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20 Exceptions and State Management
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private static void SomeMethod(String filename) { try { // Do whatevere here... } catch (IOException e) { // Add the filename to the IOException object e.Data.Add("Filename", filename); throw; } } // re-throw the same exception object that now has additional data in it
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Here is a good use of this technique: When a type constructor throws an exception that is not caught within the type constructor method, the CLR internally catches that exception and throws a new TypeInitializationException instead . This is useful because the CLR emits code within your methods to implicitly call type constructors .6 If the type constructor threw a DivideByZeroException, your code might try to catch it and recover from it but you didn t even know you were invoking the type constructor . So the CLR converts the DivideByZeroException into a TypeInitializationException so that you know clearly that the exception occurred due to a type constructor failing; the problem wasn t with your code . On the other hand, here is a bad use of this technique: When you invoke a method via reflection, the CLR internally catches any exception thrown by the method and converts it to a TargetInvocationException . This is incredibly annoying as you must now catch the TargetInvocationException object and look at its InnerException property to discern the real reason for the failure . In fact, when using reflection, it is common to see code that looks like this:
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private static void Reflection(Object o) { try { // Invoke a DoSomething method on this object var mi = o.GetType().GetMethod("DoSomething"); mi.Invoke(o, null); // The DoSomething method might throw an exception } catch (System.Reflection.TargetInvocationException e) { // The CLR converts reflection-produced exceptions to TargetInvocationException throw e.InnerException; // Re-throw what was originally thrown } }
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I have good news though: If you use C# s dynamic primitive type (discussed in 5, Primitive, Reference, and Value Types ) to invoke a member, the compiler-generated code does not catch any and all exceptions and throw a TargetInvocationException object; the originally thrown exception object simply walks up the stack . For many developers, this is a good reason to prefer using C# s dynamic primitive type rather than reflection .
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For more information about this, see the Type Constructors section in 8, Methods .
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Part IV
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unhandled Exceptions
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When an exception is thrown, the CLR climbs up the call stack looking for catch blocks that match the type of the exception object being thrown . If no catch block matches the thrown exception type, an unhandled exception occurs . When the CLR detects that any thread in the process has had an unhandled exception, the CLR terminates the process . An unhandled exception identifies a situation that the application didn t anticipate and is considered to be a true bug in the application . At this point, the bug should be reported back to the company that publishes the application . Hopefully, the publisher will fix the bug and distribute a new version of the application . Class library developers should not even think about unhandled exceptions . Only application developers need to concern themselves with unhandled exceptions, and the application should have a policy in place for dealing with unhandled exceptions . Microsoft actually recommends that application developers just accept the CLR s default policy . That is, when an application gets an unhandled exception, Windows writes an entry to the system s event log . You can see this entry by opening the Event Viewer application and then looking under Windows Logs Application node in the tree, as shown in Figure 20-1 .
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FIguRE 20-1 Windows Event log showing an application that terminated due to an unhandled exception
However, you can get more interesting details about the problem by using the Windows Action Center applet . To start the Action Center, click on the flag icon in the system tray, select Open Action Center, expand the Maintenance box, and then select the View reliability history link . From here, you can see the applications that have terminated due to an unhandled exception in the bottom pane, as shown in Figure 20-2 .
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