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п»їEverything you wanted to know about hashtables.
I want to take a step back and talk about hashtables. I use them all the time now. I was teaching someone about them after our user group meeting last night and I realized I had the same confusion about them as he had. Hashtables are really important in PowerShell so it's good to have a solid understanding of them.
The original version of this article appeared on the blog written by @KevinMarquette. The PowerShell team thanks Kevin for sharing this content with us. Please check out his blog at PowerShellExplained.com.
Hashtable as a collection of things.
I want you to first see a Hashtable as a collection in the traditional definition of a hashtable. This definition gives you a fundamental understanding of how they work when they get used for more advanced stuff later. Skipping this understanding is often a source of confusion.
What is an array?
Before I jump into what a Hashtable is, I need to mention arrays first. For the purpose of this discussion, an array is a list or collection of values or objects.
$array = @(1,2,3,5,7,11)
Once you have your items into an array, you can either use foreach to iterate over the list or use an index to access individual elements in the array.
foreach($item in $array) Write-Output $array[3]
You can also update values using an index in the same way.
$array[2] = 13.
I just scratched the surface on arrays but that should put them into the right context as I move onto hashtables.
What is a hashtable?
I'm going to start with a basic technical description of what hashtables are, in the general sense, before I shift into the other ways PowerShell uses them.
A hashtable is a data structure, much like an array, except you store each value (object) using a key. It's a basic key/value store. First, we create an empty hashtable.
$ageList = @<>
Notice that braces, instead of parentheses, are used to define a hashtable. Then we add an item using a key like this:
$key = 'Kevin' $value = 36 $ageList.add( $key, $value ) $ageList.add( 'Alex', 9 )
The person's name is the key and their age is the value that I want to save.
Using the brackets for access.
Once you add your values to the hashtable, you can pull them back out using that same key (instead of using a numeric index like you would have for an array).
$ageList['Kevin'] $ageList['Alex']
When I want Kevin's age, I use his name to access it. We can use this approach to add or update values into the hashtable too. This is just like using the add() function above.
$ageList = @<> $key = 'Kevin' $value = 36 $ageList[$key] = $value $ageList['Alex'] = 9.
There's another syntax you can use for accessing and updating values that I'll cover in a later section. If you're coming to PowerShell from another language, these examples should fit in with how you may have used hashtables before.
Creating hashtables with values.
So far I've created an empty hashtable for these examples. You can pre-populate the keys and values when you create them.
$ageList = @
As a lookup table.
The real value of this type of a hashtable is that you can use them as a lookup table. Here is a simple example.
$environments = @ $server = $environments[$env]
In this example, you specify an environment for the $env variable and it will pick the correct server. You could use a switch($env) for a selection like this but a hashtable is a nice option.
This gets even better when you dynamically build the lookup table to use it later. So think about using this approach when you need to cross reference something. I think we would see this even more if PowerShell wasn't so good at filtering on the pipe with Where-Object . If you're ever in a situation where performance matters, this approach needs to be considered.
I won't say that it's faster, but it does fit into the rule of If performance matters, test it.
Multiselection.
Generally, you think of a hashtable as a key/value pair, where you provide one key and get one value. PowerShell allows you to provide an array of keys to get multiple values.
$environments[@('QA','DEV')] $environments[('QA','DEV')] $environments['QA','DEV']
In this example, I use the same lookup hashtable from above and provide three different array styles to get the matches. This is a hidden gem in PowerShell that most people aren't aware of.
Iterating hashtables.
Because a hashtable is a collection of key/value pairs, you iterate over it differently than you do for an array or a normal list of items.
The first thing to notice is that if you pipe your hashtable, the pipe treats it like one object.
PS> $ageList | Measure-Object count : 1.
Even though the .count property tells you how many values it contains.
PS> $ageList.count 2.
You get around this issue by using the .values property if all you need is just the values.
PS> $ageList.values | Measure-Object -Average Count : 2 Average : 22.5.
It's often more useful to enumerate the keys and use them to access the values.
PS> $ageList.keys | ForEach-Object Kevin is 36 years old Alex is 9 years old.
Here is the same example with a foreach() loop.
foreach($key in $ageList.keys)
We are walking each key in the hashtable and then using it to access the value. This is a common pattern when working with hashtables as a collection.
GetEnumerator()
That brings us to GetEnumerator() for iterating over our hashtable.
$ageList.GetEnumerator() | ForEach-Object.
The enumerator gives you each key/value pair one after another. It was designed specifically for this use case. Thank you to Mark Kraus for reminding me of this one.
