Behavior of \global when used with xifthen package

Question

I'm trying to calculate the sum of some numbers in my document. I want to allow non-integer values, so I'm using lengths, which works just fine.

In my document it's often necessary to specify the values for adding to the sum from inside an environment. I learned that the \global command does the trick.

However, I encountered the problem that the \global command seems to not work properly when the xifthen package is loaded, either directly or by another package.

Here's an example:

\documentclass{article}
\usepackage{xifthen}
\makeatletter
\newcommand*{\getlength}[1]{\strip@pt#1}
\makeatother

\begin{document}
\newlength{\mylength}
\setlength{\mylength}{0pt}
\getlength{\mylength}
{
    \addtolength{\mylength}{0.5pt}
    \getlength{\mylength}
}
\getlength{\mylength}
{
    \global\addtolength{\mylength}{0.5pt}
    \getlength{\mylength}
}
\getlength{\mylength}
\end{document}

The output is:

0 0.5 0 0.5 0

Removing the xifthen package produces the expected output:

0 0.5 0 0.5 0.5

Is there any workaround to produce the second output, without having to ensure that no package which uses xifthen is loaded?