Pure PowerShell script to apply an XDT transform with a typicaly web.config found in ASP.Net.

Merge-XdtTransform.ps1

function Merge-XdtTransform {
    <#
    .SYNOPSIS
        A function to apply Xml Configuration Transforms without the need for Microsoft.Web.XmlTransform.dll.

    .NOTES
        Michael West

    .EXAMPLE
        Merge-XdtTransform -Base $xmlBase -Transform $xmlTransform
    #>
    param(
        [Parameter(Mandatory)]
        [ValidateNotNull()]
        [xml]$Base,
        [Parameter(Mandatory)]
        [ValidateNotNull()]
        [xml]$Transform
    )

    $document = $Base.Clone()

    try {
        Write-Host "Processing Xdt Transforms"
        
        #https://learn.microsoft.com/en-us/previous-versions/aspnet/dd465326(v=vs.110)
        $namespaceManager = New-Object System.Xml.XmlNamespaceManager($transform.NameTable)
        $namespaceManager.AddNamespace("xdt", "http://schemas.microsoft.com/XML-Document-Transform")

        foreach ($transformNode in $transform.SelectNodes("//*[@xdt:Transform]", $namespaceManager)) {
            $transformType = $transformNode.Attributes["xdt:Transform"]
        
            $nodesToModify = & {
                $xpath = "//$($transformNode.LocalName)"
                $locator = $transformNode.Attributes["xdt:Locator"]
                if($locator -and $locator.Value -like "Match*") {
                    #Locator="Match(comma-delimited list of one or more attribute names)"
                    #xdt:Locator="Match(key)"
                    $matchAttributeNames = $locator.Value.Replace("Match(","").Replace(")","").Split(",")
                    foreach($matchAttributeName in $matchAttributeNames) {
                        $matchAttributeValue = $transformNode.Attributes[$matchAttributeName].Value
                        $xpath += "[@$($matchAttributeName)='$($matchAttributeValue)']"
                    }
                    $document.SelectNodes($xpath)
                } elseif($locator -and $locator.Value -like "XPath*") {
                    #Locator="XPath(XPath expression)"
                    #xdt:Locator="XPath(configuration/connectionStrings/add[@name='PaymentsContext'])"
                    $xpath = $locator.Value.Replace("XPath(","").Replace(")","")
                    $document.SelectNodes($xpath)
                } elseif($locator -and $locator.Value -like "Condition*") {
                    #Locator="Condition(XPath expression)"
                    #xdt:Locator="Condition(@key='IsStaging')"

                    if ($locator.Value -match "Condition\((.+)\)") {
                        $condition = $matches[1]
                        $xpath += "[$($condition)]"
                        $document.SelectNodes($xpath)
                    }
                } elseif (!$locator) {
                    $document.SelectSingleNode($xpath)
                }               
            }
            
            $keepProcessing = $true
            $nodeQueue = New-Object System.Collections.Queue
            @($nodesToModify) | ForEach-Object { $nodeQueue.Enqueue($_) > $null }
        
            while($keepProcessing) {
                $nodeToModify = if($nodeQueue.Count -gt 0) { $nodeQueue.Dequeue() } else { $null }
                if($nodeQueue.Count -eq 0) {
                    $keepProcessing = $false
                }
                switch -Wildcard ($transformType.Value) {
                    "CommentOut" {
                        if(!$nodeToModify) {
                            Write-Host "- [CommentOut] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName) skipped" -ForegroundColor Yellow
                            continue
                        }

                        $parentNode = $nodeToModify.ParentNode

                        $comment = [string]::Concat(" ", $nodeToModify.OuterXml, " ")
                        $newNode = $document.CreateComment($comment)
                        Write-Host "- [CommentOut] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName)" -ForegroundColor Blue
                        $parentNode.InsertAfter($newNode, $nodeToModify) > $null
                        $parentNode.RemoveChild($nodeToModify) > $null
                    }
                    "SetAttributes*" {
                        if(!$nodeToModify) {
                            Write-Host "- [SetAttributes] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName) skipped" -ForegroundColor Yellow
                            continue
                        }

                        Write-Host "- [SetAttributes] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName)" -ForegroundColor Blue
                        $attributesToSet = $transformType.Value.Replace("SetAttributes","").Replace("(","").Replace(")","").Split(",", [System.StringSplitOptions]::RemoveEmptyEntries)
                        $attributeLookup = New-Object System.Collections.Generic.HashSet[string](@(,$attributesToSet))                        