BadEnumeration.
One important detail is that you can't modify a hashtable while it's being enumerated. If we start with our basic $environments example:
$environments = @
And trying to set every key to the same server value fails.
$environments.Keys | ForEach-Object An error occurred while enumerating through a collection: Collection was modified; enumeration operation may not execute. + CategoryInfo : InvalidOperation: tableEnumerator:HashtableEnumerator) [], RuntimeException + FullyQualifiedErrorId : BadEnumeration.
This will also fail even though it looks like it should also be fine:
foreach($key in $environments.keys) Collection was modified; enumeration operation may not execute. + CategoryInfo : OperationStopped: (:) [], InvalidOperationException + FullyQualifiedErrorId : System.InvalidOperationException.
The trick to this situation is to clone the keys before doing the enumeration.
$environments.Keys.Clone() | ForEach-Object.
Hashtable as a collection of properties.
So far the type of objects we placed in our hashtable were all the same type of object. I used ages in all those examples and the key was the person's name. This is a great way to look at it when your collection of objects each have a name. Another common way to use hashtables in PowerShell is to hold a collection of properties where the key is the name of the property. I'll step into that idea in this next example.
Property-based access.
The use of property-based access changes the dynamics of hashtables and how you can use them in PowerShell. Here is our usual example from above treating the keys as properties.
$ageList = @<> $ageList.Kevin = 35 $ageList.Alex = 9.
Just like the examples above, this example adds those keys if they don't exist in the hashtable already. Depending on how you defined your keys and what your values are, this is either a little strange or a perfect fit. The age list example has worked great up until this point. We need a new example for this to feel right going forward.
$person = @
And we can add and access attributes on the $person like this.
$person.city = 'Austin' $person.state = 'TX'
All of a sudden this hashtable starts to feel and act like an object. It's still a collection of things, so all the examples above still apply. We just approach it from a different point of view.
Checking for keys and values.
In most cases, you can just test for the value with something like this:
if( $person.age )
It's simple but has been the source of many bugs for me because I was overlooking one important detail in my logic. I started to use it to test if a key was present. When the value was $false or zero, that statement would return $false unexpectedly.
if( $person.age -ne $null )
This works around that issue for zero values but not $null vs non-existent keys. Most of the time you don't need to make that distinction but there are functions for when you do.
if( $person.ContainsKey('age') )
We also have a ContainsValue() for the situation where you need to test for a value without knowing the key or iterating the whole collection.
Removing and clearing keys.
You can remove keys with the .Remove() function.
$person.remove('age')
Assigning them a $null value just leaves you with a key that has a $null value.
A common way to clear a hashtable is to just initialize it to an empty hashtable.
$person = @<>
While that does work, try to use the clear() function instead.
$person.clear()
This is one of those instances where using the function creates self-documenting code and it makes the intentions of the code very clean.
All the fun stuff.
Ordered hashtables.
By default, hashtables aren't ordered (or sorted). In the traditional context, the order doesn't matter when you always use a key to access values. You may find that you want the properties to stay in the order that you define them. Thankfully, there's a way to do that with the ordered keyword.
$person = [ordered]@
Now when you enumerate the keys and values, they stay in that order.
Inline hashtables.
When you're defining a hashtable on one line, you can separate the key/value pairs with a semicolon.
$person = @
This will come in handy if you're creating them on the pipe.
Custom expressions in common pipeline commands.
There are a few cmdlets that support the use of hashtables to create custom or calculated properties. You commonly see this with Select-Object and Format-Table . The hashtables have a special syntax that looks like this when fully expanded.
$property = @ >
The name is what the cmdlet would label that column. The expression is a script block that is executed where $_ is the value of the object on the pipe. Here is that script in action:
$drives = Get-PSDrive | Where Used $drives | Select-Object -Property name, $property Name totalSpaceGB ---- ------------ C 238.472652435303.
I placed that in a variable but it could easily be defined inline and you can shorten name to n and expression to e while you're at it.
$drives | Select-Object -property name, @ >
I personally don't like how long that makes commands and it often promotes some bad behaviors that I won't get into. I'm more likely to create a new hashtable or pscustomobject with all the fields and properties that I want instead of using this approach in scripts. But there's a lot of code out there that does this so I wanted you to be aware of it. I talk about creating a pscustomobject later on.
Custom sort expression.
It's easy to sort a collection if the objects have the data that you want to sort on. You can either add the data to the object before you sort it or create a custom expression for Sort-Object .
Get-ADUser | Sort-Object -Property @ >
In this example I'm taking a list of users and using some custom cmdlet to get additional information just for the sort.
