Errors and Exception-Handling in VB.NET

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We could deal with the possible divide-by-zero error within this function by using the code shown in Listing 549
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Function Divide(ByVal Num1 As Integer, _ ByVal Num2 As Integer) As Integer Try Divide = Num1 \ Num2 Catch div0Ex As DivideByZeroException ConsoleWriteLine("Error Divide by zero") Return 0 End Try End Function Listing 549: One possible solution not ideal
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The problem that this introduces is that we do not necessarily know where this program code would be called from The program may not use the console at all (it could be a WinForms program), or it may not be an interactive program, so there will be no user to inform In a situation like this, we can only deal with the error properly if we know the context of the program it is being used in, so attempting to recover from the error, informing the user of the error or simply ignoring the error may all be inappropriate The thing to do in this situation is to pass the error to the code that called the function throw an exception Since we already have a readymade exception (div0Ex), we can simply pass this back The code in Listing 550 re-throws the exception so that it will re-appear in the code from which the function was called While this may look like we are avoiding the issue, the situation leaves us with no alternatives, and the code that calls the function is much better placed to determine an appropriate response to this error The code calling the function will of course need to provide an exception-handler, or it will be prone to crashing
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Function Divide(ByVal Num1 As Integer, _ ByVal Num2 As Integer) As Integer Try Divide = Num1 \ Num2 Catch div0Ex As DivideByZeroException Throw div0Ex Return 0 End Try End Function Listing 550: A more general solution
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554 Exception-Handling Guidelines
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Unlike the other program structures that are dictated by algorithm and program design, exception-handling is more a matter of convention Most software houses
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5 n Controlling Program Code
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impose certain stylistic guidelines on their programmers so that one programmer can easily read and understand the work done by another Exception-handling is a subject area that different programmers deal with in different ways However, there are certain aspects of exception-handling that are worth consideration by any programmer
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Vet your code for potential errors, and add exception-handling wherever these may occur As you build and test programs, you will be informed by Visual Studio of errors that caused crashes When this happens, the name of the exception is provided, and you can use this information to provide exception-handlers It is tempting to wrap an entire program up in a TryCatch e As ExceptionEnd Try block, since then it will never crash However, you will never get to know of speci c problems in the code (they will be hidden by the exception handling) and will not be able to insert code to recover from otherwise easy to handle errors Deal with exceptions on a case by case basis; protect blocks where un-validated user-input is passed into variables, where calculations may cause over ows or divide-by-zero errors or where any external device such as a printer or disk drive is used Don t use exception-handlers to replace good structured programming For example, you might be tempted to use an exception-handler so that you can write code to attempt to access properties or methods of a reference variable that may not have an object attached to it It is much more sensible to test if the reference variable is null using If Not myObject Is Nothing Then Exceptionhandlers should not be used to permit bad coding You could avoid the issue of error-handling by throwing all exceptions back to the calling code Two things should stop you doing this: 1) you cannot do this from Sub Main () (or an event-handler which acts as an entry point to a WinForms application the way Sub Main () does to a console program), since there is no calling code to catch it, and 2) it is lazy and defeats the entire principle of SEH, which is to deal with errors at the most appropriate place in program code Exception-handling is class-based that is, exceptions are created as objects in code according to the error recognized at the point things are seen to have gone wrong NET provides a good range of Exception classes to take care of general problems (eg faulty casts, arithmetic problems, bad procedure calls, etc) and others to take care of speci c problems in user-interfaces (WinForms and WebForms), databases, Internet applications, etc You should use these classes wherever possible, and only derive your own exception classes (by inheritance) when the exception classes provided are inadequate for the purpose (ie rarely)
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