Vista - Accessing Collection type items using Negative Index Numbers

It doesn't seem to be possible to access .NET collection type's item with
negative index numbers
but if i use "+=" operator, then PowerShell converts the object types to
"System.Object[]" so
negative indexing works.

Is this by design because .Net collections do not take negative index
numbers?(for consistency reasons)

Another question: Why does "+=" changes the type of the object instance
instead of using underlying "Add" method?


Case 1) Non-Generic .NET Types(Negative Indexes dont work)
[^_^]PS[24]>$m = [RegEx]::Matches('abc', 'abc')
[^_^]PS[25]>$m.Count
1
[^_^]PS[26]>$m[-1]
[^_^]PS[27]>$m.GetType().FullName
System.Text.RegularExpressions.MatchCollection
[^_^]PS[28]>$b = @("abc")
[^_^]PS[29]>$b[-1]
abc
[^_^]PS[30]>$b.GetType().FullName
System.Object[]
[^_^]PS[39]>$arr = New-Object System.Collections.ArrayList
[^_^]PS[40]>[void] $arr.Add("abc")
[^_^]PS[41]>[void] $arr.Add("def")
[^_^]PS[42]>$arr.GetType().FullName
System.Collections.ArrayList
[^_^]PS[43]>$arr[-1]
[^_^]PS[44]>rv arr
1.1) Using "+=" operator instead of "Add" method("+=" changed type of $arr
so negative index works)
[^_^]PS[45]>$arr = New-Object System.Collections.ArrayList
[^_^]PS[46]>$arr += 'abc'
[^_^]PS[47]>$arr += 'def'
[^_^]PS[48]>$arr.GetType().FullName
System.Object[]
[^_^]PS[49]>$arr[-1]
def

Case #2) Generic Object(Negative index does not work)
[^_^]PS[50]>$g = New-Object
System.Collections.ObjectModel.Collection``1"[[System.String]]"
[^_^]PS[51]>$g.Add("abc")
[^_^]PS[52]>$g.Add("def")
[^_^]PS[53]>$g.Count
2
[^_^]PS[54]>$g[-1]
[^_^]PS[55]>$g.item[-1]
Unable to index into an object of type
System.Management.Automation.PSParameterizedProperty.
At line:1 char:9
+ $g.item[- <<<< 1]
[^_^]PS[56]>rv g
2.1) Using "+=" to add an item changes object type from "Collection" to
"System.Object[]"
(Negative indexing works)
[^_^]PS[57]>$g = New-Object
System.Collections.ObjectModel.Collection``1"[[System.String]]"
[^_^]PS[58]>$g += "abc"
[^_^]PS[59]>$g += "def"
[^_^]PS[60]>$g.Count
2
[^_^]PS[61]>$g[-1]
def
[^_^]PS[62]>$g.GetType().FullName
System.Object[]

"DBMwS" <DBMwS@discussions.microsoft.com> wrote in message
news:4A1D1590-A301-43D6-A836-BCBBB703F5A5@microsoft.com...
> It doesn't seem to be possible to access .NET collection type's item with
> negative index numbers
> but if i use "+=" operator, then PowerShell converts the object types to
> "System.Object[]" so
> negative indexing works.
>
> Is this by design because .Net collections do not take negative index
> numbers?(for consistency reasons)
I think that is a safe bet. Even VB changed the way it accesses arrays and
collections to be zero based. Negative indexes are "right out".

> Another question: Why does "+=" changes the type of the object instance
> instead of using underlying "Add" method?

My guess is that the ArrayList occupies index 0 and the string 'abc'
occupies index 1 and 'def' occupies index 2. When PowerShell sees += and
the expression on the LHS doesn't represent an event then PoSH assumes you
are appending to an array. Since the types of the first two elements differ
then it can only hold the items in an object[].

--
Keith

Thanks for the response there, Keith.

