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Executing LINQ Queries in PowerShell - Part 2

And we're back!

Ok, so in the last blog we began a conversation about delegates and using LINQ in PowerShell. In today's post, I'm going to give an example of how it can be incredibly useful. Let's talk about Joins.


In my line of work, I'm constantly running into the need to combine datasets from multiple sources that relate to each other and pull out some specific properties. Say you have two internal services, one which is used to track production status and another which is used to monitor whether machines are online. To demonstrate this, let's initialize some mock data once again.

#Create empty arrays
$DatasetA = @()
$DatasetB = @()
#Initialize "status" arrays to pull random values from
$ProductionStatusArray = @('In Production','Retired')
$PowerStatusArray = @('Online','Offline')
#Loop 1000 times to populate our separate datasets
1..1000 | Foreach-Object {
    #Create one object with the current iteration attached to the name property
    #and a random power status
    $PropA = @{
        Name = "Server$_"
        PowerStatus = $PowerStatusArray[(Get-Random -Minimum 0 -Maximum 2)]
    $DatasetA += New-Object -Type PSObject -Property $PropA
    #Create a second object with the same name and a random production status
    $PropB = @{
        Name = "Server$_"
        ProductionStatus = $ProductionStatusArray[(Get-Random -Minimum 0 -Maximum 2)]
    $DatasetB += New-Object -Type PSObject -Property $PropB

Now we have two datasets with the same server names, one showing production status and the other showing power status. Our goal is to join that data together. In traditional PowerShell, we would likely iterate through one of the sets while doing a filter on the second set and then either add property members to the first set or create all new objects with a combination of properties from both sets. Something like this:

$JoinedData = @()
foreach($ServerA in $DatasetA) {
    $ServerB = $DatasetB | Where-Object Name -eq $ServerA.Name
    $Props = @{
        Name = $ServerA.Name
        PowerStatus = $ServerA.PowerStatus
        ProductionStatus = $ServerB.ProductionStatus
    $JoinedData += New-Object -Type PSObject -Property $Props

This works fine. If I wrap it in a Measure-Command it takes right around 8.82 seconds to complete. Not awful, but at enterprise level where you're dealing with ten times that amount of data, you can see how that run time could get out of control. Now let's do the same with LINQ:

$LinqJoinedData = [System.Linq.Enumerable]::Join(
    [System.Func[Object,string]] {param ($x);$x.Name},
    [System.Func[Object,string]]{param ($y);$y.Name},
        param ($x,$y); 
        New-Object -TypeName PSObject -Property @{
        Name = $x.Name; 
        PowerStatus = $x.PowerStatus; 
        ProductionStatus = $y.ProductionStatus}
$OutputArray = [System.Linq.Enumerable]::ToArray($LinqJoinedData)

This completed for me in just over 0.4 seconds! Hopefully after last week this syntax doesn't look too daunting, but let's walk through what we just did. We're calling the Join method on System.Linq.Enumerable and then passing it five parameters.

  1. The first dataset we're going to join
  2. The second dataset to join
  3. The delegate which defines the key to compare against on the first dataset
  4. The delegate which defines the key to compare against on the second dataset
  5. Finally, we pass in the delegate which defines what the output should look like

So it looks complicated, but once you use it a few times, it's really not too bad. Now you're probably wondering why I added that final line where I called "[System.Linq.Enumerable]::ToArray($LinqJoinedData)." For that we need to talk about "Deferred Execution vs. Immediate Execution." When you call the Join method, it's not actually joining the data at that time, rather it's building an expression tree which defines the relational algebra needed to perform the join. This defers the execution point to when the data is actually operated against. So in the above example, I called "ToArray()" merely to provide an accurate timespan for how long the join actually takes as opposed to the more traditional PowerShell approach we used before it. If this were production code and I wanted to see  machines with an offline status that are listed as in production, rather than that "ToArray()" line I could simply run this:

$LinqJoinedData.Where({($_.PowerStatus -eq "Offline") -and ($_.ProductionStatus -eq "In Production")})

The Join query would execute at that time and then "Where()" would filter down to just the objects I requested.

