Refactoring PowerShell – Switch Statements

Regardless of your experience within technology, the process of creating code usually starts out with a solution that is just enough to get the job done. The solution is then typically tweaked and improved continuously until it is either “production worthy” or “good enough for the requirements“. This process of improvement is called refactoring. Refactoring is a skill that all technical professionals should become proficient in, regardless if you are an IT Pro, a developer or just someone who needs to automate things.

There are many reasons to refactor code, including:

  • Add new features
  • Remove unused features
  • Improve performance
  • Increase readability, maintainability or test-ability
  • Improve design
  • Improve adherence to standards or best practices

Refactoring in Code Reviews

One of the jobs of a code reviewer is to suggest areas that could be refactored to improve some of the areas above. I’ve often found that I’ll suggest the same set of refactorings in my reviews. So rather than just putting the suggesting into the code review, I thought I’d start writing them down here in a series that I could refer contributors to as well as help anyone else who happens to come across these.

Unit Tests and Refactoring

Because refactoring requires changing code, how can we be sure that we’re not breaking functionality or introducing bugs? That is where unit testing comes in. With PowerShell, we’ll typically use the PowerShell Pester module to create unit tests that allow us to more safely refactor code. Unit testing is beyond the scope of this post.

Switch Statement Refactoring

One of the common patterns I’ve come across is PowerShell switch statements being used is to map from one values to another set of values. For example:

This converts a colour name (e.g. red, green, blue, white) to a colour value. If a colour name can not be matched then it returns 0x0 (black). Admittedly, this is a bit of an unlikely example, but it demonstrates the pattern.

This is by no means incorrect or “bad code”. It is completely valid and solves the problem perfectly well. But as you can see this requires a lot of code to perform a simple mapping.

Mapping using a Hash Table

An alternative to using a switch statement is to use a hash table:

This can simplify the code slightly by removing the break statement and braces.

Note: The break statement is not strictly required in this example from a functional perspective, but including them increases overall performance of the switch statement.

You may have noticed that the hash table above does not quite match the behavior of the switch statement: the default mapping to 0x0 is not handled. So, in this case, we’d need to include additional code to handle this:

To handle the default we’re using a quasi null coalescing operator. PowerShell doesn’t have a native null coalescing operator like many languages, but it does have a way of simulating it using the line:

You could definitely argue that using a hash table mapping with a null coalescing operator does not make the code easier to read or maintain. But the purpose here is not to define which approach is best, rather to offer alternative patterns.

One other benefit of using a hash table for mapping is that it can be separated out into a separate psd1 file. This allows editing of the mapping table elements without having to edit the code itself:

The psd1 file containing the mapping data (mapping.psd1):

Reversing the Mapping

How do we use a similar pattern to reverse the mapping? For example, mapping a colour value back to a colour name:

To implement the same functionality using a hash table also including the null coalescing operator you could use:

Using a Hash Table with Script Values

Switch blocks may contain more than just a single statement. For example:

If your switch blocks do more than just perform a mapping, you can assign script blocks to the hash table values instead:

Instead of containing a value in each hash table item, a script block is specified. The Invoke() method can then be called on the script block.

Enumerated Types

If you’re using PowerShell 5.0 or above (you hopefully are), you’re also able to use the enum keyword to define an enumerated type that can also be used to replace switch statements in some situations.

The enumerated type only needs to be declared once.

But what do we need to do if we want to have a default returned if the value is invalid in the mapping? In that case we need to use the TryParse method of the enumerated type to try and parse the value and return a default value if it is invalid:

However, we can’t assign scriptblocks to the values in an enumerated type – only constant values may be assigned. This means we can’t implement scenarios where we’d like to have the value contain more than one instruction. But this shouldn’t be too much of a problem, because if you do find yourself being limited by this, then you should probably be looking to use more advanced object oriented programming patterns such as polymorphism – which is well beyond the scope of this post. But if you’re interested to know more, review this article (not PowerShell specific).

Wrapping Up

This post is all about looking at different ways of writing the same code. It isn’t trying to say which way is better or worse. If you have a preference and it works for you, by all means, keep on using it. This is simply to provide alternative methods that may or may not make code more readable and maintainable.

Feel free to comment with alternative methods of refactoring switch statements.

 

 

 

Allow Integer Parameter to Accept Null in a PowerShell Function

One of the great things about PowerShell being based on .NET is that we get to the huge number of types built into the framework.

A problem I came across today was that I needed to have a function that took a mandatory integer parameter, but that parameter needed to allow Null. In .NET there is a generic type System.Nullable<T> that allows other types to take on a null value.

This allows me to call the function above with the following:

Set-AdapterVlan -Adapter $adapter -Vlan $null

Which will clear the Vlan ID from the virtual network adapter.

The magic is in the parameter definition:

[Parameter(Mandatory = $true)]
[AllowNull()]
[Nullable[System.Int32]]
$VlanId

The [AllowNull()] attribute allows the $VlanId parameter to accept a Null even though it is mandatory, and the [Nullable[System.Int32]] allows $VlanId to be assigned a null value.

