SPWeb being closed when getting list by powershell. What's wrong?

Question

When I run this little piece of powershell code:

$w = Get-SPWeb http://localhost
$l = $w.List["some list"]

I get Unexpected error message to my Sharepoint ULS log

Detected use of SPRequest for previously closed SPWeb object. .../long message here/...

I don't think this should happen. Is this a bug or do I do something wrong?

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On the other hand, to make it even more weird. When I wil continue in code above and write this:

$w.Dispose()
$w.Title # or any other property

I don't get that error message. I would expect I will get it. What is wrong with disposing in powershell?

Answer 1

Use of objects, such as SPWeb or SPSite, in PowerShell requires a proper memory management. Get commands dispose of these objects immediately after the pipeline finishes, but by using SPAssignment, you can assign the list of objects to a variable and dispose of the objects after they are no longer needed. Here is an example of usage of SPAssignment object, you can assign SPWeb to a variable and dispose of when they are needed to free up memory. See below:

$spAssigment = Start-SPAssignment
$spWeb = Get-SPWeb http://localhost -AssignmentCollection $spAssignment
$l = $spWeb.List["some list"]     
$spWeb.Title # or any other property
Stop-SPAssignment $spAssignment

Answer 2

I'm guessing that you are using a regular PowerShell session, probably with the SharePoint snapp-in loaded. And not the 'SharePoint Management Shell'.

With a regular powershell session each command is executed with-in its own thread. And disposable objects, like SPWeb, are automatically disposed on the end of a thread. Thus what could happen is that the Garbage Collector is quick and disposes your SPWeb before you execute the next statement.

Disposing does not clear or destroy an object, it just gives an object a chance to close connections, clean up things the GC won't clean up, etc. Most properties will still be available (like Title) but actions that require a database connection, like $w.list["listName"], will error out.

SharePoint Management Shell

The SP Management Shell is just a regular powershell session with this tiny bit of script executed when it loads:

$ver = $host | select version
if ($ver.Version.Major -gt 1)  {$Host.Runspace.ThreadOptions = “ReuseThread”}
Add-PsSnapin Microsoft.SharePoint.PowerShell
Set-location $home

Notice the setting on ThreadOptions. With the SP Management Shell you get a sligtly different powershell (in an ugly console). One that reuses the same thread for succesive commands, and thus a PowerShell that won't dispose objects 'behind your back'.

See this blog for more information.

Load C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\CONFIG\POWERSHELL\Registration\\sharepoint.ps1 , instead of just adding the SnapIn

Tip Use the PowerShell ISE, which has ThreadReuse on as default. And it is a nice tool for writing and testing scripts.