Sort a list of Hashtables.
If you have a list of hashtables that you want to sort, you'll find that the Sort-Object doesn't treat your keys as properties. We can get a round that by using a custom sort expression.
$data = @( @ @ @ @ @ @ ) $data | Sort-Object -Property @>
Splatting hashtables at cmdlets.
This is one of my favorite things about hashtables that many people don't discover early on. The idea is that instead of providing all the properties to a cmdlet on one line, you can instead pack them into a hashtable first. Then you can give the hashtable to the function in a special way. Here is an example of creating a DHCP scope the normal way.
Add-DhcpServerV4Scope -Name 'TestNetwork' -StartRange '10.0.0.2' -EndRange '10.0.0.254' -SubnetMask '255.255.255.0' -Description 'Network for testlab A' -LeaseDuration (New-TimeSpan -Days 8) -Type "Both"
Without using splatting, all those things need to be defined on a single line. It either scrolls off the screen or will wrap where ever it feels like. Now compare that to a command that uses splatting.
$DHCPScope = @ Add-DhcpServerV4Scope @DHCPScope.
The use of the @ sign instead of the $ is what invokes the splat operation.
Just take a moment to appreciate how easy that example is to read. They are the exact same command with all the same values. The second one is easier to understand and maintain going forward.
I use splatting anytime the command gets too long. I define too long as causing my window to scroll right. If I hit three properties for a function, odds are that I'll rewrite it using a splatted hashtable.
Splatting for optional parameters.
One of the most common ways I use splatting is to deal with optional parameters that come from some place else in my script. Let's say I have a function that wraps a Get-CIMInstance call that has an optional $Credential argument.
$CIMParams = @ if($Credential) Get-CIMInstance @CIMParams.
I start by creating my hashtable with common parameters. Then I add the $Credential if it exists. Because I'm using splatting here, I only need to have the call to Get-CIMInstance in my code once. This design pattern is very clean and can handle lots of optional parameters easily.
To be fair, you could write your commands to allow $null values for parameters. You just don't always have control over the other commands you're calling.
Multiple splats.
You can splat multiple hashtables to the same cmdlet. If we revisit our original splatting example:
$Common = @ $DHCPScope = @ Add-DhcpServerv4Scope @DHCPScope @Common.
I'll use this method when I have a common set of parameters that I'm passing to lots of commands.
Splatting for clean code.
There's nothing wrong with splatting a single parameter if makes you code cleaner.
$log = @ Add-Content "logging this command" @log.
Splatting executables.
Splatting also works on some executables that use a /param:value syntax. Robocopy.exe , for example, has some parameters like this.
$robo = @ robocopy source destination @robo.
I don't know that this is all that useful, but I found it interesting.
Adding hashtables.
Hashtables support the addition operator to combine two hashtables.
$person += @
This only works if the two hashtables don't share a key.
Nested hashtables.
We can use hashtables as values inside a hashtable.
$person = @ $person.location = @<> $person.location.city = 'Austin' $person.location.state = 'TX'
I started with a basic hashtable containing two keys. I added a key called location with an empty hashtable. Then I added the last two items to that location hashtable. We can do this all inline too.
$person = @ >
This creates the same hashtable that we saw above and can access the properties the same way.
$person.location.city Austin.
There are many ways to approach the structure of your objects. Here is a second way to look at a nested hashtable.
$people = @ Alex = @ >
This mixes the concept of using hashtables as a collection of objects and a collection of properties. The values are still easy to access even when they're nested using whatever approach you prefer.
PS> $people.kevin.age 36 PS> $people.kevin['city'] Austin PS> $people['Alex'].age 9 PS> $people['Alex']['City'] Austin.
I tend to use the dot property when I'm treating it like a property. Those are generally things I've defined statically in my code and I know them off the top of my head. If I need to walk the list or programmatically access the keys, I use the brackets to provide the key name.
foreach($name in $people.keys)
Having the ability to nest hashtables gives you a lot of flexibility and options.
Looking at nested hashtables.
As soon as you start nesting hashtables, you're going to need an easy way to look at them from the console. If I take that last hashtable, I get an output that looks like this and it only goes so deep:
PS> $people Name Value ---- ----- Kevin Alex.
My go to command for looking at these things is ConvertTo-JSON because it's very clean and I frequently use JSON on other things.
PS> $people | ConvertTo-Json , "Alex": >
Even if you don't know JSON, you should be able to see what you're looking for. There's a Format-Custom command for structured data like this but I still like the JSON view better.