                        $transformNode.Attributes | ForEach-Object {
                            if ($_.Name -notlike "xdt:*" -and ($attributeLookup.Count -eq 0 -or $attributeLookup.Contains($_.Name))) {
                                $nodeToModify.SetAttribute($_.Name, $_.Value) > $null
                            }
                        }
                    }
                    "Replace" {
                        if(!$nodeToModify) {
                            Write-Host "- [Replace] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName) skipped" -ForegroundColor Yellow
                            continue
                        }

                        $parentNode = $nodeToModify.ParentNode

                        $newNode = $document.ImportNode($transformNode, $true)
                        $transformNode.Attributes | ForEach-Object {
                            if ($_.Name -like "xdt:*") {
                                $newNode.RemoveAttribute($_.Name) > $null
                            }
                        }

                        Write-Host "- [Replace] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName)" -ForegroundColor Blue
                        $parentNode.ReplaceChild($newNode, $nodeToModify) > $null
                    }
                    "Insert" {
                        $parentNode = $nodeToModify.ParentNode
                        if(!$parentNode) {
                            #Node is missing so let's get the parent.
                            $parentNode = $document.SelectSingleNode("//$($transformNode.ParentNode.LocalName)")
                        }
                        if(!$parentNode) {
                            Write-Host "- [Insert] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName) skipped" -ForegroundColor Yellow
                            continue
                        }
                        $newNode = $document.ImportNode($transformNode, $true)
                        $transformNode.Attributes | ForEach-Object {
                            if ($_.Name -like "xdt:*") {
                                $newNode.RemoveAttribute($_.Name) > $null
                            }
                        }

                        Write-Host "- [Insert] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName)" -ForegroundColor Blue
                        $parentNode.AppendChild($newNode) > $null
                    }
                    "InsertIfMissing" {
                        if($nodeToModify) {
                            Write-Host "- [InsertIfMissing] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName) skipped" -ForegroundColor Yellow
                            continue
                        }
                        $parentNode = $document.SelectSingleNode("//$($transformNode.ParentNode.LocalName)")

                        $newNode = $document.ImportNode($transformNode, $true)
                        $transformNode.Attributes | ForEach-Object {
                            if ($_.Name -like "xdt:*") {
                                $newNode.RemoveAttribute($_.Name) > $null
                            }
                        }
                        
                        Write-Host "- [InsertIfMissing] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName)" -ForegroundColor Blue
                        $parentNode.AppendChild($newNode) > $null
                    }
                    "InsertBefore*" {
                        #Example: InsertBefore(/configuration/system.web/membership)
                        $parentNode = $nodeToModify.ParentNode
                        if(!$parentNode) {
                            #Node is missing so let's get the parent.
                            $parentNode = $document.SelectSingleNode("//$($transformNode.ParentNode.LocalName)")
                        }
                        if(!$parentNode) {
                            Write-Host "- [InsertBefore] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName) skipped" -ForegroundColor Yellow
                            continue
                        }
                        $path = $transformType.Value.Replace("InsertBefore(","").Replace(")","")
                        $beforeNode = $document.SelectSingleNode($path)
                        $newNode = $document.ImportNode($transformNode, $true)
                        $transformNode.Attributes | ForEach-Object {
                            if ($_.Name -like "xdt:*") {
                                $newNode.RemoveAttribute($_.Name) > $null
                            }
                        }
                        if ($beforeNode) {
                            Write-Host "- [InsertBefore] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName) before $($beforeNode.LocalName)" -ForegroundColor Blue
                            $parentNode.InsertBefore($newNode, $beforeNode) > $null
                        } else {
                            Write-Host "- [InsertBefore] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName)" -ForegroundColor Blue
                            $parentNode.AppendChild($newNode) > $null
                        }
                    }
                    "InsertAfter*" {
                        #Example: InsertAfter(/configuration/system.webServer)
                        $parentNode = $nodeToModify.ParentNode
                        if(!$parentNode) {
                            #Node is missing so let's get the parent.
                            $parentNode = $document.SelectSingleNode("//$($transformNode.ParentNode.LocalName)")
                        }
                        if(!$parentNode) {
                            Write-Host "- [InsertAfter] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName) skipped" -ForegroundColor Yellow
                            continue
                        }
                        $path = $transformType.Value.Replace("InsertAfter(","").Replace(")","")
                        $afterNode = $document.SelectSingleNode($path)
                        $newNode = $document.ImportNode($transformNode, $true)
                        $transformNode.Attributes | ForEach-Object {
                            if ($_.Name -like "xdt:*") {
                                $newNode.RemoveAttribute($_.Name) > $null
                            }
                        }
                        if ($afterNode) {
                            Write-Host "- [InsertAfter] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName) after $($afterNode.LocalName)" -ForegroundColor Blue
                            $parentNode.InsertAfter($newNode, $afterNode) > $null
                        } else {
                            Write-Host "- [InsertAfter] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName)" -ForegroundColor Blue
                            $parentNode.AppendChild($newNode) > $null
                        }
                    }
                    "Remove" {
                        if(!$nodeToModify) {
                            Write-Host "- [Remove] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName) skipped" -ForegroundColor Yellow
                            continue
                        }
                        $parentNode = $nodeToModify.ParentNode

