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Using Calculated Properties with Select-Object

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From time to time, I take a good look at how I am teaching PowerShell just to make sure that I am providing the most effective delivery.  That may be why when someone audits the class, they say they learn new material.  Besides, if I did not make changes, I would get bored.

Over the last few months I’ve been teaching how to create calculated properties with Select-Object a little differently.  This is a key skill to have so I need to make sure that we get it right.  Here is an explanation of how to create a custom property.

A custom property simply adds an additional property to an object in the PowerShell pipeline.  It does not change the original object, just the copy of it in the pipeline. Take a look at the properties of Get-Date.
PS C:\> Get-date | Select *


DisplayHint : DateTime
DateTime    : Thursday, February 11, 2016 7:46:10 PM
Date        : 2/11/2016 12:00:00 AM
Day         : 11
DayOfWeek   : Thursday
DayOfYear   : 42
Hour        : 19
Kind        : Local
Millisecond : 564
Minute      : 46
Month       : 2
Second      : 10
Ticks       : 635908167705642214
TimeOfDay   : 19:46:10.5642214
Year        : 2016

We are going to add a new property called OurDate. This new property will have a value consisting of the current year, month, and day in this format: YYYY-MM-DD.  Here is the code to accomplish this:
Get-Date |
    Select-Object -Property *,
    @{
        Name="OurDate";
        Expression={"$($_.Year)-$($_.Month)-$($_.Day)"}
    }

We first place a DateTime object into the pipeline by calling Get-Date. Next we pipe it to Select-Object.  We use the –Property parameter to select which properties to keep from the original object.  In this case, we use ‘*’ to keep all of them.  Next we add a second property.  This one is a hash table.
A hash table is a key/value pair.  In our case, a Property Name / Property Value pair.  Hash tables in PowerShell are denoted by @{}.  Inside of the curly braces is where we declare the property name and the property value.

The Name value provides us with the property name.  You can see that we set this value equal to a string of what we want to call this new calculated property, “OurDate”.

The Expression is the calculated part.  The actual calculation is done inside of its own set of curly braces ‘{}’. While calculating this property, you can reference the current object in the PowerShell pipeline by using one of two generic variables.  $_ is usable in all versions of PowerShell.  $PSItem is for PowerShell 3 and higher.  They both represent the current object in the PowerShell pipeline.  Because this variable represents the object, you have access to all the properties and methods of the object. In our example, we are creating a string that contains the values of 3 properties from the object in the pipeline.  Since we are creating this value as a string, each time we reference the object, we need to place it inside of a subexpression.  That would be the $() surrounding each call to the object.  Here is the end result.

DisplayHint : DateTime
DateTime    : Thursday, February 11, 2016 8:56:23 PM
Date        : 2/11/2016 12:00:00 AM
Day         : 11
DayOfWeek   : Thursday
DayOfYear   : 42
Hour        : 20
Kind        : Local
Millisecond : 151
Minute      : 56
Month       : 2
Second      : 23
Ticks       : 635908209831513532
TimeOfDay   : 20:56:23.1513532
Year        : 2016
OurDate     : 2016-2-11

Take a look at the last item.  It is our calculated property.
You can shorten this code up a bit:
Get-Date |
    Select-Object -Property *,
    @{
        N="OurDate";
        E={"$($_.Year)-$($_.Month)-$($_.Day)"}
    }

Or
Get-Date |
    Select-Object -Property *,
    @{N="OurDate" ; E={"$($_.Year)-$($_.Month)-$($_.Day)"}}

You can also look at the examples in the help file for Select-Object. Take a look at example #4 for another example of creating a calculated property.



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