This isn’t something I use often, but thought it was worth sharing.

Enable CORS Support in Cosmos DB using PowerShell

Support for Cross-Origin Resource Sharing (CORS) was recently added to Cosmos DB. If you want to enable CORS on an existing Cosmos DB account or create a new Cosmos DB account with CORS enabled it is very easy to do with Azure Resource Manager (ARM) templates or the Azure Portal.

But what if you’re wanting to find out the state of the CORS setting on an account or set it using PowerShell? Well, look no further.

The Cosmos DB PowerShell module (version 3.0.0 and above) supports creating Cosmos DB accounts with CORS enabled as well as updating and removing the CORS headers setting on an existing account. You can also retrieve the CORS setting for an existing Cosmos DB account.

Installing the CosmosDB Module

The first thing you need to do is install the CosmosDB PowerShell module from the PowerShell Gallery by running this in a PowerShell console:

ss_cosmosdbcors_installmodule

This will also install the Az PowerShell modules Az.Accounts and Az.Resources modules if they are not installed on your machine. The *-CosmosDbAccount functions in the CosmosDB module are dependent on these modules.

Note: The CosmosDB PowerShell module and the Az PowerShell modules are completely cross-platform and support Linux, MacOS and Windows. Running in either Windows PowerShell (Windows) or PowerShell Core (cross-platform) is supported.

Versions of the CosmosDB PowerShell module earlier than 3.0.0.0 use the older AzureRm/AzureRm.NetCore modules and do not support the CORS setting.

Authenticating to Azure with ‘Az’

Before using the CosmosDB PowerShell module accounts functions to work with CORS settings you’ll first need to authenticate to Azure using the Az PowerShell Modules. If you’re planning on automating this process you’ll want to authenticate to Azure using a Service Principal identity.

Side note: if you’re using this module in an Azure DevOps build/release pipeline the Azure PowerShell task will take care of the Service Principal authentication process for you:

ss_cosmosdbcors_azuredevopspowershelltask

But if you’re just doing a little bit of experimentation then you can just use an interactive authentication process.

To use the interactive authentication process just enter into your PowerShell console:

then follow the instructions.

ss_cosmosdbcors_authenticateaz.png

Create a Cosmos DB Account with CORS enabled

Once you have authenticated to Azure, you can use the New-CosmosDbAccount function to create a new account:

ss_cosmosdbcors_newcosmosdbaccountThis will create a new Cosmos DB account with the name dsrcosmosdbtest in the resource group dsrcosmosdbtest-rgp in the West US location and with CORS allowed origins of https://www.fabrikam.com and https://www.contoso.com.

Important: the New-CosmosDbAccount command assumes the resource group that is specified in the ResourceGroup parameter already exists and you have contributor access to it. If the resource group doesn’t exist then you can create it using the New-AzResourceGroup function or some other method.

It will take Azure a few minutes to create the new Cosmos DB account for you.

Side note: But if you want your PowerShell automation or script to be able to get on and do other tasks in the meantime, then add the -AsJob parameter to the New-CosmosDbAccountcall. This will cause the function to immediately return and provide you a Job object that you can use to periodically query the state of the Job. More information on using PowerShell Jobs can be found here.

Be aware, you won’t be able to use the Cosmos DB account until the Job is completed.

If you look in the Azure Portal, you will find the new Cosmos DB account with the CORS allowed origin values set as per your command:

ss_cosmosdbcors_cosmosdbinportalwithcors

Get the CORS Allowed Origins on a Cosmos DB Account

Getting the current CORS Allowed Origins value on an account is easy too. Just run the following PowerShell command:

ss_cosmosdbcors_getcosmosdbcors

This will return a string containing all the CORS Allowed Origins for the Cosmos DB account dsrcosmosdbtest.

You could easily split this string into an array variable by using:

ss_cosmosdbcors_getcosmosdbcorssplit

Update the CORS Allowed Origins on an existing Cosmos DB Account

To set the CORS Allowed Origins on an existing account use the Set-CosmosDbAccount function:

ss_cosmosdbcors_setcosmosdbcors

This will take a few minutes to update. So you can use the -AsJob parameter to run this as a Job.

Remove the CORS Allowed Origins from an existing Cosmos DB Account

You can remove the CORS Allowed Origins setting by setting using the Set-CosmosDbAccount function but passing in an empty string to the AllowedOrigin parameter:

ss_cosmosdbcors_removecosmosdbcors

This will take a few minutes to update as well. As always, you can use the -AsJob parameter to run this as a Job.

 

Final Words

Hopefully, you can see it is fairly simple to automate and work with the Cosmos DB CORS Allowed Origins setting using the PowerShell Cosmos DB module.

If you have any issues or queries or would like to contribute to the PowerShell Cosmos DB module, please head over to the GitHub repository.