And there you have it! If you found this interesting, I encourage you to check out these modules:

Feel free to reach out to me on Twitter or check out my personal site from time to time for other content. If you've seen my recent talk at PowerShell Summit, I'll be posting the blog I referenced there soon about turning my dog into a tea kettle.  (it's not PowerShell related, thus it will be landing somewhere other than here)

Happy tinkering!


Colecting Certificates form an Enterprise CA for use with DSC

In a domain environment auto enrollment can be used to get create unique certificates for each node that can be used with DSC.  The problem is getting the public cert to the machine that creates the DSC MOF files. I wrote a module last year to collect them directly form the Enterprise CA. If it interests you take a look


Do Anything in One Line of PowerShell

PowerShell provides a tremendous boon to productivity for computer professionals of all types. But, you have to admit: it can be a bit daunting to get up to speed! Indeed, as someone who has a fair amount of experience using it, I still find myself having to look up how to do things--frequently. So I started keeping track of the recipes I was using the most. And came up with a list of 400 or so, published in 4 parts.

Though I actually wrote these a couple years back they are certainly still relevant today, just covering a bit less of the ever-expanding PowerShell universe of discourse!

(Note that at the end of each web article listed above is a link to download it as a PDF that is more tidily formatted.)

Pitfalls of the Pipeline

Pipelining is an important concept in PowerShell. Though the idea did not originate with PowerShell (you can find it used decades earlier in Unix, for example), PowerShell does provide the unique advantage of being able to pipeline not just text, but first-class .NET objects.

Pipelining has several advantages:

  • It helps to conserve memory resources. Say you want to modify text in a huge file. Without a pipeline you might read the huge file into memory, modify the appropriate lines, and write the file back out to disk. If it is large enough you might not even have enough memory to read the whole thing.
  • It can substantially improve actual performance. Commands in a pipeline are run concurrently-even if you have only a single processor, because when one process blocks, for example, while reading a large chunk of your file, then another process in the pipeline can do a unit of work in the meantime.
  • It can have a significant effect on your end-user experience, enhancing the perceived performance dramatically. If your end-user executes a sequence of commands that takes 60 seconds, then until 60 seconds has elapsed he/she would see nothing without pipelining, whereas output could start appearing almost immediately with pipelining.

PowerShell provides a variety of techniques for using pipelining but it is all to easy to do it wrong, so you think you are pipelining but in fact you are not. In my article Ins and Outs of the PowerShell Pipeline, I discuss the most common things that can trip you up with implementing pipelining and how to avoid them.

Create Custom Monitors with PowerShell

Sometimes, as a developer, you want to be be able to keep track of free space on a drive, the size of a log, the load on your CPU, the number of users logged in, etc. With PowerShell, it is typically just a matter of finding the right cmdlet amidst the large (and rapidly growing) pool of cmdlets provided by Microsoft and by third parties. Then you just run Get-Foo to check details about the foo resource. And then you come back 5 minutes later and run it again because you want to see how it changes over time.

But wouldn't it be nice if you could just have it run automatically at regular intervals in a separate window that you could just keep in the corner of your screen? Well, I found the barebones of just such a utility sometime ago (authored by Marc van Orsouw,  aka ‘thePowerShellGuy’). His original post is no longer available, but I expanded upon his code and, over time, added features, bug fixes, and enhancements, making it more useful and more user-friendly. Here are a few screenshots of the Monitor Factory in action.

Monitor the size of a database

Start-Monitor -AsJob {
    Invoke-Sqlcmd 'DBCC SQLPERF(logspace)' |
    Select-Object 'Database Name','Log Size (MB)','Log Space Used (%)',HasErrors

Database Size Monitor

Monitor drives on a system
Drive Capacity Monitor

Monitor longest running DB queries
Long-runnning DB Query Monitor

Build Your Own Resource Monitor in a Jiffy reveals how quick and easy it is to get started with the Monitor Factory.

