4: Methods and Parameters in Visual C#.NET

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4: Methods and Parameters
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ConsoleWrite("Enter your first name: "); firstName = ConsoleReadLine(); ConsoleWrite("Enter your age: "); ageText = ConsoleReadLine(); try { age = intParse(ageText); ConsoleWriteLine( "Hi {0}! You are {1} months old", firstName, age*12); } finally { ConsoleWriteLine("Goodbye {0}", firstName); } return result; } }
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OUTPUT 414:
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Enter your first name: Inigo Enter your age: forty-two Unhandled Exception: SystemFormatException: Input string was not in a correct format at SystemNumberStringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal) at SystemNumberParseInt32(String s, NumberStyles style, NumberFormatInfo info) at ExceptionHandlingMain() Goodbye Inigo
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When this code executes, the finally block executes before printing an unhandled exception to the console (an unhandled exception dialog may also appear)
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ADVANCED TOPIC Exception Class Inheritance Starting in C# 20, all exceptions derive from SystemException Therefore, they can be handled by the catch(SystemException exception) block It
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Basic Error Handling with Exceptions
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is preferable, however, to include a catch block that is specific to the most derived type (SystemFormatException, for example), because then it is possible to get the most information about an exception and handle it less generically In so doing, the catch statement that uses the most derived type is able to handle the exception type specifically, accessing data related to the exception thrown, and avoiding conditional logic to determine what type of exception occurred This is why C# enforces that catch blocks appear from most derived to least derived For example, a catch statement that catches SystemException cannot appear before a statement that catches SystemFormat Exception because SystemFormatException derives from SystemException A method could throw many exception types Table 42 lists some of the more common ones within the framework
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TABLE 42: Common Exception Types
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Exception Type
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SystemException
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Description
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A generic exception from which other exceptions derive A means of indicating that one of the parameters passed into the method is invalid Indicates that a particular parameter is null and that this is not valid for that parameter To be avoided Originally the idea that you might want to have one kind of handling for system exceptions and another for application exceptions, although plausible, doesn t actually work well in the real world Indicates that the string format is not valid for conversion Indicates that an attempt was made to access an array element that does not exist
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Continues
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SystemArgumentException
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SystemArgumentNullException
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SystemApplicationException
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SystemFormatException
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SystemIndexOutOfRangeException
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4: Methods and Parameters
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TABLE 42: Common Exception Types (Continued)
Exception Type
SystemInvalidCastException
Description
Indicates that an attempt to convert from one data type to another was not a valid conversion Indicates that although the method signature exists, it has not been fully implemented Throws when code tries to find the object referred to by a reference (such as a variable) which is null Indicates an invalid math operation, not including divide by zero Occurs when attempting to store an element of the wrong type into an array Generally indicates that there is an infinite loop in which a method is calling back into itself (known as recursion)
SystemNotImplementedException
SystemNullReferenceException
SystemArithmeticException
SystemArrayTypeMismatchException
SystemStackOverflowException
ADVANCED TOPIC Generic catch It is possible to specify a catch block that takes no parameters, as shown in Listing 422
Listing 422: General Catch Blocks
try { age = intParse(ageText); SystemConsoleWriteLine( "Hi {0}! You are {1} months old", firstName, age*12); } catch (SystemFormatException exception)
Basic Error Handling with Exceptions
{ SystemConsoleWriteLine( "The age entered ,{0}, is not valid", ageText); result = 1; } catch(SystemException exception) { SystemConsoleWriteLine( "Unexpected error: {0}", exceptionMessage); result = 1; } catch { SystemConsoleWriteLine( "Unexpected error!"); result = 1; } finally { SystemConsoleWriteLine("Goodbye {0}", firstName); }
A catch block with no data type, called a generic catch block, is equivalent to specifying a catch block that takes an object data type: for instance, catch(object exception){} And since all classes ultimately derive from object, a catch block with no data type must appear last Generic catch blocks are rarely used because there is no way to capture any information about the exception In addition, C# doesn t support the ability to throw an exception of type object (Only libraries written in languages such as C++ allow exceptions of any type) The behavior starting in C# 20 varies slightly from the earlier C# behavior In C# 20, if a language allows non-SystemExceptions, the object of the thrown exception will be wrapped in a SystemRuntime CompilerServicesRuntimeWrappedException which does derive from SystemException Therefore, all exceptions, whether deriving from SystemException or not, will propagate into C# assemblies as derived from SystemException The result is that SystemException catch blocks will catch all exceptions not caught by earlier blocks, and a general catch block, following a
4: Methods and Parameters SystemException catch block, will never be invoked Because of this,
following a SystemException catch block with a general catch block in C# 20 or later will result in a compiler warning indicating that the general catch block will never execute
Reporting Errors Using a throw Statement Just as intParse() can throw an exception, C# allows developers to throw exceptions from their code, as demonstrated in Listing 423 and Output 415