Cannot find an overload for "ToString" and the argument count: "1" - powershell

Question

i'm not understanding what i'm doing wrong here since i seem to do the same thing but only one works.
i have a text file with a number list that i want to process (round the values):
39.145049
40.258140
41.400803
42.540093
43.664530
and here my script:
$a = get-content "input.txt"
$b = $a -join ','
$b | % {$_.ToString("#.###")}
this results in the following error:
Cannot find an overload for "ToString" and the argument count: "1".
At D:\script.ps1:9 char:9
+ $b | % {$_.ToString("#.###")}
+ ~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [], MethodException
+ FullyQualifiedErrorId : MethodCountCouldNotFindBest
however if i take the result after joining which is:
39.145049,40.258140,41.400803,42.540093,43.664530
and create the following script:
$b = 39.145049,40.258140,41.400803,42.540093,43.664530
$b | % {$_.ToString("#.###")}
it works just fine and outputs:
39.145
40.258
41.401
42.54
43.665
where am i going wrong on this one?

Answer 1

This happens as the inputs are not of the same type.
$b1 = $a -join ','
$b2 = 39.145049,40.258140,....
$b1.GetType().Name
String
$b2.GetType().Name
Object[]
As the input in the first case is a single string, foreach loop doesn't process it as a collection of decimal values but a single string. Thus,
$b | % {$_.ToString("#.###")}
Is going to do (as pseudocode):
'39.145049,40.258140,41.400803,42.540093,43.664530'.ToString("#.###")
Whilst the array version is doing
39.145049.ToString("#.###")
40.258140.ToString("#.###")
41.400803.ToString("#.###")
Powershell's able to figure out in the later case that the values are numbers. In the first case, it's just a string and thus the automatic conversion doesn't work.
What actually works in the first case is to cast the values as nubmers. Like so,
$a | % {$([double]$_).ToString("#.###")}
39,145
40,258
41,401
42,54
43,665