Bad Font-Measuring Performance under Windows 8.1

Question

Why does the following little loop (in c#) perform so bad using current version of WPF (.NET 4.5.2) if i change the font-family from "Segoe UI" to "Arial" (or something else...tried "Times New Roman" and "Courier New"
- Same problem)?
var tb = new TextBlock {Text = "Testtext", FontFamily = new FontFamily("Arial")};
for(int i = 0; i < 100000; i++)
tb.InvalidateMeasure();
tb.Measure(new Size(double.MaxValue, double.MaxValue));
With font-family set to Arial this block of code takes about 7.6s on my machine. Font-family set to "Segoe UI" takes about 1.9s. Why do (most) other fonts than Segoe UI perform so bad during measurement? Is there any tweak around, that does avoid
this enormous loss in performance?
As i found found out, "Tahoma", "Lucida Sans" and "Microsoft Sans Serif" are being measured really fast, also. Is this some "System-internal font"-thing?
Yes, i know, this is really constructed and broken down to a minimal reproducable example. The whole component is a custom datagrid with complex ui- and data-virtualization - much too big to post here. If i set the font of my
grid to, let's say "Arial", the scrolling-performance gets really bad. Using Visual Studio's Profiler i tracked the problem down to the measurement of my single grid-cells, which basically measure single textblocks and so i wrote the little test-code
above. Please keep in mind: My problem is NOT, that the code above is slow. (I know, this loop is totally senseless...it's for demonstration purposes only) My problem IS, that changing the font impacts measurement in such an enormous amount...
What is the difference between fonts like "Segoe UI", "Tahoma", "Lucida Sans" or "Microsoft Sans Serif" and fonts like "Arial", "Times New Roman" or "Courier New" that causes this huge
impact in measurement?
Btw.: This problem not only arises within my own grid component, it can also be reproduced with WPF's internal datagrid. Scrolling performance degrades dramatically when using "Arial" as the font-family.

Answer 1

Hi Max
I realise that WPF doesn't use Win32, but I suspected something similar (i.e. the font is being loaded and unloaded everytime a measure takes place rather than using a cached font). I just looked up TextBlock in ILSpy and can see a MeasureOverride method
which does quite a lot of work with the font before using it to measure text and then discards all of this information when it's finished. That's obviously where the bottleneck is. I don't program in WPF so I have no idea how to fix it.
protected sealed override Size MeasureOverride(Size constraint)
this.VerifyReentrancy();
this._textBlockCache = null;
this.EnsureTextBlockCache();
Follow the path of EnsureTextBlockCache() to see how much information is being processed and then dumped every time a measure operation is processed.
Mick Doherty
http://dotnetrix.co.uk
http://glassui.codeplex.com