CODEWITHSUNDEEP
powershell
KFL
KFL

Reputation: 17870

A gotcha of using .NET collections (ArrayList, List,..) in PowerShell

I just realized this and I've learned it the hard way, so I want to share. Consider you have the following list of strings:

>$list = New-Object -TypeName System.Collections.Generic.List[string]
>$list.Add("x")
>$list.Add("yy")
>$list.Add("zzz")
>$list
x
yy
zzz

Now if you want to get the number of items in the list, you would access the .Count property, and it gives you "3" as expected.

>$list.Count
3

However, sometimes you would make mistake by accessing .Length instead of .Count. In other languages, like Python or C#, you would get an error saying this property is not available. But in PowerShell, it turns out that, if it's not found on that object, it iterates through the contained objects and access the property on them. So you end up having a list of Lengthes! Surprise!!

>$list.Length
1
2
3

You can even invoke methods!

>$list.ToUpper()
X
YY
ZZZ

Note that, as I've tried, this "feature" also works for System.Collections.ArrayList. But it does not work for PowerShell's builtin array type.

What is this "feature" called? And why is it designed such way? It's very surprising and error-prone.

Upvotes: 18

Views: 18837

Answers (1)

KFL
KFL

Reputation: 17870

Keith Hill mentioned in the comment that this is a new feature called Member Enumeration in V3.

Refer to "New V3 Language Features" on devblogs.microsoft.com.

Upvotes: 6

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