Advanced Windows PowerShell Scripting Video Training

Advanced Windows PowerShell Scripting Video Training
Advanced Windows PowerShell Scripting Video Training

Wednesday, January 13, 2016

Reading and Resolving PowerShell Errors - Part 10

This is part 10 of my series of the most common PowerShell errors that are made in my PowerShell classes.  This is the last one!
Today’s error: My data file does not have my data.

Here is our starting code:
Get-Process |
    Measure-Object -Property CPU -sum |
    Select-Object -Property Name, CPU |
    Export-CSV -Path c:\temp\data.csv

And here is the full error:
No error, just the wrong data.  Here is the contents of the CSV.
PS C:\temp> import-csv -Path C:\temp\Data.csv

Name CPU
---- ---

The problem here is that the programmer did not keep track of what is in the PowerShell Pipeline. I see this very often.  When you are constructing a pipeline, you need to keep track of what is in the pipeline.  After each statement, pipe to object to Get-Member and verify that you are still working with the object type that you expect.
PS C:\temp> Get-Process | Get-Member

   TypeName: System.Diagnostics.Process

Here you can see that the original object is System.Diagnostics.Process.  Now look at the member information after the execution of the second command.
PS C:\temp> Get-Process |
    Measure-Object -Property CPU -sum | Get-Member

   TypeName: Microsoft.PowerShell.Commands.GenericMeasureInfo

Name        MemberType Definition                               
----        ---------- ----------                               
Equals      Method     bool Equals(System.Object obj)           
GetHashCode Method     int GetHashCode()                        
GetType     Method     type GetType()                           
ToString    Method     string ToString()                        
Average     Property   System.Nullable[double] Average {get;set;}
Count       Property   int Count {get;set;}                     
Maximum     Property   System.Nullable[double] Maximum {get;set;}
Minimum     Property   System.Nullable[double] Minimum {get;set;}
Property    Property   string Property {get;set;}               
Sum         Property   System.Nullable[double] Sum {get;set;}

The object typename is now Microsoft.PowerShell.Commands.GenericMeasureInfo.  In other words, you have a completely new object.  Many cmdlets alter the object in the pipeline. The next command, Select-Object will attempt to filter out all but two properties.  The problem here is that the properties that are being asked for are from the object System.Diagnostics.Process, not the current object in the pipeline.
To fix this, remove the Measure-Object cmdlet.
Get-Process |
    Select-Object -Property Name, CPU |
    Export-CSV -Path c:\temp\data.csv

This mistake is most often the result of not wanting to make the effort.  Once you are experienced working with a set of cmdlets, you will know what they put into the pipeline.  Until you are experienced with a set of cmdlets, make sure you check the pipeline often.  This will save you a lot of work in debugging.

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