                        Write-Host "- [Remove] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName)" -ForegroundColor Blue
                        $parentNode.RemoveChild($nodeToModify) > $null
                    }
                    "RemoveAll" {
                        # TODO: Should RemoveAll consider the element type?
                        $parentNode = $nodeToModify.ParentNode
                        if(!$parentNode) {
                            Write-Host "- [RemoveAll] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName) skipped" -ForegroundColor Yellow
                            continue
                        }
                        Write-Host "- [RemoveAll] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName)" -ForegroundColor Blue
                        if($transformNode.Attributes["xdt:Locator"]) {
                            $parentNode.RemoveChild($nodeToModify) > $null
                        } else {
                            $matchingNodes = $parentNode.SelectNodes("$($transformNode.LocalName)")
                            $matchingNodes | ForEach-Object { $parentNode.RemoveChild($_) > $null }
                            #$parentNode.RemoveAll() > $null
                        }
                    }
                    "RemoveAttributes*" {
                        if(!$nodeToModify) {
                            Write-Host "- [RemoveAttributes] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName) skipped" -ForegroundColor Yellow
                            continue
                        }
                        Write-Host "- [RemoveAttributes] $($transformNode.ParentNode.LocalName)/$($transformNode.LocalName)" -ForegroundColor Blue
                        $attributesToRemove = $transformType.Value.Replace("RemoveAttributes","").Replace("(","").Replace(")","").Split(",")
                        if(!$attributesToRemove) {
                            $nodesToModify.Attributes.RemoveAll() > $null
                        } else {
                            @($attributesToRemove) | ForEach-Object {
                                $nodeToModify.Attributes.Remove($nodeToModify.Attributes[$_]) > $null
                            }
                        }
                    }
                }
            }
        }
    } catch {
        Write-Error "Error applying XDT transform: $_"
        return $Base
    }
    return $document   
}

Microsoft.Web.XmlTransform.dll

TransformTests.ps1

. .\Merge-XdtTransform.ps1

$XdtDllPath = ".\Microsoft.Web.XmlTransform.dll"
Add-Type -Path $XdtDllPath

function Invoke-XdtTransform {
  param(
    [string]$Base,
    [string]$Transform,
    [ref]$Output
  )

  $stringReader = New-Object System.IO.StringReader($Base)
  $xmlReader = [System.Xml.XmlReader]::Create($stringReader)

  $target = New-Object Microsoft.Web.XmlTransform.XmlTransformableDocument
  $target.PreserveWhitespace = $true
  $target.Load($xmlReader)

  $transformation = New-Object Microsoft.Web.XmlTransform.XmlTransformation($Transform, $false, $null);
    
  if (!$transformation.Apply($target))
  {
    return $Base
  }

  $stringWriter = New-Object System.IO.StringWriter
  $xmlWriter = New-Object System.XMl.XmlTextWriter $stringWriter

  $target.WriteContentTo($xmlWriter)
  $xmlWriter.Flush()
  $stringWriter.Flush()
  return [xml]($stringWriter.ToString())
}