Creating objects.
Sometimes you just need to have an object and using a hashtable to hold properties just isn't getting the job done. Most commonly you want to see the keys as column names. A pscustomobject makes that easy.
$person = [pscustomobject]@ $person name age ---- --- Kevin 36.
Even if you don't create it as a pscustomobject initially, you can always cast it later when needed.
$person = @ [pscustomobject]$person name age ---- --- Kevin 36.
I already have detailed write-up for pscustomobject that you should go read after this one. It builds on a lot of the things learned here.
Reading and writing hashtables to file.
Saving to CSV.
Struggling with getting a hashtable to save to a CSV is one of the difficulties that I was referring to above. Convert your hashtable to a pscustomobject and it will save correctly to CSV. It helps if you start with a pscustomobject so the column order is preserved. But you can cast it to a pscustomobject inline if needed.
$person | ForEach-Object | Export-CSV -Path $path.
Again, check out my write-up on using a pscustomobject.
Saving a nested hashtable to file.
If I need to save a nested hashtable to a file and then read it back in again, I use the JSON cmdlets to do it.
$people | ConvertTo-JSON | Set-Content -Path $path $people = Get-Content -Path $path -Raw | ConvertFrom-JSON.
There are two important points about this method. First is that the JSON is written out multiline so I need to use the -Raw option to read it back into a single string. The Second is that the imported object is no longer a [hashtable] . It's now a [pscustomobject] and that can cause issues if you don't expect it.
Watch for deeply-nested hashtables. When you convert it to JSON you might not get the results you expect.
@ >>> | ConvertTo-Json > >
Use Depth parameter to ensure that you have expanded all the nested hashtables.
@ >>> | ConvertTo-Json -Depth 3 > > >
If you need it to be a [hashtable] on import, then you need to use the Export-CliXml and Import-CliXml commands.
Converting JSON to Hashtable.
If you need to convert JSON to a [hashtable] , there's one way that I know of to do it with the JavaScriptSerializer in .NET.
[Reflection.Assembly]::LoadWithPartialName("System.Web.Script.Serialization") $JSSerializer = [System.Web.Script.Serialization.JavaScriptSerializer]::new() $JSSerializer.Deserialize($json,'Hashtable')
Beginning in PowerShell v6, JSON support uses the NewtonSoft JSON.NET and adds hashtable support.
' ' | ConvertFrom-Json -AsHashtable Name Value ---- ----- a b.
PowerShell 6.2 added the Depth parameter to ConvertFrom-Json . The default Depth is 1024.
Reading directly from a file.
If you have a file that contains a hashtable using PowerShell syntax, there's a way to import it directly.
$content = Get-Content -Path $Path -Raw -ErrorAction Stop $scriptBlock = [scriptblock]::Create( $content ) $scriptBlock.CheckRestrictedLanguage( $allowedCommands, $allowedVariables, $true ) $hashtable = ( & $scriptBlock )
It imports the contents of the file into a scriptblock , then checks to make sure it doesn't have any other PowerShell commands in it before it executes it.
On that note, did you know that a module manifest (the psd1 file) is just a hashtable?
Keys can be any object.
Most of the time, the keys are just strings. So we can put quotes around anything and make it a key.
$person = @ $person['full name']
You can do some odd things that you may not have realized you could do.
$person.'full name' $key = 'full name' $person.$key.
Just because you can do something, it doesn't mean that you should. That last one just looks like a bug waiting to happen and would be easily misunderstood by anyone reading your code.
Technically your key doesn't have to be a string but they're easier to think about if you only use strings. However, indexing doesn't work well with the complex keys.
$ht = @ $ht Name Value ---- ----- a.
Accessing a value in the hashtable by its key doesn't always work. For example:
$key = $ht.keys[0] $ht.$($key) a $ht[$key] a.
When the key is an array, you must wrap the $key variable in a subexpression so that it can be used with member access ( . ) notation. Or, you can use array index ( [] ) notation.
Use in automatic variables.
$PSBoundParameters.
$PSBoundParameters is an automatic variable that only exists inside the context of a function. It contains all the parameters that the function was called with. This isn't exactly a hashtable but close enough that you can treat it like one.
That includes removing keys and splatting it to other functions. If you find yourself writing proxy functions, take a closer look at this one.
PSBoundParameters gotcha.
One important thing to remember is that this only includes the values that are passed in as parameters. If you also have parameters with default values but aren't passed in by the caller, $PSBoundParameters doesn't contain those values. This is commonly overlooked.
$PSDefaultParameterValues.
