title: A gotcha of using .NET collections (ArrayList, List,..) in PowerShell
body: <p>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:</p>
  
  <pre><code>&gt;$list = New-Object -TypeName System.Collections.Generic.List[string]
  &gt;$list.Add("x")
  &gt;$list.Add("yy")
  &gt;$list.Add("zzz")
  &gt;$list
  x
  yy
  zzz
  </code></pre>
  
  <p>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.</p>
  
  <pre><code>&gt;$list.Count
  3
  </code></pre>
  
  <p>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!!</p>
  
  <pre><code>&gt;$list.Length
  1
  2
  3
  </code></pre>
  
  <p>You can even invoke methods!</p>
  
  <pre><code>&gt;$list.ToUpper()
  X
  YY
  ZZZ
  </code></pre>
  
  <p>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.</p>
  
  <p>What is this "feature" called? And why is it designed such way? It's very surprising and error-prone.</p>