[xml]$xmlBase = @"
<?xml version="1.0" encoding="utf-8"?>
<!--
  For more information on how to configure your ASP.NET application, please visit
  https://go.microsoft.com/fwlink/?LinkId=169433
  -->
<configuration>
  <configSections>
    <section name="sitecore" type="Sitecore.Configuration.RuleBasedConfigReader, Sitecore.Kernel" />
  </configSections>
  <connectionStrings>
    <add name="WebContext" connectionString="" providerName="System.Data.SqlClient" />
    <add name="PaymentsContext" connectionString="" providerName="System.Data.SqlClient" />
  </connectionStrings>
  <appSettings file="Base.config">
    <add key="Environment" value="" />
    <add key="IsTestMode" value="true" />
    <add key="IsStaging" value="true" />
    <add key="IsDisabled" value="true" />
  </appSettings>
  <rewrite>
    <rules>
        <rule />
        <rule />
    </rules>
  </rewrite>
  <system.web>
    <compilation defaultLanguage="c#" debug="false" targetFramework="4.8" />
    <membership defaultProvider="sitecore" hashAlgorithmType="SHA1">
    </membership>
    <compilation>
      <assemblies>
      </assemblies>
    </compilation>
    <application coldstart="true" user="root" />
  </system.web>
  <system.webServer>
    <urlCompression doDynamicCompression="true" />
    <httpCookies httpOnlyCookies="true" requireSSL="false" />
  </system.webServer>
</configuration>
"@

[xml]$xmlTransform = @"
<?xml version="1.0" encoding="utf-8"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <configSections>
    <section xdt:Transform="Replace" xdt:Locator="Match(name)" name="sitecore" type="Scms.Foundation.Configuration.EnvironmentVariablesConfigReader, Scms.Foundation"/>
  </configSections>
  <connectionStrings>
    <add xdt:Transform="SetAttributes" xdt:Locator="Match(name)" name="WebContext" connectionString="data source=CHS-CADEVDB01;initial catalog=Web;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework" />
    <add xdt:Transform="Replace" xdt:Locator="XPath(configuration/connectionStrings/add[@name='PaymentsContext'])" name="PaymentsContext" connectionString="data source=CHS-CADEVDB01;initial catalog=Payments;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework" />
  </connectionStrings>
  <appSettings>
    <add xdt:Transform="SetAttributes" xdt:Locator="Match(key)" key="Environment" value="Development" />
    <add xdt:Transform="CommentOut" xdt:Locator="Match(key)" key="IsTestMode" />
    <add xdt:Locator="Condition(@key='IsStaging' or @key='IsDisabled')" xdt:Transform="Remove"/>
  </appSettings >
  <rewrite>
    <rules>
        <rule xdt:Transform="RemoveAll" />
    </rules>
  </rewrite>
  <system.web>
    <machineKey xdt:Transform="Remove" />
    <machineKey xdt:Transform="InsertBefore(/configuration/system.web/membership)" validationKey="42F21E06B91BC207DAFFF53FDD44375515938CCEF1BF7F2CA70052B3B024C009B385A6E86D816ED1B0144D7BDEAEEECD9C14B20BBD02A29B1C0B1683493497D6" decryptionKey="DF97132E23EE6B974E7EC9FA3701B71BC4EBA440F1A56C744179529786C6EE58" validation="SHA1" decryption="AES" />
    <compilation>
      <assemblies xdt:Locator="Condition(count(*) = 0)" xdt:Transform="Remove"/>
    </compilation>
    <application coldstart="true" user="root" xdt:Transform="Remove" />
    <compilation xdt:Locator="Condition(count(*) = 0)" xdt:Transform="Remove" />
    <application xdt:Transform="Replace" user="guest" type="web" />
  </system.web>
  <location xdt:Transform="InsertAfter(/configuration/system.webServer)" path="sitecore">
    <system.web>
      <authorization>
        <deny users="*"/>
      </authorization>
    </system.web>
  </location>
  <system.webServer>
    <urlCompression xdt:Transform="Insert" doDynamicCompression="true" />
    <httpCookies xdt:Transform="InsertIfMissing" httpOnlyCookies="true" requireSSL="true" />
  </system.webServer>
  <compilation xdt:Transform="RemoveAttributes(debug)" />
</configuration>
"@


function Format-Xml ([xml]$xml, $indent=2) {
    $StringWriter = New-Object System.IO.StringWriter
    $XmlWriter = New-Object System.XMl.XmlTextWriter $StringWriter
    $xmlWriter.Formatting = "indented"
    $xmlWriter.Indentation = $Indent
    $xml.WriteContentTo($XmlWriter)
    $XmlWriter.Flush()
    $StringWriter.Flush()
    Write-Output $StringWriter.ToString()
}

