dotnet/wpf

TextFormatter.FormatLine has O(n^2) performance when called multiple times

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#6,304 opened on Mar 23, 2022

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Performancehelp wanted

Description

  • .NET Core Version: .net 6
  • Windows version: 11
  • Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes
  • Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? no

Actual behavior:

TextFormatter.FormatLine has O(n^2) performance

Expected behavior:

TextFormatter.FormatLine exhibits linear performance

Minimal repro: WpfFontRepro.zip

Remarks

Disposing the formatted lines or creating new TextFormatter instances prevents the O(n^2) behavior.

Contributor guide

Tech stack
csharp
Domain
desktopperformance
Issue type
performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C#WPFPerformance profiling
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
Investigate the TextFormatter.FormatLine method in the WPF codebase (likely in TextFormatter.cs). The issue suggests that creating new TextFormatter instances or disposing formatted lines avoids the O(n^2) behavior, indicating improper caching or resource management. Review how FormattedLine objects are stored and disposed, and consider implementing a fix that ensures linear time complexity across multiple calls. The minimal repro provided in the issue can be used to validate the fix.