Fluent Theme in .NET 10 Plan

[dipeshmsft]

In .NET 9, we laid the foundation for integrating the Fluent theme into WPF applications. While there are still some areas to improve to make it comparable to the WinUI Fluent theme, as well as achieving parity with Aero2 and other WPF themes, we are actively working to address these gaps.

We have been maintaining a project to track all the Fluent related work items via https://github.com/orgs/dotnet/projects/147 in .NET 9 and have updated it again to keep in focus the timelines of our deliverables for .NET 10. Below are few of the pending work items sorted by their priority -

1. Missing Control Styles in Windows 11 Themes #8698
2. Fixing Fluent Style Issues #10393
3. Testing for Fluent Theme #8552

The aforementioned tasks are currently in progress, and we will begin submitting PRs for these items shortly. After completing these tasks, we plan to proceed with additional important tasks for Fluent (not presented in any order), which will enable more developer scenarios :

1. Enabling Fluent as Default System Theme - Next Steps ? #9283
2. Window Backdrop Support : Mica and Acrylic #8545
3. Rounded Corners #8544

Please let us know your thoughts if we have missed anything.

/cc @dotnet/dotnet-wpf-triage @robert-abeo @MichaeIDietrich @pomianowski @riverar

[lindexi]

Something about Performance: as we know, the DynamicResource may cause the performance issues. How about the feature of ThemeResource and so on? See #8599

[robert-abeo]

I'm using a Fluent theme in a large production app with the heavy merged dictionary and DynamicResource design. It works just fine and there are no performance issues noticeable to the end user. Would love to see actual data on comments like this rather than theoretical discussions.

[dipeshmsft]

@robert-abeo, I will get some metrics when I work on the task of Enabling Fluent as Default, I have some metrics for the current approach as well, but there is nothing to compare with.

However, @lindexi is right that DynamicResources and merged dictionary approach is slower as compared to loading them as default styles.

[miloush]

At some point we will have to deal with the merged dictionary situation (which is also discussed in #9283)

[dipeshmsft]

Will pick it up soon...

[dipeshmsft]

Update : .NET 10 Preview 2

• During this time frame, our main focus was on adding Missing Fluent Styles for controls in WPF. Although there haven't been a lot of activity yet, but we are planning to get this completed by .NET 10 Preview 3.
• As an other parallel, we were working on setting up the plan for Fixing Fluent styles for controls and have started with DataGrid. We will continue working on this in Preview 3.
• We are still stuck on the testing infra for Fluent styles and weren't able to work more on that in the past month. Also planning to take this up in next month.

[aquinn39]

The plan looks good! And visually, these new themes look really nice and almost identical to WinUI, so they fit in well with Windows 11. However, just had a couple of questions about some things:

• In the new themes, a new way of defining system brushes and colours was added, such that apps which use SystemColors now break in dark mode due to these colours not updating in dark mode. Is there a plan to address this issue to make it easier to migrate existing apps and so apps can easily support the older themes as well as the newer ones?

• Is there any plan to bring the new themes to older versions of .NET and the .NET Framework via a NugetPackage or update? There are still reasons to use these frameworks such as backwards compatibility and the fact that .NET Framework comes preinstalled in Windows, allowing for lightweight exes that don't need you to install any prerequisites.

• When the theme is on by default, will it be the default no matter the OS like it is now when you opt-in to them? For example, .NET 9 is compatible with Windows 10 and above, and now, in .NET 9, if you opt-in to the new themes as per here, it uses it on Windows 10 and 11. Will this be the same behaviour in .NET 10?

• Will there be any plans to update the behaviours in WPF to be more modern, in line with the new themes, such as smooth vertical scrolling support by taking into account the delta value of mouse wheel events for example? I detailed some of these old behaviours that haven't been updated to follow the rest of the operating system here: Windows 11 Themes: Update the behaviours of various controls to be more modern/inline with WinUI #9203.