Working with breakpoints in Visual Basic .NET

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Working with breakpoints
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That s what I want, so the script is now debugged. I don t need to remove all the Write-Debug statements, either. In fact, I may want to leave them there in case I ever need to debug this again. For now, I ll just set $DebugPreference back to its default of SilentlyContinue, which will suppress the output of Write-Debug. (Doing that in no way harms the performance of the script, and it s a very convenient way to switch between production mode and debugging mode.) Here s the script one last time:
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$DebugPreference = "SilentlyContinue" $name = Read-Host "Enter computer name" write-debug "`$name contains $name" if (test-connection $name -quiet) { write-debug "Test-connection was True" get-wmiobject win32_bios -computername $name } else { write-debug "Test-connection was False" }
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Whenever I begin working on a script, I add the Write-Debug statements as I go. I know there will be a bug or two in there eventually, so building in the debugging from the start makes debugging faster when the time comes. Here are my guidelines:
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Whenever I change the contents of a variable, I use Write-Debug to output the variable, just so I can check those contents. Whenever I m going to read the value of a property or a variable, I use WriteDebug to output that property or variable, so that I can see what s going on inside the script. Any time I have a loop or logic construct, I build it in such a way that I get a Write-Debug message no matter how the loop or logic works out. In this example, I added an Else section specifically to have debug output the Else portion of the construct has no other purpose.
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Of course, in a really long script, having to wade through a lot of debug messages to track down a problem can be time-consuming. Is there a more efficient technique You bet there is!
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23.4 Working with breakpoints
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We re going to revert back to the original script in listing 23.1. It isn t a long script, but it will suffice to illustrate breakpoints, a great feature of PowerShell v2. Note that this discussion will focus solely on what s available in the PowerShell console host and the PowerShell ISE; third-party editors often provide similar (and better) breakpoint functionality, but they typically implement it in a different way. You ll have to consult your product s manual if you re using one of those editors.
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Debugging techniques
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A breakpoint is a defined area where a script will pause its execution, allowing you to examine the environment that the script is running within. PowerShell can be configured to break when
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Your script reaches a certain line A variable is read and/or changed A specific command is executed
In the first instance, you must specify the script file that you re referring to. In the second and third situations, you can choose to specify a script, and the breakpoint will only be active for that script. If you don t, the breakpoint will occur globally throughout the shell when that variable is read or written, or that command is executed. Going back to listing 23.1, suppose I want to have the script stop immediately after line 1 finishes executing, meaning that I want to break before line 2. I ve saved the script as C:\Demo.ps1, so in the shell I ll run this command:
PS C:\> set-psbreakpoint -script c:\demo.ps1 -line 2 ID Script -- -----0 demo.ps1 Line Command ---- ------2 Variable -------Action ------
The shell confirms that it has set the breakpoint. I also want to be notified whenever the $name variable is accessed, so I ll run this:
PS C:\> set-psbreakpoint -script c:\demo.ps1 -variable name -mode read ID Script -- -----1 demo.ps1 Line Command ---- ------Variable -------name Action ------
Again, the shell confirms. Notice that the variable s name is just name, and not $name. Variable names don t include the dollar sign; $ is just a cue to the shell telling it that you wish to use the contents of a variable. In this case, I don t want to refer to the contents of $name; I want to refer to the variable name itself. With those two breakpoints set, I ll run the script. After entering the computer name, the script will break. The shell modifies the command-line prompt, indicating that I m in suspend mode. Here I can examine the contents of variables, execute commands, and so on. When I m done, I can run Exit to resume script execution. Here s how it all looks:
PS C:\> ./demo Entering debug mode. Use h or for help. Hit Line breakpoint on 'C:\demo.ps1:2' demo.ps1:3 $name = Read-Host "Enter computer name" [DBG]: PS C:\>>> exit Enter computer name: server-r2 Hit Variable breakpoint on 'C:\demo.ps1:$name' (Read access)
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