Examples of Calculated Properties in PowerShell

Overview

Calculated properties in PowerShell have been around for a while, about as long as PowerShell has been around in version 1.0. Calculated Properties give you the ability to dynamically add properties and output them to the pipeline, as explained in the Calculated Properties docs. Although this feature is readily available and can make work so much easier by adding and outputting custom properties as a column in a file or output stream, I often have trouble identifying when to use them. In this post I will work out a few basic examples.

Examples using Select-Object

Creating an AccessedToday property

A quick example of a calculated property use-case would be comparing the last access time of a file to a date – or today. We need to pipe each iteration of Get-ChildItem to Select-Object and list the built-in properties we will be using, so Name, LastAccessTime, Size, FullName and DirectoryName. Thes properties are from the System.IO.FileSystemInfo class instance returned by Get-ChildItem during each iteration in the Foreach-Object loop. Additionally, we create our calculated property named AccessedToday using the LastAccessTime property and comparing it to Get-Date. You need to remove the trailing comma and close the loop with a } to run this code as-is.

$myCustomObj = Get-ChildItem -Path $PSScriptRoot -File -Recurse `
| foreach-object {
    $_ | Select-Object Name, LastAccessTime, Length, FullName, DirectoryName, `
    @{
        Name="AccessedToday";
        Expression={ $_.LastAccessTime.toString('yyyy-MM-dd') -eq ((Get-date).ToString('yyyy-MM-dd'))} 
    },

Computed Property Syntax

When creating a computed property using Select-Object cmdlet, you would comma separate the properties, then for the computed property, use a hash table with Name (n) and Expression (e) keys. Just a note the Expression key in the hash table takes a script block as the value, so you can further refactor your script.

Creating a Property to Dynamically Calculate the Size of the File

Let’s continue to add to our output stream with another calculated property called CustomFileSize. This property dynamically calculates the output of the file size if a more readable format. It will return a formatted string with the file size, rounded to two decimal places, in Bytes, KB, MB, or GB. I am using if conditional blocks but feel free to try it with a switch statement.

• {0}: This refers to the first argument after the format string, which in this case is the number you want to format.
• :N2: This is the format specifier:
  • N: Indicates that the number should be formatted as a number.
  • 2: Specifies that two decimal places should be displayed.

@{
        Name="CustomFileSize";
        Expression={
            $s = $_.Length
            if($s -ge 1GB){

                $s = "{0:N2} GB" -f ($s/1GB)
            } elseif($s -ge 1MB){

                $s = "{0:N2} MB" -f ($s/1MB)
            } elseif($s -ge 1KB){

                $s = "{0:N2} KB" -f ($s/1KB)
            } else {
                
                $s = "{0:N2} Bytes" -f $s
            }
            return $s
        }
    },

Get MD5 File Hash as a Computed Property

Continuing to add to our object, we will add an MD5Hash computed property using the Get-FileHash cmdlet to output the MD5 hash. Once again within our hashtable, we create the Name, MD5Hash, and the Expression, the logic to dynamically add the custom property to our object.

@{
    Name="MD5Hash";
    Expression= { (Get-FileHash -Path $_.FullName -Algorithm MD5).Hash }
}

Converting Our Custom Object to HTML

Ok, it is great to write the output stream to the console, but lets convert it to an HTML file and send it to our co-workers, yea? We can use ConvertTo-HTML for this, retrieve all the properties of our object we want to display in columns, then pipe it to Out-File.

Write-Output $myCustomObj

$myCustomObj `
| ConvertTo-Html Name, AccessedToday, CustomFileSize, FullName, MD5Hash `
| Out-File "output.html" -Force

Completed Code

$myCustomObj = Get-ChildItem -Path $PSScriptRoot -File `
| foreach-object {
    $_ | Select-Object Name, LastAccessTime, Length, FullName, DirectoryName, `
    @{
        Name="AccessedToday";
        Expression={ $_.LastAccessTime.toString('yyyy-MM-dd') -eq ((Get-date).ToString('yyyy-MM-dd'))} 
    },
    @{
        Name="CustomFileSize";
        Expression={
            $s = $_.Length
            if($s -ge 1GB){

                $s = "{0:N2} GB" -f ($s/1GB)
            } elseif($s -ge 1MB){

                $s = "{0:N2} MB" -f ($s/1MB)
            } elseif($s -ge 1KB){

                $s = "{0:N2} KB" -f ($s/1KB)
            } else {
                
                $s = "{0:N2} Bytes" -f $s
            }
            return $s
        }
    },
    @{
        Name="MD5Hash";
        Expression= { (Get-FileHash -Path $_.FullName -Algorithm MD5).Hash }
    }
}

Write-Output $myCustomObj

$myCustomObj `
| ConvertTo-Html Name, AccessedToday, CustomFileSize, FullName, MD5Hash `
| Out-File "output.html" -Force

Conclusion

In this PowerShell script, we’ve crafted a comprehensive tool to gather and present file details from a script’s directory. By utilizing Get-ChildItem, we fetched file attributes like name, size, and last access time. We enhanced the user experience by converting file sizes to readable formats like KB, MB, or GB. Additionally, the script calculates the MD5 hash for each file, ensuring data integrity. The cherry on top? We seamlessly transformed this data into an HTML format, streamlining its presentation and making it easily accessible. Through this code, we’ve not only automated file management tasks but also enriched our understanding of PowerShell’s versatility for data manipulation and presentation.