A date with PowerShell

At the beginning of July, we welcomed our 3rd son into the world. As days past my wife and I would say, "wow, he's 11 days old. Can you believe it?!". I'm sure parents out there are relating to this!
This gave me an idea for a fun script that would get your age in years, months and days, tell you how many days until your birthday and your star sign.

I wanted date of birth passed to the function as 'dd/MM/yy'. To keep to this format, I’m using the 'ValidatePattern' Advanced Parameter with a Regular Expression (Regex). The regular expression, "^(0[1-9]|[12]\d|3[01])/(0[1-9]|1[0-2])/(\d{2})$", will only allow a date in the format of 01/01/16, for example.

Briefly, here is regex syntax I used in some of the expression:

^ Start of string
( .. ) Capturing group
(0[1-9] Match two digits that make up the day. This accepts numbers from 01 to 09
| Acts like a Boolean OR.
/d match any digital character
[12] match any character in the set
/ used to divide the date numbers
{2} Exactly two times
$ End of string

Now that my function parameter variable $Bday has a date, its passed to get-date to be converted from a string to a date. The date in variable $cDate will look like this, '01 January 2016 00:00:00'. The next line in the code will use todays date and subtract the date passed in $cDate variable. The $diff variable will contain the following data which we will use to get our age in years, months and days:

Days : 212
Hours : 12
Minutes : 40
Seconds : 20
Milliseconds : 533
Ticks : 183624205335135
TotalDays : 212.528015434184
TotalHours : 5100.67237042042
TotalMinutes : 306040.342225225
TotalSeconds : 18362420.5335135
TotalMilliseconds : 18362420533.5135

I've contained this first part in our Begin block. The Process block does the main code.

Now I need to get my age in Years, Months and Days. This is where the [math] data type is used. I'm using the 'Truncate' property as I don't want to do anything fancy like round up my numbers. Adding the .typename of Days to my $diff variable and dividing by $daysInYear variable I can get my age in years.

The next two, months and days required a tweak to the algorithm.

I ended up using a maths term called a 'Mod'. Now I’m not talking about youth culture and style in the sixties (Mods and rockers anyone ??), but the Modulus Math Operator. Basically the Modulus Operator returns the remainder when the first number is divided by the second. So for example:
1 mod 3 = 1 (or 1 % 3 = 1)
2 mod 3 = 2
3 mod 3 = 0
4 mod 3 = 1

The operator sign used is % for Modulus. Not to be confused for the alias of foreach in PowerShell. For days in a month, I used the average of 30.

I thought it would be fun to add the star sign as well. I was after something that could tell me, "is this date in this date range?". One of the properties of 'get-date' is DayOfYear.
Finding if a number is in a range is pretty straight forward, For example:

 5 -in 1..10 

Which gives a Boolean result.

Now if I convert my date ranges into days of the year then I can match the day of the year I was born against the ranges of days for star signs. I've used a switch statement to check against multiple conditions. Within a scriptblock I’ve asked if the value I’m passing is 'in' the array of dates for each star sign. The match will return the star sign and is held in the $starSign variable.

The Final part of the process block is to work out how many days until your next birthday. By capturing the current date, formatting the date of birth by removing the year born, adding the current year and finally subtract the amended date of birth against the current date. Phew!
This will leave a number of days until your next birthday. The 'if' statement is added if your birthday has already happened at the time of the code, it simply reverses the sum to give a positive number.

The end block displays the three captured results to the host.

