PowerShell Iterators
When doing an iteration with a ForEach loop, internally, at every loop the get_Current() is called.
See this statement:
foreach (V v in x) {scriptblock}
Translate to this:
E e = X.GetEnumerator();
try {
T o;
while (e.MoveNext()) {
o = e.Current;
{scriptblock}
}
}
finally {
… // Dispose e
}
Order Execution
Suppose you want to loop in a collection, but need to initialize some data before the loop, it can be done in GetEnumerator()
Let’s create a small collection:
Class T : Collections.IEnumerator{
[System.Collections.IEnumerator]GetEnumerator() {
$a = @(1,2,3)
$a | % { Write-Host "__T::IEnumerator::GetEnumerator $_" }
# Add Logic here...
return $a.GetEnumerator()
}
[Int]$count = 0
[Object]Clone(){ return [T]@{count=$this.count} }
[void]Reset(){ $this.count = 0 }
[bool]MoveNext(){ return ++$this.count -lt 4 }
[Object]get_Current(){
Write-Host "T::IEnumerator get_Current $($this.count)"
return $this.count
}
}
If you put a write-host in get_Current() you will get an ‘interlaced’ output.
ForEach($i in [T]::new()){
Write-Host "ForEach Script Loop #$i"
}
Outputs this:
T::IEnumerator get_Current 1
ForEach Script Loop #1
T::IEnumerator get_Current 2
ForEach Script Loop #2
T::IEnumerator get_Current 3
ForEach Script Loop #3
Whereas looping using GetEnumerator() :
ForEach($i in [T]::new().GetEnumerator()){
Write-Host "ForEach Script Loop #$i"
}
Outputs this:
__T::IEnumerator::GetEnumerator 1
__T::IEnumerator::GetEnumerator 2
__T::IEnumerator::GetEnumerator 3
ForEach Script Loop #1
ForEach Script Loop #2
ForEach Script Loop #3
So for data initialization before looping in said data, it can be done in GetEnumerator() . All pre-loop evaluation is be done in GetEnumerator() . If you add dynamic custom logic, do it there.
Sample Custom Collection
class __FooBar : System.Collections.IEnumerator {
[int]$_count = 0
[int]$_size = 0
[object]get_Current() {
return $this._count
}
[bool]MoveNext(){
$this._count++;
return ($this._count -lt $this._size);
}
[void] Log([string]$str) {
$s = "[__FooBar 0x{0:x}]" -f [int]$this.GetHashCode()
Write-Host "$s" -f DarkRed -n
Write-Host "::$str" -f DarkYellow
}
[void] Reset() {
$this._size = 0
$this._count = 0
}
[void] Dispose() {
# Do nothing
}
__FooBar(){
#$this.Log("__FooBar()")
}
}
class FooBar : __FooBar, System.Collections.Generic.IEnumerator[int] {
FooBar([int]$size){
$this._size = $size
}
[int]get_Current() {
return $this._count
}
}
Looping with
function Test-foreach{
$StopWatch = [system.diagnostics.stopwatch]::StartNew()
ForEach($i in [FooBar]::new(500)){
}
$StopWatch.Stop()
return $StopWatch.Elapsed.TotalMilliseconds
}
function Test-foreachobject{
$StopWatch = [system.diagnostics.stopwatch]::StartNew()
[FooBar]::new(500) | foreach-object {
}
$StopWatch.Stop()
return $StopWatch.Elapsed.TotalMilliseconds
}
Write-Host "==================" -f DarkRed
Write-Host " Test-foreach " -f DarkYellow
Write-Host "==================" -f DarkRed
$v1=Test-foreach
$v2=Test-foreachobject
"Test-foreach `t{1:n5}ms`nTest-foreachobject`t{2:n5}ms`n------`nDiff fe/feo`t`t{3:n5}ms" -f $v0,$v1, $v2, ($v1-$v2)
Test-foreach 15.60630ms
Test-foreachobject 8.18050ms
------
Diff fe/feo 7.42580ms
This Collection inheriting from IEnumerator and IEnumerable
class __FooBar : System.Collections.IEnumerator,System.Collections.IEnumerable {
[int]$_count = 0
[int]$_size = 0
[System.Collections.Generic.List[int]]$_list = [System.Collections.Generic.List[int]]::new()
[object]get_Current() {
return $this._count
}
[System.Collections.IEnumerator]GetEnumerator() {
return $this._list.GetEnumerator()
}
[bool]MoveNext(){
$this._count++;
return ($this._count -lt $this._size);
}
[void] Log([string]$str) {
$s = "[__FooBar 0x{0:x}]" -f [int]$this.GetHashCode()
Write-Host "$s" -f DarkRed -n
Write-Host "::$str" -f DarkYellow
}
[void] Reset() {
$this._count = 0
}
[void] Init([int]$size) {
$this._size = $size
0..$this._size | % { $this._list.Add($_) }
}
[void] Dispose() {
# Do nothing
}
__FooBar(){
}
}
class FooBar : __FooBar, System.Collections.Generic.IEnumerator[int] {
FooBar([int]$size){ $_size = $size ; Write-Host "CONSTRUCTOR FooBar" ; $this.Init($size)}
[int]get_Current() {
return $this._count
}
}
Iterate with :
$toBeIterated = [FooBar]::new(500)
##################################
# Iteration Loop Model 1
[Collections.IEnumerable]$iEnumerableOftoBeIterated = $toBeIterated
foreach($element in $iEnumerableOftoBeIterated){
Write-Host "Current value is $element"
}
$toBeIterated.Reset()
##################################
# Iteration Loop Model 2
[Collections.IEnumerator] $iEnumeratorOftoBeIterated = $toBeIterated.GetEnumerator();
while ($iEnumeratorOftoBeIterated.MoveNext()){
Write-Host "Current value is $($iEnumeratorOftoBeIterated.Current)"
}
