This question addresses the following scenario:
Can custom tab-completion for a given command dynamically determine completions based on the value previously passed to another parameter on the same command line, using either a parameter-level [ArgumentCompleter()] attribute or the Register-ArgumentCompleter cmdlet?
If so, what are the limitations of this approach?
Example scenario:
A hypothetical Get-Property command has an -Object parameter that accepts an object of any type, and a -Property parameter that accepts the name of a property whose value to extract from the object.
Now, in the course of typing a Get-Property call, if a value is already specified for -Object, tab-completing -Property should cycle through the names of the specified object's (public) properties.
$obj = [pscustomobject] @{ foo = 1; bar = 2; baz = 3 }
Get-Property -Object $obj -Property # <- pressing <tab> here should cycle
# through 'foo', 'bar', 'baz'
@mklement0, regarding first limitation stated in your answer
The custom-completion script block (
{ ... }) invoked by PowerShell fundamentally only sees values specified via parameters, not via the pipeline.
I struggled with this, and after some stubbornness I got a working solution.
At least good enough for my tooling, and I hope it can make life easier for many others out there.
This solution has been verified to work with PowerShell versions 5.1 and 7.1.2.
Here I made use of $cmdAst (called $commandAst in the docs), which contains information about the pipeline. With this we can get to know the previous pipeline element and even differentiate between it containing only a variable or a command. Yes, A COMMAND, which with help of Get-Command and the command's OutputType() member method, we can get (suggested) property names for such as well!
PS> $obj = [pscustomobject] @{ foo = 1; bar = 2; baz = 3 }
PS> $obj | Get-Property -Property # <tab>: bar, baz, foo
PS> "la", "na", "le" | Select-String "a" | Get-Property -Property # <tab>: Chars, Context, Filename, ...
PS> 2,5,2,2,6,3 | group | Get-Property -Property # <tab>: Count, Values, Group, ...
Note that apart from now using $cmdAst, I also added [Parameter(ValueFromPipeline=$true)] so we actually pick the object, and PROCESS {$Object.$Property} so that one can test and see the code actually working.
param(
[Parameter(ValueFromPipeline=$true)]
[object] $Object,
[ArgumentCompleter({
param($cmdName, $paramName, $wordToComplete, $cmdAst, $preBoundParameters)
# Find out if we have pipeline input.
$pipelineElements = $cmdAst.Parent.PipelineElements
$thisPipelineElementAsString = $cmdAst.Extent.Text
$thisPipelinePosition = [array]::IndexOf($pipelineElements.Extent.Text, $thisPipelineElementAsString)
$hasPipelineInput = $thisPipelinePosition -ne 0
$possibleArguments = @()
if ($hasPipelineInput) {
# If we are in a pipeline, find out if the previous pipeline element is a variable or a command.
$previousPipelineElement = $pipelineElements[$thisPipelinePosition - 1]
$pipelineInputVariable = $previousPipelineElement.Expression.VariablePath.UserPath
if (-not [string]::IsNullOrEmpty($pipelineInputVariable)) {
# If previous pipeline element is a variable, get the object.
# Note that it can be a non-existent variable. In such case we simply get nothing.
$detectedInputObject = Get-Variable |
Where-Object {$_.Name -eq $pipelineInputVariable} |
ForEach-Object Value
} else {
$pipelineInputCommand = $previousPipelineElement.CommandElements[0].Value
if (-not [string]::IsNullOrEmpty($pipelineInputCommand)) {
# If previous pipeline element is a command, check if it exists as a command.
$possibleArguments += Get-Command -CommandType All |
Where-Object Name -Match "^$pipelineInputCommand$" |
# Collect properties for each documented output type.
ForEach-Object {$_.OutputType.Type} | ForEach-Object GetProperties |
# Group properties by Name to get unique ones, and sort them by
# the most frequent Name first. The sorting is a perk.
# A command can have multiple output types. If so, we might now
# have multiple properties with identical Name.
Group-Object Name -NoElement | Sort-Object Count -Descending |
ForEach-Object Name
}
}
} elseif ($preBoundParameters.ContainsKey("Object")) {
# If not in pipeline, but object has been given, get the object.
$detectedInputObject = $preBoundParameters["Object"]
}
if ($null -ne $detectedInputObject) {
# The input object might be an array of objects, if so, select the first one.
# We (at least I) are not interested in array properties, but the object element's properties.
if ($detectedInputObject -is [array]) {
$sampleInputObject = $detectedInputObject[0]
} else {
$sampleInputObject = $detectedInputObject
}
# Collect property names.
$possibleArguments += $sampleInputObject | Get-Member -MemberType Properties | ForEach-Object Name
}
# Refering to about_Functions_Argument_Completion documentation.
# The ArgumentCompleter script block must unroll the values using the pipeline,
# such as ForEach-Object, Where-Object, or another suitable method.
# Returning an array of values causes PowerShell to treat the entire array as one tab completion value.
$possibleArguments | Where-Object {$_ -like "$wordToComplete*"}
})]
[string] $Property
)
PROCESS {$Object.$Property}