Resources

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Resources
Often, styles and controls will need to share resources such as (but not limited to) brushes and colors. You can put such resources in the Resources dictionary which is available on every style and control and then refer to these resources elsewhere.
Declaring resources
If a resource is to be available to your entire application, you can define it in App.xaml/App.axaml:
<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="MyApp.App">
  <Application.Resources>
    <SolidColorBrush x:Key="Warning">Yellow</SolidColorBrush>
  </Application.Resources>
</Application>
Alternatively you can declare resources on a Window or UserControl: the resource will be available to the Window/UserControl and its children:
<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="MyApp.MyUserControl">
  <UserControl.Resources>
    <SolidColorBrush x:Key="Warning">Yellow</SolidColorBrush>
  </UserControl.Resources>
</UserControl>
Or in fact any control at all:
<Window xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="MyApp.MainWindow">
  <StackPanel>
    <StackPanel.Resources>
      <SolidColorBrush x:Key="Warning">Yellow</SolidColorBrush>
    </StackPanel.Resources>
  </StackPanel>
</Window>
You can also declare resources on styles:
<Style Selector="TextBlock.warn">
  <Style.Resources>
    <SolidColorBrush x:Key="Warning">Yellow</SolidColorBrush>
  </Style.Resources>
</Style>
Referencing resources
You can references resources from controls using the {DynamicResource} markup extensions, e.g.:
<Border Background="{DynamicResource Warning}">
  Look out!
</Border>
Alternatively there is the StaticResource markup extension which has a few limitations with respect to DynamicResource:
It will not respond to changes in the resource
The resource needs to be declared in the same XAML file
In return, StaticResource doesn't need to add an event handler to listen for changes in resources which means it uses slightly less memory.
Overriding resources
Resources are resolved by walking up the logical tree from the point of the DynamicResource or StaticResource until a resource with the requested key is found. This means that resources can be "overridden" in sub-trees of the application, for example:
<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="MyApp.MyUserControl">
  <UserControl.Resources>
    <SolidColorBrush x:Key="Warning">Yellow</SolidColorBrush>
  </UserControl.Resources>
​
  <StackPanel>
    <StackPanel.Resources>
      <SolidColorBrush x:Key="Warning">Orange</SolidColorBrush>
    </StackPanel.Resources>
​
    <Border Background="{DynamicResource Warning}">
      Look out!
    </Border>
  </StackPanel>
</UserControl>
Here's the Border's background will be Orange because its parent StackPanel has "overridden" the Warning resource from the Yellow declared on the UserControl.
Merged resource dictionaries
The Resources property on each control and style is of type ResourceDictionary. Resource dictionaries can also include other resource dictionaries by making use of the MergedDictionaries property. To include a resource dictionary in another you can use the ResourceInclude class, e.g.:
<Window.Resources>
  <ResourceDictionary>
    <ResourceDictionary.MergedDictionaries>
      <ResourceInclude Source='/AnotherResourceDictionary.xaml'/>
    </ResourceDictionary.MergedDictionaries>
    <SolidColorBrush x:Key="Warning">Yellow</SolidColorBrush>
  </ResourceDictionary>
</Window.Resources>
Where AnotherResourceDictionary is a XAML file with a root of ResourceDictionary and is included as an asset in the application.
Resource resolution
As mentioned above, resources are resolved by walking up the logical tree from the point of the DynamicResource or StaticResource until a resource with the requested key is found. However the presence of styles and merged dictionaries complicates this somewhat. The precedence is as follows:
Control resources
Merged dictionaries
Style resources
Merged dictionaries
For the example application below, resource lookup for a resource defined on the Border control would follow the order indicated in [] brackets:
Application
 |- Resources [11]
     |- Merged dictionary [12]
     |- Merged dictionary [13]
 |- Styles
     |- Resources [14]
         |- Merged dictionary [15]
         |- Merged dictionary [16]
​
Window
 |- Resources [6]
     |- Merged dictionary [7]
 |- Styles
     |- Resources [8]
         |- Merged dictionary [9]
         |- Merged dictionary [10]
 |- StackPanel
     |- Resources [1]
         |- Merged dictionary [2]
         |- Merged dictionary [3]
     |- Styles
         |- Resources [4]
             |- Merged dictionary [5]
     |- Border
Theme resources
Themes will usually define brushes, colors and other settings as resources. By changing these resources one can e.g. switch from a dark to a light theme. The resources defined will usually be specific to the theme in use but you can see the resources defined by the default theme here.
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Troubleshooting
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Outline
Declaring resources
Referencing resources
Overriding resources
Merged resource dictionaries
Resource resolution
Theme resources