How to use ThemeResource from code?

[StevoSM]

In XAML, I can style a Line with Stroke="{ThemeResource SystemControlForegroundBaseMediumLowBrush}". If I am composing a Line in C++ code, how would I accomplish the same thing?

As in line.Stroke(-- what goes here? --);

[castorix]

You can read Application.Current.Resources

In C# :

  var brush = (Brush)Application.Current.Resources["SystemControlForegroundBaseMediumLowBrush"];
  line.Stroke = brush;

I cannot test in C++, but if I ask the translation to ChatGPT, it gives me (not always 100% correct...) :

  auto brush = unbox_value<Brush>(Application::Current().Resources().Lookup(winrt::box_value(L"SystemControlForegroundBaseMediumLowBrush")));
  line.Stroke(brush);

[Zakariathr22]

I think that the provided solution uses StaticResource, not ThemeResource. This means it won't dynamically respond to theme changes (like switching between light and dark mode).

I raised a proposal in the Microsoft UI XAML GitHub repository addressing the need for direct color changes using ThemeResource in C# for WinUI3. In the meantime, I’ve found a workaround that works in C# by leveraging Border controls to hold theme brushes and applying them in the Loaded and ActualThemeChanged events (you can find the workaround here).

Notes:
- I am not sure if this is the best practice for C#, but at least it is doing the job.
- The workaround still needs a translation to C++.

[NicholasChrzan]

Can't you just create a style in your resource dictionary that uses theme resources then apply that in code behind?

[Zakariathr22]

@NicholasChrzan
Yes, this is actually an easier way, I don't know how I didn't think of it before 😂.
I edited the workaround.
Thanks for your help!

[StevoSM]

Thanks everyone for your contributions. Given the above, I ended up with a solution that leaves the stroke defined in XAML and loaded that as a custom control. The actual controls (implemented in more than one control) are more than just the Line, and ended up being a more direct and efficient solution than doing much of it in code as I initially planned.