PowerShell Comma Operator: Deferred Enumeration Explained

The PowerShell comma operator (,) is the array-construction operator; used as the unary comma it returns a single-element array that wraps the object. That wrapper changes how PowerShell’s automatic enumeration and parameter-binding see the value, so a lazy IEnumerable<T> (your permutation generator) is treated as a single object and not expanded immediately — i.e., enumeration is deferred until something explicitly iterates it. That’s why ,[Permutation]$Input keeps the generator lazy and runs in milliseconds, while returning the raw Permutation triggers full enumeration and takes seconds.

Contents

• PowerShell comma operator: what it does (unary vs binary)
• PowerShell enumeration and pipeline behavior
• Why the unary comma prevents immediate enumeration (step‑by‑step)
• Practical examples and patterns
• Performance implications and best practices
• Sources
• Conclusion

PowerShell comma operator: what it does (unary vs binary)

Short version: the comma is an array operator. In expression (unary) mode a leading comma produces an array with exactly one element; in binary mode commas separate items to make a multi-element array. The official documentation describes this behavior: the about_Operators documentation says the unary comma “creates an array with just one member.”

Examples:

# binary comma -> array of three numbers
$a = 1, 2, 3 # $a is an object[] with 3 integers

# unary comma -> single-element array whose item is the value
$b = ,5 # $b is an object[] containing 5

# unary comma wrapping a hashtable preserves the hashtable object
$h = @{ a=1; b=2 }
$h # enumerates keys by default (a, b)
,$h # returns an array whose only element is the hashtable

Because ,$value always produces an object[] wrapper, you can use it to preserve the original object identity when PowerShell would otherwise enumerate or unwrap the value. Community writeups show this is the practical trick people use to stop unwrapping of hash tables and other collections (see the hashtable example in the Evotec post linked below).

PowerShell enumeration and pipeline behavior

Why does wrapping matter? PowerShell automatically enumerates objects that implement IEnumerable when those objects cross a pipeline boundary. The about_Pipelines documentation explains the core rule: when an object is piped to a cmdlet, PowerShell enumerates any IEnumerable and sends each element down the pipeline one at a time.

That yields two practical differences:

• Piped form (Get-Thing | Cmdlet) — automatic enumeration happens as the object is passed between commands.
• Parameter form (Cmdlet -InputObject (Get-Thing)) — the expression in parentheses is evaluated first and the result is bound as the parameter value; PowerShell does not automatically enumerate the inner IEnumerable at that binding moment.

So context matters: whether a value is being sent across the pipeline or supplied as a parameter/expression influences whether enumeration happens immediately.

Why the unary comma prevents immediate enumeration (step‑by‑step)

Let’s follow your permutation example in plain terms. Your generator class implements IEnumerable<T> and produces a huge sequence lazily. Two different things can happen when your function returns that generator:

1. Without the comma

• The function emits the generator instance directly.
• At the pipeline/assignment boundary PowerShell sees an object that implements IEnumerable, so it treats it as a collection and begins enumerating its elements immediately.
• The generator therefore runs to produce every permutation (heavy, slow); the caller receives the fully enumerated results.

2. With the unary comma

• The function emits an object[] whose single element is the generator instance.
• PowerShell now sees the outer array as the item being passed. The array contains one element (the generator), and the pipeline/binder treats that element as a single object value at that stage.
• The generator’s own IEnumerable is not expanded right away; iteration on that generator only happens when some downstream code explicitly iterates its elements.
• Result: no immediate, full enumeration — execution stays fast until/if someone enumerates the permutations.

Two ways to think of it: the comma creates a one-item wrapper (a sentinel) that changes what the engine recognizes at the pipeline boundary, and PowerShell’s automatic enumeration is applied to what is being transferred at that boundary, not recursively into every nested enumerable at once. The about_Pipelines page and the StackOverflow discussion linked below both call out this exact behavior and how wrapping with , defers enumeration.

Practical examples and patterns

Your function (simplified):

function Get-Permutation {
 param(
 [Parameter(ValueFromPipeline=$true)]
 [string[]]$InputObject
 )
 # returns a Permutation object that implements IEnumerable<T>
 if ($Input) { ,[Permutation]$Input } else { ,[Permutation]$InputObject }
}

Behavior:

• Get-Permutation (1..9) without the comma will cause PowerShell to enumerate the returned generator while building the assignment result — expensive.
• Get-Permutation (1..9) with the leading comma returns a one-element array containing the generator; assignment completes quickly because the heavy work is deferred.

Other useful patterns:

• Preserve a hashtable instance instead of enumerating keys: ,$hashtable (Evotec blog shows this practical trick: Evotec example).
• Explicit, clear alternative in PowerShell 7+: Write-Output -NoEnumerate $obj (this prevents enumeration on output in a readable way; see the StackOverflow discussion that compares , to -NoEnumerate: StackOverflow example).
• Force eager enumeration when you want it: wrap an expression in @(...) or call an explicit conversion/ToArray on the enumerable — that materializes the entire sequence.

Small caution: wrapping with a comma changes the shape you return. Downstream code that expects a stream of items will see a single element (the wrapped object). That can break callers which expect automatic streaming behavior.

Performance implications and best practices

Why you saw ~8 seconds vs ~15 ms: without the comma PowerShell was forced to iterate the generator to produce pipeline output (eager enumeration). With the comma you returned a single object (the lazy generator) and avoided doing the expensive work until someone actually iterated it. That difference is about lazy vs eager execution — and it’s the reason for the speed change.

Guidance:

• Use the unary comma when you want to return or pass a lazy IEnumerable as a single object (preserve identity; defer work).
• Don’t use it if you want the function to stream items downstream immediately — omit the comma to let automatic enumeration produce items.
• If readability matters, prefer explicit alternatives where available: Write-Output -NoEnumerate (PowerShell 7+) or document the function’s contract clearly so callers know whether they receive a generator or actual items.
• When a caller wants all items realized (and you want to force eager evaluation), explicitly materialize the sequence (e.g., .ToArray(), @( ... )) rather than relying on incidental pipeline behavior.

A final tip: think in terms of data shape. ,$x changes the data shape you hand across boundaries from “this enumerable” to “an array containing this enumerable”. That small shape change is what changes enumeration and performance.

Sources

• about_Operators - PowerShell | Microsoft Learn
• about_Pipelines - PowerShell | Microsoft Learn
• PowerShell - Few tricks about HashTables and Arrays I wish I knew when I started (Evotec)
• What exactly does the Comma operator “,” do (StackOverflow)
• Why is a leading comma required when creating an array? (StackOverflow)
• Putting an array on the pipeline with the unary comma operator (Fleex’s Lab)

Conclusion

The PowerShell comma operator is more than a syntactic oddity: the unary comma produces a single-element array wrapper that changes how the engine applies automatic enumeration. Wrapping a lazy IEnumerable<T> with , makes PowerShell treat it as one value (deferring work until iteration), which is exactly why your permutation generator stayed lazy and became fast when you used ,[Permutation]$Input. Use the comma (or explicit -NoEnumerate) when you need to preserve an object or defer expensive enumeration — and omit it when you want automatic streaming of elements.