#if(Merge-XdtTransform -Base $xmlBase -Transform $xmlTransform) {
#  Format-Xml -Xml $xmlBase
#}

function Assert-AreEqual {
  param(
      [string]$TestName,
      [object]$Expected,
      [object]$Actual
  )
  $sb = New-Object System.Text.StringBuilder
  
  if ($Expected -ne $Actual) {
      $sb.AppendLine("[$($TestName)] Failed") > $null
      $sb.AppendLine("Expected") > $null
      $sb.AppendLine((Format-Xml -Xml $Expected)) > $null
      $sb.AppendLine("Actual") > $null
      $sb.AppendLine((Format-Xml -Xml $Actual)) > $null
      Write-Host $sb.ToString() -ForegroundColor Red -BackgroundColor White
      return $false
  } else {
    $sb.AppendLine("[$($TestName)] Succeeded") > $null
    Write-Host $sb.ToString() -ForegroundColor Green
  }
  return $true
}

[xml]$replaceTestBaseXml = @"
<?xml version="1.0"?>
<configuration>
  <system.web>
    <compilation debug="true" strict="true" batch="true" />

    <application coldstart="true" user="root">
      <subtag>old content</subtag>
      <subtag2>old content</subtag2>
    </application>
    
  </system.web>
</configuration>
"@

[xml]$replaceTestTransformXml = @"
<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <system.web>
    <!-- Replaces the whole tag -->
    <application xdt:Transform="Replace" user="guest" type="web" >
      <sub>new content</sub>
    </application>
  </system.web>
</configuration>
"@

$testExpectedXml = Invoke-XdtTransform -Base $replaceTestBaseXml.OuterXml -Transform $replaceTestTransformXml.OuterXml
$testActualXml = Merge-XdtTransform -Base $replaceTestBaseXml -Transform $replaceTestTransformXml
Assert-AreEqual -TestName "Test_Replace" -Expected $testExpectedXml.OuterXml -Actual $testActualXml.OuterXml > $null

[xml]$removeTestBaseXml = @"
<?xml version="1.0"?>
<configuration>
  <appSettings>
    <add key="IsStaging" value="True" />
    <add key="Environment" value="Dev" />
    <add key="Version" value="14" />
  </appSettings>
</configuration>
"@

[xml]$removeTestTransformXml = @"
<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <appSettings>
    <add key="Environment"
         xdt:Locator="Match(key)" xdt:Transform="Remove" />
  </appSettings>
</configuration>  
"@

$testExpectedXml = Invoke-XdtTransform -Base $removeTestBaseXml.Clone().OuterXml -Transform $removeTestTransformXml.OuterXml
$testActualXml = Merge-XdtTransform -Base $removeTestBaseXml -Transform $removeTestTransformXml
Assert-AreEqual -TestName "Test_Remove" -Expected $testExpectedXml.OuterXml -Actual $testActualXml.OuterXml > $null

[xml]$removeAllTestBaseXml = @"
<?xml version="1.0"?>
<configuration>
  <appSettings>
    <add key="IsStaging" value="True" />
    <add key="Environment" value="Dev" />
    <add key="Version" value="14" />
    <object key="" />
  </appSettings>
</configuration>
"@

[xml]$removeAllTestTransformXml = @"
<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <appSettings>
    <add xdt:Transform="RemoveAll" />
  </appSettings>
</configuration>
"@

$testExpectedXml = Invoke-XdtTransform -Base $removeAllTestBaseXml.OuterXml -Transform $removeAllTestTransformXml.OuterXml
$testActualXml = Merge-XdtTransform -Base $removeAllTestBaseXml -Transform $removeAllTestTransformXml
Assert-AreEqual -TestName "Test_RemoveAll" -Expected $testExpectedXml.OuterXml -Actual $testActualXml.OuterXml > $null

[xml]$removeAllMatchTestBaseXml = @"
<?xml version="1.0"?>
<configuration>
  <appSettings>
    <add key="IsStaging" value="True" />
    <add key="Environment" value="Dev" />
    <add key="SiteStyle" value="dark" />
    <add key="Environment" value="Prod" />
    <add key="Version" value="14" />
    <add key="Environment" value="Staging" />
  </appSettings>
</configuration>
"@

[xml]$removeAllMatchTestTransformXml = @"
<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <appSettings>
    <add key="Environment"
         xdt:Locator="Match(key)" xdt:Transform="RemoveAll"/>
  </appSettings>
</configuration>

