Why ResourceDictionary.ThemeDictionaries keys are not being found?

[fitdev]

I was following the explanation in the official docs and defined:

  <Application.Resources>
    <ResourceDictionary>
      <ResourceDictionary.ThemeDictionaries>
          <ResourceDictionary x:Key="Light">
            <SolidColorBrush x:Key="TextContrastBack_Brush">DimGray</SolidColorBrush>
          </ResourceDictionary>
          <ResourceDictionary x:Key="Dark">
            <SolidColorBrush x:Key="TextContrastBack_Brush">White</SolidColorBrush>
          </ResourceDictionary>
        </ResourceDictionary.ThemeDictionaries>
    </ResourceDictionary>
  </Application.Resources>

in my App.axaml. Then I use the resource like so:

  <TextBox>
    <ToolTip.Tip>
      <StackPanel>
        <StackPanel.Styles>
          <Style Selector="Run">
            <Setter Property="Background" Value="{StaticResource TextContrastBack_Brush}" />
        </StackPanel.Styles>
      </StackPanel>
    </ToolTip.Tip>
  </TextBox>

However I get an exception at runtime saying that key TextContrastBack_Brush was not found despite my theme being of either Dark or Light variant.

However if I simply define the resource with the same key outside of <ResourceDictionary.ThemeDictionaries> then everything works.

Am I missing something?

[maxkatz6]

Statically, there is no Dark or Light theme during app initialization and styles processing. If your theme dictionary had "Default" key - it would be used, though.

But better practice is to use DynamicResource for anything like this, what can be changed in runtime.

[fitdev]

I see, that makes sense since themes are dynamic. I have submitted a tiny PR to update the docs on the subject.