the handling catch block executes . This code typically performs some operations to deal with the exception . At the end of the catch block, you have three choices:
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Re-throw the same exception, notifying code higher up in the call stack of the exception . Throw a different exception, giving richer exception information to code higher up in the call stack . Let the thread fall out of the bottom of the catch block .
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Later in this chapter, I ll offer some guidelines for when you should use each of these techniques . If you choose either of the first two techniques, you re throwing an exception, and the CLR behaves just as it did before: It walks up the call stack looking for a catch block whose type matches the type of the exception thrown . If you pick the last technique, when the thread falls out of the bottom of the catch block, it immediately starts executing code contained in the finally block (if one exists) . After all of the code in the finally block executes, the thread drops out of the finally block and starts executing the statements immediately following the finally block . If no finally block exists, the thread continues execution at the statement following the last catch block . In C#, you can specify a variable name after a catch type . When an exception is caught, this variable refers to the System.Exception-derived object that was thrown . The catch block s code can reference this variable to access information specific to the exception (such as the stack trace leading up to the exception) . Although it s possible to modify this object, you shouldn t; consider the object to be read-only . I ll explain the Exception type and what you can do with it later in this chapter . Note Your code can register with AppDomain s FirstChanceException event to receive notifications as soon as an exception occurs within an AppDomain . This notification occurs before the CLR searches for any catch blocks . For more information about this event, see 22, CLR Hosting and AppDomains .
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