"@

$testExpectedXml = Invoke-XdtTransform -Base $removeAllMatchTestBaseXml.OuterXml -Transform $removeAllMatchTestTransformXml.OuterXml
$testActualXml = Merge-XdtTransform -Base $removeAllMatchTestBaseXml -Transform $removeAllMatchTestTransformXml
Assert-AreEqual -TestName "Test_RemoveAll_Match" -Expected $testExpectedXml.OuterXml -Actual $testActualXml.OuterXml > $null

[xml]$insertTestBaseXml = @"
<?xml version="1.0"?>
<configuration>
  <appSettings>
    <add key="IsStaging" value="True" />
    <add key="Environment" value="Dev" />
  </appSettings>
</configuration>
"@

[xml]$insertTestTransformXml = @"
<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <appSettings>
    <add key="SiteStyle" value="dark"
         xdt:Locator="Match(key)" xdt:Transform="Insert" />
  </appSettings>
</configuration>
"@

$testExpectedXml = Invoke-XdtTransform -Base $insertTestBaseXml.OuterXml -Transform $insertTestTransformXml.OuterXml
$testActualXml = Merge-XdtTransform -Base $insertTestBaseXml -Transform $insertTestTransformXml
Assert-AreEqual -TestName "Test_Insert" -Expected $testExpectedXml.OuterXml -Actual $testActualXml.OuterXml > $null

[xml]$insertIfMissingTestBaseXml = @"
<?xml version="1.0"?>
<configuration>
  <appSettings>
    <add key="IsStaging" value="True" />
    <add key="Environment" value="Dev" />
  </appSettings>
</configuration>
"@

[xml]$insertIfMissingTestTransformXml = @"
<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <appSettings>
    <!-- Will not be inserted since already existing in the source-->
    <add key="Environment" value="Prod"
         xdt:Locator="Match(key)" xdt:Transform="InsertIfMissing" />

    <add key="SiteStyle" value="Dark"
         xdt:Locator="Match(key)" xdt:Transform="InsertIfMissing" />
  </appSettings>
</configuration>
"@

$testExpectedXml = Invoke-XdtTransform -Base $insertIfMissingTestBaseXml.OuterXml -Transform $insertIfMissingTestTransformXml.OuterXml
$testActualXml = Merge-XdtTransform -Base $insertIfMissingTestBaseXml -Transform $insertIfMissingTestTransformXml
Assert-AreEqual -TestName "Test_InsertIfMissing" -Expected $testExpectedXml.OuterXml -Actual $testActualXml.OuterXml > $null

[xml]$insertBeforeTestBaseXml = @"
<?xml version="1.0"?>
<configuration>
  <appSettings>
    <add key="IsStaging" value="True" />
    <add key="Environment" value="Dev" />
    <add key="Version" value="14" />
  </appSettings>
</configuration>
"@

[xml]$insertBeforeTestTransformXml = @"
<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <appSettings>
    <add key="SiteStyle" value="dark"
         xdt:Transform="InsertBefore(/configuration/appSettings/add[@key='Environment'])"/>
  </appSettings>
</configuration>
"@

$testExpectedXml = Invoke-XdtTransform -Base $insertBeforeTestBaseXml.OuterXml -Transform $insertBeforeTestTransformXml.OuterXml
$testActualXml = Merge-XdtTransform -Base $insertBeforeTestBaseXml -Transform $insertBeforeTestTransformXml
Assert-AreEqual -TestName "Test_InsertBefore" -Expected $testExpectedXml.OuterXml -Actual $testActualXml.OuterXml > $null

[xml]$insertAfterTestBaseXml = @"
<?xml version="1.0"?>
<configuration>
  <appSettings>
    <add key="IsStaging" value="True" />
    <add key="Environment" value="Dev" />
    <add key="Version" value="14" />
  </appSettings>
</configuration>
"@

[xml]$insertAfterTestTransformXml = @"
<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <appSettings>
    <add key="SiteStyle" value="dark"
         xdt:Transform="InsertAfter(/configuration/appSettings/add[@key='Environment'])"/>
  </appSettings>
</configuration>
"@

$testExpectedXml = Invoke-XdtTransform -Base $insertAfterTestBaseXml.OuterXml -Transform $insertAfterTestTransformXml.OuterXml
$testActualXml = Merge-XdtTransform -Base $insertAfterTestBaseXml -Transform $insertAfterTestTransformXml
Assert-AreEqual -TestName "Test_InsertAfter" -Expected $testExpectedXml.OuterXml -Actual $testActualXml.OuterXml > $null