I hope you have enjoyed this post and can see the many options possible for dates in PowerShell.
Feel free to download the script from my GitHub

function Get-age {
                   HelpMessage="Date must be written as dd/mm/yy",

Begin {
    # use 'get-date' to convert '$Bday' Variable
    $cDate = (get-date -Date $Bday)

    # from today's date subtract birth date
    $diff = (Get-Date).Subtract($cDate)

Process {

    # Work out Years, months and days
    [int]$daysInYear = '365'
    [int]$averageMonth = '30'
    # years
    $totalYears = [math]::Truncate( $($diff.Days) / $daysInYear ) 
    $totalMonths = [math]::Truncate( $($diff.Days) % $daysInYear / $averageMonth ) 
    # days
    $remainingDays = [math]::Truncate( $($diff.Days) % $daysInYear % $averageMonth ) 

    # Your star sign
    $thisYear = (get-date).Year
    $starSign = 
    switch ($cDate.DayOfYear) {
        { $_ -in @( ((get-date 22/12/$thisYear).DayOfYear)..365; 0..((get-date 19/01/$thisYear).DayOfYear) ) } { "Capricorn" }
        { $_ -in @( ((get-date 20/01/$thisYear).DayOfYear)..((get-date 18/02/$thisYear).DayOfYear) ) } { "Aquarius" }
        { $_ -in @( ((get-date 19/02/$thisYear).DayOfYear)..((get-date 20/03/$thisYear).DayOfYear) ) } { "Pisces" }
        { $_ -in @( ((get-date 21/03/$thisYear).DayOfYear)..((get-date 19/04/$thisYear).DayOfYear) ) } { "Aries" }
        { $_ -in @( ((get-date 20/04/$thisYear).DayOfYear)..((get-date 20/05/$thisYear).DayOfYear) ) } { "Taurus" }
        { $_ -in @( ((get-date 21/05/$thisYear).DayOfYear)..((get-date 20/06/$thisYear).DayOfYear) ) } { "Gemini" }
        { $_ -in @( ((get-date 21/06/$thisYear).DayOfYear)..((get-date 22/07/$thisYear).DayOfYear) ) } { "Cancer" }
        { $_ -in @( ((get-date 23/07/$thisYear).DayOfYear)..((get-date 22/08/$thisYear).DayOfYear) ) } { "Leo" }
        { $_ -in @( ((get-date 23/08/$thisYear).DayOfYear)..((get-date 22/09/$thisYear).DayOfYear) ) } { "Virgo" }
        { $_ -in @( ((get-date 23/09/$thisYear).DayOfYear)..((get-date 22/10/$thisYear).DayOfYear) ) } { "Libra" }
        { $_ -in @( ((get-date 23/10/$thisYear).DayOfYear)..((get-date 21/11/$thisYear).DayOfYear) ) } { "Scorpio" }
        { $_ -in @( ((get-date 22/10/$thisYear).DayOfYear)..((get-date 21/12/$thisYear).DayOfYear) ) } { "Sagittarius" }
    # Work out how many days until birthday    
    $now = [DateTime]::Now   
    $dm = get-date $Bday -UFormat "%m/%d/" 
    $Days = [Datetime]($dm + $now.Year) – $Now

    # If birthday has happened this year change sum
    if (!($Days -ge 0)) { $Days = $now - [Datetime]($dm + $now.Year) }               
End {
    # display
    "`nYou are {0} year(s), {1} month(s) and {2} day(s)" -f $totalYears, $totalMonths, $remainingDays
    "Your Star sign is: " + $starSign
    # and...
    if ($cDate.Year -eq (get-date).Year) { 
        "You have another $($daysInYear - $diff.Days) days until your birthday" # If you are under 1 years old
    } else { 
        "You have another $($Days.days) days until your birthday" # over the age of 1 

}# Function End

5 Tips for Writing DSC Resources in PowerShell 5

PowerShell 5 brought class based DSC Resources, which majorly simplifies the process of writing custom DSC resources.

During my time working on some custom resources, I developed some tips a long the way which should save you some time and pain during your DSC journey.

The tips cover:

  • Structuring your class based DSC Resources
  • Making it easier to get IntelliSense based on your DSC resources without constantly copying them into the module path
  • Using PowerShell ISE IntelliSense when writing DSC configuration
  • Troubleshooting resources which aren't being exposed correctly from your DSC Module
  • Testing classed based resources with Pester

Head over to to take a look at the tips.

