Can I determine if a PowerShell function is running as part of a pipeline?

Question

Can a PowerShell function determine if it is being run as part of a pipeline? I have a function which populates an array with instances of FileInfo which I would li

Answer 1

The "idiomatic" way to do this is to output a specific object type and then define formatting data for that object. The object can be a custom (C#/VB-based object) or a named PSObject. The advantage of this approach is that you can just output these objects and if there is no further pipeline output formatting (ie use of a Format command) then you're defined default output formatting will get used. Otherwise, one of the Format commands can override that default formatting. Here's an example:

PS> $obj = new-object psobject -Property @{FName = 'John'; LName = 'Doe'; `
                                           BirthDate = [DateTime]"5/7/1965"}
PS> $obj.psobject.TypeNames.Insert(0, "MyNamespace.MyCustomTypeName")
PS> $obj

BirthDate                               FName                            LName
---------                               -----                            -----
5/7/1965 12:00:00 AM                    John                             Doe


PS> Update-FormatData .\MyCustomFormatData.ps1xml
PS> $obj

FName                     LName                     BirthDate
-----                     -----                     ---------
John                      Doe                       5/7/1965 12:00:00 AM

Notice how the default output is different the second time we sent $obj down the pipe. That's because it used the custom formatting instructions provided since there were no explicit formatting commands used.

BTW there is always a pipeline even for $obj because that is implicitly piped to Out-Default.

Here is the custom format definition that is defined in a .ps1xml file that I named MyCustomFormatData.xml:

  
    
      MyNamespace.MyCustomTypeName
      
        MyNamespace.MyCustomTypeName
      
      
        
          
            FName
            25
            left
          
          
            LName
            25
            left
          
          
            BirthDate
            25
            left
          
        
        
          
            
              
                FName
              
              
                LName
              
              
                BirthDate
              
            
          
        
      
    
  

For more examples of how to format custom objects look at the files output by this command:

Get-ChildItem $pshome\*.format.ps1xml