function Using-Object
{
[CmdletBinding()]
param (
[Parameter(Mandatory = $true)]
[AllowEmptyString()]
[AllowEmptyCollection()]
[AllowNull()]
[Object]
$InputObject,
[Parameter(Mandatory = $true)]
[scriptblock]
$ScriptBlock
)
try
{
. $ScriptBlock
}
finally
{
if ($null -ne $InputObject -and $InputObject -is [System.IDisposable])
{
$InputObject.Dispose()
}
}
}
I don’t know why I did this. Performance is better if you use try/finally directly, rather than passing script blocks around. But, if you want PowerShell “using” syntax that looks similar to the C# examples you see on MSDN / etc, enjoy!
# Demonstrating the usage of Using-Object
Using-Object ($streamWriter = New-Object System.IO.StreamWriter("$pwd\newfile.txt")) {
$streamWriter.WriteLine('Line written inside Using block.')
}
# The second call to $streamWriter.WriteLine() produces an error, because
# the stream is already closed.
$streamWriter.WriteLine('Line written outside Using block.')
Get-Content .\newfile.txt
More examples on…C# Using Statement
Brian
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Great post, thanks!
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Pingback: Wrapping using statement with dispose in powershell - Alexander Williamson
I like this idea. But this implementation never disposes $streamWriter because $streamWriter is never passed to Using-Object. Unlike for C#, variable assignments in PowerShell evaluate to $null so the assignment to $streamWriter occurs but the result is $null.
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The parentheses in the example matter. 🙂 When you put an assignment expression in parentheses, in PowerShell, the value also surfaces as output. You can try it out at a console:
$i = 1 # outputs nothing
($i = 2) # outputs 2
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The only problem with this is that it’s not a true “using” statement – it’s basically a glorified function. The moment you enter it, scope goes bye bye.
You can fix a hack with a hack – use [ref] on variables out of scope.
function Main {
$test1 = “foo”;
$test2 = “foo”;
Using-Object ($streamWriter = New-Object System.IO.StreamWriter(“$pwd\newfile.txt”)) {
$streamWriter.WriteLine(‘Line written inside Using block.’)
$test1 = “bar”;
([ref]$test2).Value = “bar”;
}
Write-Host $test1;
Write-Host $test2;
}
Main;
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If the Using-Object function is defined in a script module that’s separate from where the call comes from, it works fine. Another workaround if it’s defined in the same file / scope is to dot-source the call to Using-Object: . Using-Object ($object = Whatever) { $someVariable = ‘Value }
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