Practical PowerShell Unit-Testing

By the time you are using PowerShell to automate an increasing amount of your system administration, database maintenance, or application-lifecycle work, you will likely come to the realization that PowerShell is indeed a first-class programming language and, as such, you need to treat it as such. That is, you need to do development in PowerShell just as you would with other languages, and in particular to increase robustness and decrease maintenance cost with unit tests and--dare I say--test-driven development (TDD). I put together several articles on getting started with unit tests and TDD in PowerShell using Pester, the leading test framework for PowerShell. This series introduces you to Pester and provides what I like to call "tips from the trenches" on using it most effectively, along with a gentle prodding towards a TDD style.

Part 1: Getting Started with the Pester Framework

Starting with the ubiquitous "Hello, World", this introduces Pester, showing how to execute tests, how to start writing tests, and the anatomy of a test.

Part 2: Mock Objects and Parameterized Test Cases

To be able to create true unit tests, you need to be able to isolate your functions and modules to be able to focus on the component under test; mocks provide great support for doing that. Another topic of "power" unit tests is making them parameterizable, i.e. being able to run several scenarios through a single test simply by providing different inputs.

Part 3: Validating Data and Call History

The final part of this series provides a "how-to" for several other key parts of Pester: how to validate data, how to determine if something was called appropriately, and how to address a particular challenge with Pester, validating arrays. I've included a library for array validation to supplement Pester.

For a more general treatment of unit tests, I refer you to Roy Osherove's canonical text on the subject, The Art of Unit Testing.

... you wanted to know about Unit Testing in .NET | Coding in .NET

Making Awesome Dashboards from Windows Performance Counters

Having an understanding of your systems performance is a crucial part of running IT infrastructure.

If a user comes to us and says "why is my application running slowly?", where do we start? Is it their machine? Is it the database server? Is it the file server?

The first thing we usually do is open up perfmon.exe and take a look at some performance counters. You then see the CPU on the database server is 100% and think  "was the CPU always at 100% or did this issue just start today? Was it something I changed? If only I could see what was happening at this time yesterday when the application was running fine!". It might take you a few hours to find the performance issue on your infrastructure, and you are probably going to need to open up perfmon.exe on a couple of other systems. There is a better way!

What if you could turn your Windows performance counters into dashboards that look like this? How much time would you save?

Full Hyper-V Dashboard

Using a combination of the open source tools InfluxDB to store the performance counter data, Grafana to graph the data and the Telegraf agent to collect Windows performance counters, you will be a master of your metrics in no time!

Read the detailed walk through over at



Documenting your PowerShell API--solved!

Long has it been known how to easily document your PowerShell source code simply by embedding properly formatted documentation comments right along side your code, making maintenance relatively painless...


Sample Doc-Comments for PowerShell source

But if you advanced to writing your PowerShell cmdlets in C#, you have largely been on your own, either hand-crafting MAML files or using targeted MAML editors far removed from your source code. But not anymore. With the advent of Chris Lambrou's open-source XmlDoc2CmdletDoc, the world has been righted upon its axis once more: it allows instrumenting your C# source with doc-comments just like any other C# source:

csharp doc-comment sample

All of the above provides fuel for Get-Help, i.e. providing help one cmdlet at a time. But we are a civilized people; we also need a web-based version of our full custom PowerShell API. That is, a hierarchical and indexed set of Get-Help pages for all the cmdlets in our module. For this task, my own open-source effort, DocTreeGenerator, nicely fills the gap, requiring very little beyond the doc-comments described above to do the complete job.

I have written extensively on using both XmlDoc2CmdletDoc and DocTreeGenerator, and just this week, released a one-page wallchart that shows how all the pieces work together:

doc wallchart thumbnail

Here's the link to get you started on this fun journey:

Unified Approach to Generating Documentation for PowerShell Cmdlets

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