 

Use Pester to Test Azure Resource Manager Templates for Best Practices

Recently I came across the amazing Secure DevOps Kit for Azure (AzSK). This contains a really useful AzSK PowerShell Module that contains cmdlets for performing different types of security scanning on Azure Resources, Subscriptions and Resource Manager Templates.

The feature of this module that I was most interested in for my current project was being able to scan ARM templates for best practice violations. The module contains several

To install the module, open a PowerShell Window and run:

Important: At the time of writing this post, the AzSK module has dependencies on the AzureRM.Profile and other AzureRM.* PowerShell modules. As of December 2018, the AzureRM.* PowerShell Modules are going to be renamed to Az.* (see this post). The AzureRM and Az modules can not be installed side-by-side, so if you’ve installed the Az PowerShell modules on your system then the installation of AzSK will fail because the AzureRM modules will also be installed and a conflict will occur.

The cmdlet we’re most interested in is the Get-AzSKARMTemplateSecurityStatus. It can be used to scan one or more ARM templates or entire folders of ARM templates for best practice violations:

ss_azsk_scanning

This will scan the ARM templates and produce a CSV report in a folder Microsoft\AzSKLogs\ARMChecker within your $ENV:LOCALAPPDATA folder and open the folder in Explorer. This isn’t ideal for automation scenarios or using during Continuous Integration or Continuous Delivery pipelines. I’ve raised an issue with the AzSK team on GitHub to see if this can be improved.

In my case, I wanted to be able to use the PowerShell Pester Module, a PowerShell testing framework, to execute tests on the output and then use the nUnit output Pester generates to publish into a Continuous Integration pipeline. To do that I needed to create a custom test script that would take the CSV report, count the failures of each level (High, Medium or Low) and fail if any are counted in the specific level.

This is what the script looks like:

You can download the script from GitHub Gist directly or get it from the PowerShell Gallery by running:

Install-Script -Name AzSKARMTemplateSecurityStatus.Test

To use it you will need to install Pester 4.3.0 and AzSK 3.6.1 modules:

Once that is done, you can use Invoke-Pester and pass in the TemplatePath and Severity parameters to the test script:

This will execute the Pester tests in the file above on the specified ARM template. The tests will fail when there are any best practice violations with the specified Severity or above. If you didn’t pass in a Severity then it will default to failing on Medium and High.

ss_azsk_invokepester

If you use the OutputFile and OutputFormat parameters to cause Pester to output an NUnit format file that most Continuous Integration tools will happily accept and use to display the output of the tests.

If you installed the script from the PowerShell Gallery, you can also run the tests like this:

AzSKARMTemplateSecurityStatus.Test -TemplatePath D:\101-webapp-basic-windows\azuredeploy.json

Finally, if you’re using Azure DevOps, you can also get this function as part Secure DevOps Kit (AzSK) CICD Extensions for Azure in the Azure DevOps Marketplace.

Which ever way you choose to consume AzSK, it is a great module and well worth including in your CI/CD pipelines to ensure your ARM templates meet best practices.

 

 

List Global Assembly Cache using PowerShell

The list of assemblies stored in the Global Assembly Cache (GAC) can be found in the registry under the HKEY_CLASSES_ROOT\Installer\Assemblies\Global key.

If you want to get a list of the assemblies registered in the GAC using PowerShell you can use this snippet:

ss_gac_getcontent

The first line registers a new drive called HKCR in PowerShell that maps to the HKEY_CLASSES_ROOT in the registry. This is required because, by default only the HKEY_CURRENT_USER and HKEY_LOCAL_MACHINE registry hives are registered as drives in PowerShell.

The second line just gets the list of registry properties in the HKEY_CLASSES_ROOT\Installer\Assemblies\Global key and displays them.

 

 

Get the ForceChangePassword Office 365 User Setting with PowerShell

Recently I was asked by a friend if I knew of a way to get the value of the setting that forces a user to change their password when the next log in to Office 365. The friend wanted to get this value for all users using PowerShell.

Changing this setting is fairly straight forward either in the Office 365 portal or using the Set-MsolUserPassword cmdlet in the MSOnline PowerShell module:

ss_o365_setmsoluserpassword

However retrieving the current value of the setting isn’t possible using Get-MoslUser cmdlet – the attribute does not appear in the returned object:

ss_o365_getmsoluser

Instead, we need to use the Get-AzureADUser cmdlet in the AzureAD PowerShell Module to query the Azure Active Directory for the Office 365 tenant.

If you don’t have the AzureAD module installed, use Install-Module cmdlet to install it from the PowerShell Gallery:

Then connect to the AzureAD using the Connect-AzureAD cmdlet. Once connected you can run the following command to get the user object and show only the appropriate property (ForceChangePasswordNextLogin of the PasswordProfile object):

ss_o365_getazureadallforcechangepasswordnextlogin

This is all fairly straight forward once you figure out which object in Azure AD contains the information required.