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Part IV
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FCL-Defined Exception Classes
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The Framework Class Library (FCL) defines many exception types (all ultimately derived from System.Exception) . The following hierarchy shows the exception types defined in the MSCorLib .dll assembly; other assemblies define even more exception types . (The application used to obtain this hierarchy is shown in 23, Assembly Loading and Reflection . )
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System.Exception System.AggregateException System.ApplicationException System.Reflection.InvalidFilterCriteriaException System.Reflection.TargetException System.Reflection.TargetInvocationException System.Reflection.TargetParameterCountException System.Threading.WaitHandleCannotBeOpenedException System.InvalidTimeZoneException System.IO.IsolatedStorage.IsolatedStorageException System.Runtime.CompilerServices.RuntimeWrappedException System.SystemException System.AccessViolationException System.AppDomainUnloadedException System.ArgumentException System.ArgumentNullException System.ArgumentOutOfRangeException System.DuplicateWaitObjectException System.Globalization.CultureNotFoundException System.Text.DecoderFallbackException System.Text.EncoderFallbackException System.ArithmeticException System.DivideByZeroException System.NotFiniteNumberException System.OverflowException System.ArrayTypeMismatchException System.BadImageFormatException System.CannotUnloadAppDomainException System.Collections.Generic.KeyNotFoundException System.ContextMarshalException System.DataMisalignedException System.ExecutionEngineException System.FormatException System.Reflection.CustomAttributeFormatException System.IndexOutOfRangeException System.InsufficientExecutionStackException System.InvalidCastException System.InvalidOperationException System.ObjectDisposedException System.InvalidProgramException System.IO.IOException System.IO.DirectoryNotFoundException System.IO.DriveNotFoundException System.IO.EndOfStreamException System.IO.FileLoadException System.IO.FileNotFoundException System.IO.PathTooLongException System.MemberAccessException System.FieldAccessException
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20 Exceptions and State Management
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System.MethodAccessException System.MissingMemberException System.MissingFieldException System.MissingMethodException System.MulticastNotSupportedException System.NotImplementedException System.NotSupportedException System.PlatformNotSupportedException System.NullReferenceException System.OperationCanceledException System.Threading.Tasks.TaskCanceledException System.OutOfMemoryException System.InsufficientMemoryException System.RankException System.Reflection.AmbiguousMatchException System.Reflection.ReflectionTypeLoadException System.Resources.MissingManifestResourceException System.Resources.MissingSatelliteAssemblyException System.Runtime.InteropServices.ExternalException System.Runtime.InteropServices.COMException System.Runtime.InteropServices.SEHException System.Runtime.InteropServices.InvalidComObjectException System.Runtime.InteropServices.InvalidOleVariantTypeException System.Runtime.InteropServices.MarshalDirectiveException System.Runtime.InteropServices.SafeArrayRankMismatchException System.Runtime.InteropServices.SafeArrayTypeMismatchException System.Runtime.Remoting.RemotingException System.Runtime.Remoting.RemotingTimeoutException System.Runtime.Remoting.ServerException System.Runtime.Serialization.SerializationException System.Security.Cryptography.CryptographicException System.Security.Cryptography.CryptographicUnexpectedOperationException System.Security.HostProtectionException System.Security.Policy.PolicyException System.Security.Principal.IdentityNotMappedException System.Security.SecurityException System.Security.VerificationException System.Security.XmlSyntaxException System.StackOverflowException System.Threading.AbandonedMutexException System.Threading.SemaphoreFullException System.Threading.SynchronizationLockException System.Threading.ThreadAbortException System.Threading.ThreadInterruptedException System.Threading.ThreadStartException System.Threading.ThreadStateException System.TimeoutException System.TypeInitializationException System.TypeLoadException System.DllNotFoundException System.EntryPointNotFoundException System.TypeUnloadedException System.UnauthorizedAccessException System.Security.AccessControl.PrivilegeNotHeldException System.Threading.LockRecursionException System.Threading.Tasks.TaskSchedulerException System.TimeZoneNotFoundException
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Part IV
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Microsoft s original idea was that System.Exception would be the base type for all exceptions and that two other types, System.SystemException and System. ApplicationException, would be the only two types immediately derived from Exception . Furthermore, exceptions thrown by the CLR would be derived from SystemException, and all application-thrown exceptions would be derived from ApplicationException . This way, developers could write a catch block that catches all CLR-thrown exceptions or all application-thrown exceptions . However, as you can see, this rule was not followed very well; some exception types are immediately derived from Exception (IsolatedStorageException), some CLR-thrown exceptions are derived from ApplicationException (TargetInvocationException), and some application-thrown exceptions are derived from SystemException (FormatException) . So it is all a big mess, and the result is that the SystemException and ApplicationException types have no special meaning at all . At this point, Microsoft would like to remove them from the exception class hierarchy, but they can t because it would break any code that already references these two types .
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Throwing an Exception
When implementing your own methods, you should throw an exception when the method cannot complete its task as indicated by its name . When you want to throw an exception, there are two issues that you really need to think about and consider . The first issue is about deciding what Exception-derived type are you going to throw . You really want to select a type that is meaningful here . Consider the code that is higher up the call stack and how that code might want to determine that a method failed in order to execute some graceful recovery code . You can use a type that is already defined in the FCL, but there may not be one in the FCL that matches your exact semantics . So you ll probably need to define your own type, ultimately derived from System.Exception . If you want to define an exception type hierarchy, it is highly recommended that the hierarchy be shallow and wide in order to create as few base classes as possible . The reason is that base classes act as a way of treating lots of errors as one error, and this is usually dangerous . Along these lines, you should never throw a System.Exception object,3 and you should use extreme caution if you throw any other base class exception type .
In fact, the System.Exception class should have been marked as abstract, which would forbid code that tried to throw it from even compiling .
20 Exceptions and State Management
Important There are versioning ramifications here, too . If you define a new exception type derived from an existing exception type, then all code that catches the existing base type will now catch your new type as well . In some scenarios this may be desired and in some scenarios, it may not be desired . The problem is that it really depends on how code that catches the base class responds to the exception type and types derived from it . Code that never anticipated the new exception may now behave unpredictably and open security holes . The person defining the new exception type can t know about all the places where the base exception is caught and how it is handled . And so, in practice, it is impossible to make a good intelligent decision here .
The second issue is about deciding what string message are you going to pass to the exception type s constructor . When you throw an exception, you should include a string message with detailed information indicating why the method couldn t complete its task . If the exception is caught and handled, this string message is not seen . However, if the exception becomes an unhandled exception, this message is usually logged . An unhandled exception indicates a true bug in the application, and a developer must get involved to fix the bug . An end user will not have the source code or the ability to fix the code and recompile it . In fact, this string message should not be shown to an end user . So these string messages can be very technically detailed and as geeky as is necessary to help developers fix their code . Furthermore, since all developers have to speak English (at least to some degree, since programming languages and the FCL classes and methods are in English), there is usually no need to localize exception string messages . However, you may want to localize the strings if you are building a class library that will be used by developers who speak different languages . Microsoft localizes the exception messages thrown by the FCL, since developers all over the world will be using this class library .
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