Question:
I’m working on some short demo scripts for a presentation on powershell and of course one of my own simple demos has me perplexed. The purpose of this demo is to show different ways to create objects in powershell. Here’s my code:
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$file = ls 'C:\temp' | Where-Object {$_.Extension -eq '.txt'} $file.FullName $path = 'C:\temp\jdh.txt' $newfile = New-Object -TypeName System.IO.FileInfo -ArgumentList $path $newfile.FullName $file.GetType() $newfile.GetType() $file $newfile $file | Get-Content | out-null $newfile | Get-Content | out-null |
Simple right? Create two FileInfo objects via different methods and read them. The output shows the files are identical:
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C:\temp\jdh.txt C:\temp\jdh.txt IsPublic IsSerial Name BaseType -------- -------- ---- -------- True True FileInfo System.IO.FileSystemInfo True True FileInfo System.IO.FileSystemInfo LastWriteTime : 4/10/2013 3:38:29 PM Length : 46046 Name : jdh.txt LastWriteTime : 4/10/2013 3:38:29 PM Length : 46046 Name : jdh.txt |
However, when Get-Content tries to read the $newfile object, I get the following error:
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Get-Content : The input object cannot be bound to any parameters for the command either because the command does not take pipeline input or the input and its properties do not match any of the parameters that take pipeline input. At line:16 char:12 + $newfile | Get-Content | out-null + ~~~~~~~~~~~ + CategoryInfo : InvalidArgument: (C:\temp\jdh.txt:PSObject) [Get-Content], ParameterBindingException + FullyQualifiedErrorId : InputObjectNotBound,Microsoft.PowerShell.Commands.GetContentCommand |
So I noticed in the error it says PSObject which confuses me because in my ouput it seems $newfile is a FileInfo object. So the next logical thing I figured to do would be to typecast $newfile into type [System.IO.FileInfo]. That yielded the same error, except now instead of PSObject it shows FileInfo as the type. So how is it that I’m getting that error if $file and $newfile appear to be identical and get-content worked on $file? Is there some subtle difference when you use the new-object command?
Answer:
I traced the binding with the object created using New-Object and the problem is that Get-Content couldn’t bind to any properties on the FileInfo info object.
You can check this out with this command:
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Trace-Command -psHost -Name ParameterBinding {$newFile | gc} Trace-Command -psHost -Name ParameterBinding {$file | gc} |
So when you look at the trace of the one that binds successfully you’ll see that Literal path is bound to the property starting with Microsoft.PowerShell.Core\FileSystem:: which is PsPath. So on closer inspection comparing the object properties when piping to Get-Member you’ll see the one that works has these note properties where as the one created with New-Object does not –
- PSChildName
- PSDrive
- PSIsContainer
- PSParentPath
- PSPath
- PSProvider
Taking a look at the Get-Content cmdlet parameters we can see that LiteralPath has an alias of PsPath:
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(Get-Command -Name Get-Content).ParameterSets Parameter Name: LiteralPath ParameterType = System.String[] Position = 0 IsMandatory = True IsDynamic = False HelpMessage = ValueFromPipeline = False ValueFromPipelineByPropertyName = True ValueFromRemainingArguments = False Aliases = {PSPath} Attributes = System.Management.Automation.AliasAttribute System.Management.Automation.ParameterAttribute |
If you add a property to the
New-Object created FileInfo it will work:|
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$newFile | Add-Member -MemberType NoteProperty -Name PsPath -Value 'Microsoft.PowerShell.Core\FileSystem::C:\temp\jdh.txt' |