New-ADUser and SAMAccountName limit - powershell

Question

I've put together a script that creates accounts in AD and assigns a bunch of info including group membership based on input from a CSV. I use the .Substring method to create a temporary password using parts of the new hire's name. But when I try to set the SamAccountName using the .Substring method to grab the first 20 characters of the username with the same syntax as when I set the password, I'm treated to errors.
Within the same New-ADUser command, why does the below set the password as the first character of their FirstName followed by the first character of their LastName just like I would expect
-AccountPassword (convertto-securestring($Firstname.Substring(0,1)+$Lastname.Substring(0,1))
-AsPlainText -Force)
While the below does not set the SamAccountName as the first 20 characters of the Username? I get errors no matter how I've tried to do it.
-SamAccountName ($Username.Substring(0,20))
Error below:
Exception calling "Substring" with "2" argument(s): "Index and length
must refer to a location within the string.Parameter name: length"At
C:\Create_AD_Users.ps1:28 char:7+
New-ADUser `+ ~~~~~~~~~~~~ + CategoryInfo :
NotSpecified: (:) [], MethodInvocationException +
FullyQualifiedErrorId : ArgumentOutOfRangeException

Answer 1

here's a demo of what i meant by using regex to grab UP TO the limit instead of trying to grab ALL the limit.
what the code does ...
creates an array of strings to work with
just use your own when you get ready to do this for real.
sets the max number of chars to capture
iterates thru the collection of strings
shows the original string
uses regex to capture up to $MaxChars from the string
if you want details on the exact working of the regex pattern, please go to ...
regex101: build, test, and debug regex
== https://regex101.com/
the box in the upper right will explain the meaning of any regex pattern you put into the box in the upper center.
displays that result
draws a divider that is 20 chars long
the code ...
#region >>> make some strings to work on
# the below strings are 29, 30, 20, and 10 chars long
$InStuff = #'
Abcdefghi_Bcdefghij_Cdefghijk
123456789_123456789_123456789_
Qwertyuio_Asdfghjkl_
123456789_
'# -split [System.Environment]::NewLine
#endregion >>> make some strings to work on
$MaxChars = 17
foreach ($IS_Item in $InStuff)
{
$IS_Item
$IS_Item -replace "^(.{0,$MaxChars}).*$", '$1'
# yes, you can multiply strings [*grin*]
'=' * 20
}
output ...
Abcdefghi_Bcdefghij_Cdefghijk
Abcdefghi_Bcdefgh
====================
123456789_123456789_123456789_
123456789_1234567
====================
Qwertyuio_Asdfghjkl_
Qwertyuio_Asdfghj
====================
123456789_
123456789_
====================