PowerShell is a *power*ful shell as well as a language
but still "scripting" side of nature might be a bit degraded
had it not supported negative indexing at all..

I tend not to use negative indexes when i write long scripts.
But I personally think that, not being able to use negative index number to
retrieve last collection item would be too much to type sometimes in an *
interactive * mode

Negative indexes not working on a collection is just a bug. Please post on
the connect site.

--
Bruce Payette [MSFT]
Windows PowerShell Technical Lead
Microsoft Corporation
This posting is provided "AS IS" with no warranties, and confers no rights.
Visit the Windows PowerShell Team blog at:
http://blogs.msdn.com/PowerShell
Visit the Windows PowerShell ScriptCenter at:
http://www.microsoft.com/technet/scr.../hubs/msh.mspx


"DBMwS" <DBMwS@discussions.microsoft.com> wrote in message
news:4A1D1590-A301-43D6-A836-BCBBB703F5A5@microsoft.com...
> It doesn't seem to be possible to access .NET collection type's item with
> negative index numbers
> but if i use "+=" operator, then PowerShell converts the object types to
> "System.Object[]" so
> negative indexing works.
>
> Is this by design because .Net collections do not take negative index
> numbers?(for consistency reasons)
>
> Another question: Why does "+=" changes the type of the object instance
> instead of using underlying "Add" method?
>
>
> Case 1) Non-Generic .NET Types(Negative Indexes dont work)
> [^_^]PS[24]>$m = [RegEx]::Matches('abc', 'abc')
> [^_^]PS[25]>$m.Count
> 1
> [^_^]PS[26]>$m[-1]
> [^_^]PS[27]>$m.GetType().FullName
> System.Text.RegularExpressions.MatchCollection
> [^_^]PS[28]>$b = @("abc")
> [^_^]PS[29]>$b[-1]
> abc
> [^_^]PS[30]>$b.GetType().FullName
> System.Object[]
> [^_^]PS[39]>$arr = New-Object System.Collections.ArrayList
> [^_^]PS[40]>[void] $arr.Add("abc")
> [^_^]PS[41]>[void] $arr.Add("def")
> [^_^]PS[42]>$arr.GetType().FullName
> System.Collections.ArrayList
> [^_^]PS[43]>$arr[-1]
> [^_^]PS[44]>rv arr
> 1.1) Using "+=" operator instead of "Add" method("+=" changed type of $arr
> so negative index works)
> [^_^]PS[45]>$arr = New-Object System.Collections.ArrayList
> [^_^]PS[46]>$arr += 'abc'
> [^_^]PS[47]>$arr += 'def'
> [^_^]PS[48]>$arr.GetType().FullName
> System.Object[]
> [^_^]PS[49]>$arr[-1]
> def
>
> Case #2) Generic Object(Negative index does not work)
> [^_^]PS[50]>$g = New-Object
> System.Collections.ObjectModel.Collection``1"[[System.String]]"
> [^_^]PS[51]>$g.Add("abc")
> [^_^]PS[52]>$g.Add("def")
> [^_^]PS[53]>$g.Count
> 2
> [^_^]PS[54]>$g[-1]
> [^_^]PS[55]>$g.item[-1]
> Unable to index into an object of type
> System.Management.Automation.PSParameterizedProperty.
> At line:1 char:9
> + $g.item[- <<<< 1]
> [^_^]PS[56]>rv g
> 2.1) Using "+=" to add an item changes object type from "Collection" to
> "System.Object[]"
> (Negative indexing works)
> [^_^]PS[57]>$g = New-Object
> System.Collections.ObjectModel.Collection``1"[[System.String]]"
> [^_^]PS[58]>$g += "abc"
> [^_^]PS[59]>$g += "def"
> [^_^]PS[60]>$g.Count
> 2
> [^_^]PS[61]>$g[-1]
> def
> [^_^]PS[62]>$g.GetType().FullName
> System.Object[]
>