how to generate qr code in asp.net using c# LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT in Visual Basic .NET

Draw QR Code ISO/IEC18004 in Visual Basic .NET LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT

CHAPTER 2 LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT
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variables that are assigned at the end of the method. The following rewritten DoSomething method can cope with exceptions: public void DoSomething( int param1, int param2) { int temp = param1; AssignState( param2); _variable = temp; } In the rewritten example of DoSomething, the parameter param1 is written to a temporary variable that s stored locally within the function. The variable _variable is only assigned once all the child methods have been called. The solution is correct, but a new coding style is defined, and the rules of thumb are defined as follows: Assign state as late as possible: Assigning state as late as possible dramatically reduces the chances that an exception will arise that violates state. If state must be assigned use a try . . . finally block: Using a try . . . finally block, it s possible to catch any thrown exception and fix up the modified state to the original state before the method is called. In general, write code that modifies as little global state as possible: Often it isn t possible to write code that doesn t modify state, but making an effort to work with local objects reduces the chances that a state error will occur.
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Implementing an Exception Strategy
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When something goes wrong in an application, the best way to indicate that a failure has occurred is to use an exception. Exceptions are generally expensive in computational terms, but remember exceptions only occur when something goes wrong. Ideally, nothing goes wrong in an application and only a few exceptions are generated. If too many exceptions are generated, then there are problems in the design of the application.
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Exceptions 101
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In the .NET environment, all exception classes are subclasses of the type Exception. Trying to throw an exception using a type that doesn t directly or indirectly inherit from the Exception type generates an error when the code is compiled. The following example generates an exception: public void ThrowException() { throw new Exception( "My error"); } You use the keyword throw to generate an exception, and an instantiated class is thrown. In the preceding example, the constructor parameter is a message that indicates why the exception was thrown. The Exception type can still be thrown if you don t supply any type of message. However, if you don t provide a message, then when the exception is thrown, the exception catcher has no idea what the problem was. To catch an exception, you need to define a try and catch block; an example follows:
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CHAPTER 2 LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT
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public void CatchException() { try { ThrowException () } catch( Exception ex) { // Do something with exception } } The method CatchException has an embedded try and catch block that s used to trap an exception that s thrown. All methods called within the try block are tracked for possible exceptions to trap. The exceptions that are caught depend on the contents of the catch method call. I put the quotes around the word method in the context of catch because the catch statement defines a sub-block. The sub-block is executed if the types of the catch method match, or are a subclass of, the exception being thrown. In the example, the type is Exception, which means the sub-block will catch all exceptions that can be thrown. The Exception type has the following properties, which you can query or assign: InnerException: References the embedded exception, which the following section will explain in greater detail. Message: A string property that s a textual representation of the exception s cause. Source: A string property that represents the name of the application that causes the exception. StackTrace: A string property that contains the function stack state at the time the exception was thrown. This property is useful when performing a postmortem analysis of what went wrong. TargetSite: References an object instance of type MethodBase that references the method that threw the exception. An exception can embed other exceptions. For example, imagine writing an assembly that uses an externally developed assembly. If an exception arises within the external assembly, the assembly will catch it. The problem is: what does the assembly do with the exception The simplest solution is to catch the exception, analyze it, and throw the exception again. It s possible to process the exception, but if the error is fatal then the calling application needs to know about the problem. The following example analyzes a caught exception and then throws it again: public void MyAssembly() { try { externalAssembly.externalMethod() } catch( Exception ex) { // Do something throw ex; } }
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