This automatic variable lets you assign default values to any cmdlet without changing the cmdlet. Take a look at this example.
$PSDefaultParameterValues["Out-File:Encoding"] = "UTF8"
This adds an entry to the $PSDefaultParameterValues hashtable that sets UTF8 as the default value for the Out-File -Encoding parameter. This is session-specific so you should place it in your $profile .
I use this often to pre-assign values that I type quite often.
$PSDefaultParameterValues[ "Connect-VIServer:Server" ] = 'VCENTER01.contoso.local'
This also accepts wildcards so you can set values in bulk. Here are some ways you can use that:
$PSDefaultParameterValues[ "Get-*:Verbose" ] = $true $PSDefaultParameterValues[ "*:Credential" ] = Get-Credential.
For a more in-depth breakdown, see this great article on Automatic Defaults by Michael Sorens.
Regex $Matches.
When you use the -match operator, an automatic variable called $matches is created with the results of the match. If you have any sub expressions in your regex, those sub matches are also listed.
$message = 'My SSN is 123-45-6789.' $message -match 'My SSN is (.+)\.' $Matches[0] $Matches[1]
Named matches.
This is one of my favorite features that most people don't know about. If you use a named regex match, then you can access that match by name on the matches.
$message = 'My Name is Kevin and my SSN is 123-45-6789.' if($message -match 'My Name is (?.+) and my SSN is (?.+)\.')
In the example above, the (?.*) is a named sub expression. This value is then placed in the $Matches.Name property.
Group-Object -AsHashtable.
One little known feature of Group-Object is that it can turn some datasets into a hashtable for you.
Import-CSV $Path | Group-Object -AsHashtable -Property email.
This will add each row into a hashtable and use the specified property as the key to access it.
Copying Hashtables.
One important thing to know is that hashtables are objects. And each variable is just a reference to an object. This means that it takes more work to make a valid copy of a hashtable.
Assigning reference types.
When you have one hashtable and assign it to a second variable, both variables point to the same hashtable.
PS> $orig = @ PS> $copy = $orig PS> $copy.name = 'copy' PS> 'Copy: []' -f $copy.name PS> 'Orig: []' -f $orig.name Copy: [copy] Orig: [copy]
This highlights that they're the same because altering the values in one will also alter the values in the other. This also applies when passing hashtables into other functions. If those functions make changes to that hashtable, your original is also altered.
Shallow copies, single level.
If we have a simple hashtable like our example above, we can use .Clone() to make a shallow copy.
PS> $orig = @ PS> $copy = $orig.Clone() PS> $copy.name = 'copy' PS> 'Copy: []' -f $copy.name PS> 'Orig: []' -f $orig.name Copy: [copy] Orig: [orig]
This will allow us to make some basic changes to one that don't impact the other.
Shallow copies, nested.
The reason why it's called a shallow copy is because it only copies the base level properties. If one of those properties is a reference type (like another hashtable), then those nested objects will still point to each other.
PS> $orig = @ > PS> $copy = $orig.Clone() PS> $copy.person.name = 'copy' PS> 'Copy: []' -f $copy.person.name PS> 'Orig: []' -f $orig.person.name Copy: [copy] Orig: [copy]
So you can see that even though I cloned the hashtable, the reference to person wasn't cloned. We need to make a deep copy to truly have a second hashtable that isn't linked to the first.
Deep copies.
There are a couple of ways to make a deep copy of a hashtable (and keep it as a hashtable). Here's a function using PowerShell to recursively create a deep copy:
function Get-DeepClone foreach($key in $InputObject.keys) return $clone > else > >
It doesn't handle any other reference types or arrays, but it's a good starting point.
Another way is to use .Net to deserialize it using CliXml like in this function:
function Get-DeepClone.
For extremely large hashtables, the deserializing function is faster as it scales out. However, there are some things to consider when using this method. Since it uses CliXml , it's memory intensive and if you are cloning huge hashtables, that might be a problem. Another limitation of the CliXml is there is a depth limitation of 48. Meaning, if you have a hashtable with 48 layers of nested hashtables, the cloning will fail and no hashtable will be output at all.
Anything else?
I covered a lot of ground quickly. My hope is that you walk away leaning something new or understanding it better every time you read this. Because I covered the full spectrum of this feature, there are aspects that just may not apply to you right now. That is perfectly OK and is kind of expected depending on how much you work with PowerShell.
Collaborate with us on GitHub.
The source for this content can be found on GitHub, where you can also create and review issues and pull requests. For more information, see our contributor guide.
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