[xml]$commentOutTestBaseXml = @"
<?xml version="1.0"?>
<configuration>
  <appSettings>
    <add key="IsStaging" value="True" />
    <add key="Environment" value="Dev" />
    <add key="Version" value="14" />
  </appSettings>
</configuration>
"@

[xml]$commentOutTestTransformXml = @"
<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <appSettings>
    <add key="Environment"
         xdt:Locator="Match(key)" xdt:Transform="CommentOut" />
  </appSettings>
</configuration>
"@

$testExpectedXml = Invoke-XdtTransform -Base $commentOutTestBaseXml.OuterXml -Transform $commentOutTestTransformXml.OuterXml
$testActualXml = Merge-XdtTransform -Base $commentOutTestBaseXml -Transform $commentOutTestTransformXml
Assert-AreEqual -TestName "Test_CommentOut" -Expected $testExpectedXml.OuterXml -Actual $testActualXml.OuterXml > $null

[xml]$removeAttributesTestBaseXml = @"
<?xml version="1.0"?>
<configuration>
  <system.web>
    <application coldstart="true" user="root" />
    <compilation debug="true" strict="true" batch="true" />
  </system.web>
</configuration>
"@

[xml]$removeAttributesTestTransformXml = @"
<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <system.web>
    <!-- Remove all attributes-->
    <application xdt:Transform="RemoveAttributes"  />

    <!-- Remove specific attributes-->
    <compilation xdt:Transform="RemoveAttributes(debug,strict)" />
  </system.web>
</configuration>
"@

$testExpectedXml = Invoke-XdtTransform -Base $removeAttributesTestBaseXml.OuterXml -Transform $removeAttributesTestTransformXml.OuterXml
$testActualXml = Merge-XdtTransform -Base $removeAttributesTestBaseXml -Transform $removeAttributesTestTransformXml
Assert-AreEqual -TestName "Test_RemoveAttributes" -Expected $testExpectedXml.OuterXml -Actual $testActualXml.OuterXml > $null

[xml]$setAttributesTestBaseXml = @"
<?xml version="1.0"?>
<configuration>
  <system.web>
    <application coldstart="true" user="root" />

    <compilation debug="true" strict="true" batch="true" />
  </system.web>
</configuration>
"@

[xml]$setAttributesTestTransformXml = @"
<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <system.web>
    <!-- Add or updates all attributes-->
    <application user="guest" newAttribute="value"
    xdt:Transform="SetAttributes"  />

    <!-- Add or updates specific attributes-->
    <compilation  debug="false" strict="false" batch="false"
    xdt:Transform="SetAttributes(debug,strict)" />
  </system.web>
</configuration>
"@

$testExpectedXml = Invoke-XdtTransform -Base $setAttributesTestBaseXml.OuterXml -Transform $setAttributesTestTransformXml.OuterXml
$testActualXml = Merge-XdtTransform -Base $setAttributesTestBaseXml -Transform $setAttributesTestTransformXml
Assert-AreEqual -TestName "SetAttributes_Element" -Expected $testExpectedXml.OuterXml -Actual $testActualXml.OuterXml > $null

[xml]$setAttributesMatchTestBaseXml = @"
<?xml version="1.0"?>
<configuration>
  <appSettings>
    <add key="IsStaging" value="True" />
    <add key="Environment" value="Dev" />
    <add key="SiteStyle" value="dark" />
  </appSettings>
</configuration>
"@

[xml]$setAttributesMatchTestTransformXml = @"
<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <appSettings>
    <add key="Environment" value="Prod" 
         xdt:Locator="Match(key)" xdt:Transform="SetAttributes"/>
  </appSettings>
</configuration>
"@

$testExpectedXml = Invoke-XdtTransform -Base $setAttributesMatchTestBaseXml.OuterXml -Transform $setAttributesMatchTestTransformXml.OuterXml
$testActualXml = Merge-XdtTransform -Base $setAttributesMatchTestBaseXml -Transform $setAttributesMatchTestTransformXml
Assert-AreEqual -TestName "Test_SetAttributesMatch" -Expected $testExpectedXml.OuterXml -Actual $testActualXml.OuterXml > $null

# TODO: Add tests for Condition and XPath

Found this neat website to perform transforms in the browser. Convenient to compare with results of this script.

Xdt Playground