Keeping support functions private in VB.NET

Generating Data Matrix in VB.NET Keeping support functions private

21.5 Keeping support functions private
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If you saved either listing 21.5 or 21.6 as a module and imported it, take a look at the shell s FUNCTION: drive by running dir function:. You ll notice that your worker functions show up, which isn t what you want. Ideally, users should only see the public function, and the worker function should be hidden. There s an easy way to achieve that: by default, importing a module makes every function inside of it available to users. But if the module includes specific instructions for what should be visible, then only those things will be. Listing 21.7 is a revision of listing 21.6, and you ll see where I added a few specific instructions.
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Listing 21.7 Making a script module that has private functions
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function RebootWork { param([string]$computername) Get-WmiObject Win32_OperatingSystem -computer $computername | Invoke-WmiMethod -name Reboot | Out-Null } function Reboot-Server {
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Creating your own cmdlets and modules
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[CmdletBinding(SupportsShouldProcess=$True, ConfirmImpact='High')] param ( [Parameter(Mandatory=$True, ValueFromPipeline=$True, ValueFromPipelineByPropertyName=$True)] [Alias('host')] [string[]]$computername ) BEGIN {} PROCESS { foreach ($computer in $computername) { if ($pscmdlet.ShouldProcess($computer)) { RebootWork -computername $computer } } } END {} } New-Alias rbt Reboot-Server Export-ModuleMember -function Reboot-Server Export-ModuleMember -alias rbt
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The last three lines of code are the only additions. I ve defined an alias, rbt, for the Reboot-Server function. I ve also specified that only that alias and the RebootServer function should be visible to someone who imports this function into their shell; the RebootWork function will remain hidden and inaccessible to users.
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21.6 Lab
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I have two tasks for you to accomplish in this lab. These might take you longer than you have left in the current lunch hour, so feel free to spread them out over a couple of days, if needed. First, write a filtering function. The ideal use for a filtering function is when you need to pipe in some objects, analyze them in some way, and possibly remove some from the pipeline. In this case, I want your function to accept computer names and ping them. If the computer names are reachable, your function should output them. If they aren t reachable, drop them by simply not outputting them to the pipeline. Call your function something like Test-Host or Ping-Host. You can use the TestConnection cmdlet to perform the ping. Second, write an advanced function (or script cmdlet, if you prefer that term) that accepts computer names, either from the pipeline or through a -computerName parameter. For each computer, display the drive letter and free space (in megabytes) of any local, fixed disk that has less than 10 percent free space. Here s a hint: you ll query the WMI Win32_LogicalDisk class and filter the results so that only those drives having a DriveType property of 3 are included. Keep in mind that any given computer might have more than one local disk, so you ll have to account for that and filter the
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Ideas for on your own
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results accordingly. Your worker function should, however, consist entirely of a parameter declaration and a one-line command. There s no need to use an If, ForEach, or other construct.
21.7 Ideas for on your own
We re coming close to the end of this book, with just a few more chapters to go. Hopefully, you re starting to think of some real-world tasks that you d like to accomplish in PowerShell. Which of those might involve writing a filtering function, an advanced function, or a script module Start making a list of things you d like to create, and that list will be a great starting point once you ve wrapped up the next few chapters.
Trapping and handling errors
Anytime you re dealing with computers, errors are bound to occur: network problems, permission denied, server not found ... you know what I m talking about. Fortunately, your PowerShell commands and scripts can plan for those errors and deal with them, rather than spewing out a bunch of red text.
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