SystemArgumentNullException in C#

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SystemArgumentNullException
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SystemApplicationException
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SystemFormatException
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Exception Type
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SystemIndexOutOfRangeException
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Description Indicates that an attempt was made to access an array element that does not exist 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 Thrown when code tries to access a variable that does not yet contain any data 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)
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SystemInvalidCastException
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SystemNotImplementedException
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SystemArithmeticException SystemArrayTypeMismatchException
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SystemStackOverflowException
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Advanced Topic: Generic catch
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It is possible to specify a catch block that takes no parameters, as shown in Listing 420
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Listing 420 General Catch Blocks
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try { age = intParse(ageText); SystemConsoleWriteLine( "Hi {0}! You are {1} months old", firstName, age*12); } catch (SystemFormatException exception) { SystemConsoleWriteLine( "The age entered ,{0}, is not valid", ageText); result = 1; } catch(SystemException exception) { SystemConsoleWriteLine( "Unexpected error: {0}", exceptionMessage); result = 1; } catch { SystemConsoleWriteLine(
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"Unexpected error!"); result = 1; } finally { SystemConsoleWriteLine("Goodbye {0}", firstName); }
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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 like C++ allow exceptions of any type) The behavior 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 SystemRuntimeCompilerServicesRuntimeWrappedException which does derive from System Exception 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 SystemException catch block, will never be invoked Because of this, following a SystemException catch block with a general catch block in C# 20 will result in a compiler warning indicating the general catch block will never execute
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Reporting Errors Using a throw Statement
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Just as intParse() can throw an exception, C# allows developers to throw exceptions from their code, as demonstrated in Listing 421 and Output 415
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Listing 421 Throwing an Exception
Output 415
Begin executing Throw exception Unexpected error: Arbitrary exception Shutting down
As the arrows in Listing 421 depict, throwing an exception jumps execution from where the exception is thrown into the first catch block within the stack that is compatible with the thrown exception type In this case, the second catch block handles the exception and writes out an error message In Listing 421, there is no finally block, so execution falls through to the SystemConsoleWriteLine() statement following the try/catch block In order to throw an exception, it is necessary to have an instance of an exception Listing 421 creates an instance using the keyword new followed by the data type of the exception Most exception types allow a message as part of throwing the exception so that when the exception occurs, the message can be retrieved Sometimes a catch block will trap an exception but be unable to handle it appropriately or fully In these circumstances, a catch block can rethrow the exception using the throw statement without specifying any exception, as shown in Listing 422
Listing 422 Rethrowing an Exception
catch(Exception exception) { ConsoleWriteLine( "Rethrowing unexpected error: {0}", exceptionMessage); throw; }
Avoid Using Exception Handling to Deal with Expected Situations
As with most languages, C# incurs a performance hit when throwing an exception, especially the first time the error-handling infrastructure needs to be loaded For example, running Listing 418 and entering an invalid age demonstrates a noticeable pause while the program throws and handles the exception Because of the performance cost associated with throwing exceptions, developers should make an effort to avoid throwing exceptions for expected conditions or normal control flow For example, developers should expect users to enter invalid text when specifying their age[2] Therefore, instead of relying on an exception to validate data entered by the user, developers should provide a means of checking the data before attempting the conversion Better yet, you should prevent the user from entering invalid data in the first place
In general, developers should expect their users to perform unexpected actions, and therefore they should code defensively to handle "stupid user tricks"
Advanced Topic: Numeric Conversion with TRyParse()
One of the problems with the Parse() method is that the only way to determine whether the conversion will be successful is to attempt the cast and then catch the exception if it doesn't work Because throwing an exception is a relatively expensive operation, it is better to attempt the conversion without exception handling In the first release of C#, the only data type that enabled this was a double method called doubleTryParse() However, the CLI added this method to all numeric primitive types in the CLI 20 version It requires the use of the out keyword because the return from the tryParse() function is a bool rather than the converted value Here is a code listing that demonstrates the conversion using intTryParse()
if (intTryParse(ageText, out age)) { SystemConsoleWriteLine( "Hi {0}! You are {1} months old", firstName, age * 12); } else { SystemConsoleWriteLine( "The age entered ,{0}, is not valid", ageText); }
With the tryParse() method, it is no longer necessary to include a try/catch block simply for the purpose of handling the string-to-numeric conversion
Summary
This chapter discussed the details of declaring and calling methods In many ways, this construct is identical to its declaration in C-like languages However, the addition of the keywords out and ref are more like COM (the predecessor to CLI technology) than C-like language features In addition to method declaration, this chapter introduced exception handling Methods are a fundamental construct that is a key to writing readable code Instead of writing large methods with lots of statements, you should use methods for "paragraphs" within your code, whose lengths target roughly 10 lines or less The process of breaking large functions into smaller pieces is one of the ways you can refactor your code to make it more readable and maintainable The next chapter considers the class construct and how it encapsulates methods (behavior) and fields (data) into a single unit