The Surprising Working of TrimEnd

Time to read: ~ 2 minutes

Words: 397

A couple of days ago, I was running some unit tests across a piece of PowerShell code for work and a test was failing where I didn’t expect it to.

After realising that the issue was with the workings of TrimEnd and my thoughts on how TrimEnd works (versus how it actually works), I wondered if it was just me being a bit stupid.

So I put a poll up on Twitter, and I’m not alone! 60% of the people answering the poll had the wrong idea as well.

Let’s have some code to show what we mean.


Incorrect Ideas

The vast majority of code that I have seen out in the wild has strings as the inner portion of TrimEnd


The code works how I thought that it would, removing the “sqlserver” portion of the string at the end. Now, let’s try it again and remove the underscore as well.


See! Where has my “s” and “e” gone?!

Let’s look at the overload definitions for TrimEnd by running the code without the brackets after the method.


No overload definition takes a string; they either take a char or an array of chars. Is that what’s happening here?

# Takes an array of chars
'Shanes_sqlserver'.TrimEnd('_', 's', 'q', 'l', 'e', 'r', 'v')

# Turns a string into an array of chars

# Order doesn't matter either

A New Way of Thinking

So TrimEnd takes the characters that we provide inside the method and removes them from the end until it reaches the first non-matching character.

This example explains why our first example, with TrimEnd('sqlserver'), removes everything up to the underscore.

# -----^ First non-matching character (_)

However, when we include the underscore, the first non-matching character shuffles back.

# --^ First non-matching character (n)

Initial Problem

Now that we have a new understanding of how TrimEnd works, how can we remove the “_sqlserver” part of the string?

Split it in two.

# -----^  First non-matching character (_)
# ----^  First non-matching character after first TrimEnd (s)

This rewrite works for us since we have a defined character that acts as a stop-gap. If that stop-gap isn’t possible, then -replace may be our best option.

'Shanes_sqlserver' -replace '_sqlserver'

Always good to get a better understanding of PowerShell. If my tests catch more of these misunderstandings that I can learn from, then I’m OK with that!