c# barcode zebra printer Dealing with errors you just knew were going to happen in Visual Basic .NET

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22.1 Dealing with errors you just knew were going to happen
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To be clear, I m not talking about errors that you make, such as typos, using the wrong syntax, or something like that. Your errors are called bugs, and we ll deal with those in the next chapter. This chapter is going to deal with the errors that are out of your control, but that you can usually anticipate. Here are some examples:
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A file not found error A permission denied error The RPC server not found error that Get-WmiObject can produce Other errors related to network connectivity
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You can t necessarily prevent these errors from happening, but when they happen you may want to take some specific action. For example, you might want to log the names of computers that can t be reached, or prompt for a different filename if the one specified can t be found. PowerShell offers you a number of ways to deal with these kinds of errors, and we ll cover the two most commonly used ways in this chapter.
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22.2 Errors and exceptions
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First, we need to get some terminology straight. Try running this command:
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Get-WmiObject Win32_BIOS -computer notonline,localhost
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The $ErrorActionPreference variable
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Assuming you don t have a computer named NOTONLINE on your network, this command will produce an error message, or what I ll refer to as an error. It does that because you didn t tell it to do anything else, and because the shell s default action for a nonterminating problem is to display an error and try to keep going. Nonterminating simply means that, although the problem interrupted this particular operation, the command is able to continue executing. In this example, it can continue trying the next computer name that was specified. PowerShell doesn t give you a way to deal with errors like this. It shouldn t; after all, the whole point of its default behavior is to report the problem and keep going. In many cases, that s perfectly acceptable. When you re running a command from the command line, for example, error messages tell you what went wrong, and that s sufficient. But if you re running a script especially a script that might be scheduled to run unattended you won t be around to see the error message, and you ll want the option to do something about the problem, like logging it to a file. To do that, you need to turn the error into an exception. Another way of saying this is that you need to turn the nonterminating problem into a terminating one, forcing the shell to stop executing the command, and to instead do what you tell it.
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22.3 The $ErrorActionPreference variable
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The shell s default error-handling behavior is defined by a built-in variable called $ErrorActionPreference. When you open a new shell session, this variable is set to Continue. Its possible values, and their functions, are as follows:
SilentlyContinue For nonterminating problems, don t display an error mes-
sage just keep going. Continue For nonterminating problems, display an error message and keep going. Inquire For nonterminating problems, ask what to do using an interactive prompt to which the user must respond. Stop Stop executing and throw an exception.
Anytime a command runs into a terminating problem from which it can t recover and continue, the behavior is always Stop. The exception thrown by Stop is something you can trap and handle. Please, please, please, please, don t ever put this at the top of a script:
$ErrorActionPreference = 'SilentlyContinue'
People do that (you ll see it in internet examples all the time) because they anticipate their script having a problem, and they know it s safe to ignore it, and they don t want to see an error message. This is an incredibly poor practice, because it also suppresses any error messages that might help you debug the script. For example, if you edit the script and make a typo somewhere, you won t see an error message when you run the
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