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Listing 101 throws a SystemApplicationException, not the SystemException type demonstrated in 4 C# allows code to throw any type that derives (perhaps indirectly) from SystemException
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10: Exception Handling
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The code for throwing any exception is simply to prefix the exception instance with the keyword throw The type of exception used is obviously the type that best describes the circumstances surrounding the error that caused the exception For example, consider the TextNumberParserParse() method in Listing 101
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Listing 101: Throwing an Exception
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public sealed class TextNumberParser { public static int Parse(string textDigit) { string[] digitTexts = { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" }; int result = ArrayIndexOf( digitTexts, textDigitToLower()); if (result < 0) { throw new ArgumentException( "The argument did not represent a digit", "textDigit"); } return result; } }
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Instead of throwing SystemException, it is more appropriate to throw ArgumentException because the type itself indicates what went wrong and includes special parameters for identifying which parameter was at fault Two similar exceptions are ArgumentNullException, and NullReferenceException ArgumentNullException should be thrown for the inappropriate passing of null arguments This is a special case of an invalid parameter exception that would more generally (when it wasn t null) be thrown as an ArgumentException or an ArgumentOutOfRangeException NullReferenceException is generally something that only the underlying runtime will throw with an attempt to dereference a null value to call a member on an object whose value is null Instead of causing a NullReferenceException, programmers should check parameters for null before
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accessing them and then throw an ArgumentNullException, which can provide more contextual information such as the parameter name
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Throwing a particular exception type enables the type itself to identify the problem It is not necessary, in other words, to catch the exception and use a switch statement on the exception message to determine what action to take in light of the exception Instead, C# allows for multiple catch blocks, each targeting a specific exception type, as Listing 102 shows
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Listing 102: Catching Different Exception Types
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public sealed class Program { public static void Main() { try { // throw new ApplicationException( "Arbitrary exception"); // } catch (NullReferenceException exception) { // Handle NullReferenceException } catch (ArgumentException exception) { // Handle ArgumentException } catch (ApplicationException exception) { // Handle ApplicationException } catch (SystemException exception) { // Handle SystemException } catch (Exception exception) { // Handle Exception } } }
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10: Exception Handling
Listing 102 has five catch blocks, each handling a different type of exception When an exception occurs, the execution will jump to the catch block with the exception type that most closely matches The closeness of a match is determined by the inheritance chain For example, even though the exception thrown is of type SystemException, this is a relationship occurs through inheritance because SystemApplicationException derives from SystemException Since ApplicationException most closely matches the exception thrown, catch(ApplicationException ) will catch the exception instead of the catch(Exception) block Catch blocks must appear in order, from most specific to most general, to avoid a compile error For example, moving the catch(Exception ) block before any of the other exceptions will result in a compile error, since all prior exceptions derive from SystemException at some point in their inheritance chain
Language Contrast: Java Exception Specifiers
C# has no equivalent for Java s exception specifiers With exception specifiers, the Java compiler is able to verify that all possible exceptions thrown within a function (or a function s call hierarchy) are either caught or declared as possibly rethrown The C# team considered this option and concluded that the maintenance burden that it imposed was not worth the perceived benefit Therefore, it is not necessary to maintain a list of all possible exceptions throughout a particular call stack, but neither is it possible to easily determine the possible exceptions (As it turns out, this wasn t possible for Java either Calling virtual methods or using late binding, such as reflection, made it impossible to fully resolve at compile time what exceptions a method could possibly throw)