ArrayList not returning expected result - powershell

Question

Why does this code not work?
$method = {
[System.Collections.ArrayList]$array;
for ($i=0; $i -le 50; $i++) { $array += $i }
}
Executing the scriptblock with:
&$method
Shows on the console:
1, 2, 3, 4, 5, 6
when it should print 50 numbers?

Answer 1

The Add() method of the ArrayList type emits the index at which a new item was inserted. Cast the expression to [void], pipe it to Out-Null or assign it to $null to supress this output:
[void]$array.Add($i)
# or
$array.Add($i) |Out-Null
# or
$null = $array.Add($i)
Avoid piping to Out-Null if you do this many times, casting or assigning is much faster than piping.

Answer 2

The code you posted shouldn't generate any output at all, unless you already have a variable $array defined in the parent scope. The statement
[System.Collections.ArrayList]$array
casts the value of the variable $array to the type ArrayList and echoes it. If the variable has a value in the parent scope the statement will output the value as an array list, otherwise the output will be null. The subsequent loop will not use that variable, but instead increment a new (local) variable $array. You can verify that by placing a statement $array.GetType().FullName at the end of the scriptblock. You'll get System.Int32, not System.Collections.ArrayList (or System.Object[]) as you might expect.
If you want to instantiate an ArrayList object, add numbers to it, and output that list at the end of the scriptblock, you need to change your code to something like this:
$method = {
[Collections.ArrayList]$array = #()
for ($i=0; $i -le 50; $i++) { $array += $i }
$array
}
Note the assignment operation in the first statement.
Demonstration:
PS C:\> $sb = { [Collections.ArrayList]$a; 0..50 | % { $a += $_ }; $a.GetType().FullName; $a }
PS C:\> &$sb
System.Int32
1275
PS C:\> $sb = { [Collections.ArrayList]$a = #(); 0..50 | % { $a += $_ }; $a.GetType().FullName; $a }
PS C:\> &$sb
System.Collections.ArrayList
0
1
...
49
50