How can I use StaticResources defined in App.xaml in other ResourceDictionaries?

Question

I defined some StaticResource styles inside App.xaml to use it across the app:

<Application x:Class="TaskListApp.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:TaskListApp"
             StartupUri="MainWindow.xaml">
<Application.Resources>
    <ResourceDictionary>
        <SolidColorBrush x:Key="primaryColor" Color="#B5B4D9"/>
        <SolidColorBrush x:Key="secondaryColor" Color="#393E59"/>
        <SolidColorBrush x:Key="backgroundColor" Color="#2a2e42"/>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="./Themes/CheckboxTheme.xaml"/>
            <ResourceDictionary Source="./Themes/TasksTheme.xaml"/>
            <ResourceDictionary Source="./Themes/TaskListTheme.xaml"/>
            <ResourceDictionary Source="./Themes/MenuButtonTheme.xaml"/>
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>

Using it in MainWindow.xaml is making no errors and works just fine:

<Grid Grid.Row="2"
      Background="{StaticResource backgroundColor}"
</Grid>

But when I added it inside a ResourceDictionary (in this case TaskListTheme.xaml), I got an exception:

Exception: Cannot find resource named 'primaryColor'. Resource names are case sensitive.

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Style TargetType="ListBoxItem" x:Key="TaskListTheme">
    <Setter Property="Margin" Value="0,0,0,5"/>
    <Setter Property="Padding" Value="0"/>
    <Setter Property="BorderThickness" Value="0"/>
    <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
    <Setter Property="ContentTemplate">
        <Setter.Value>
            <DataTemplate>
                <Grid>
                    <Grid.RowDefinitions>
                        <RowDefinition Height="50"/>
                    </Grid.RowDefinitions>
                    <Label Grid.Column="1" Content="{Binding TaskListItemName}"
                             Margin="0"
                             Height="50"
                             Background="{StaticResource primaryColor}"
                             Padding="10,0,0,0"
                             Foreground="{StaticResource secondaryColor}"
                             FontSize="15"
                             FontWeight="SemiBold"
                             HorizontalContentAlignment="Stretch"
                             VerticalContentAlignment="Center"
                             BorderThickness="0"/>
                </Grid>
            </DataTemplate>
        </Setter.Value>
    </Setter>
</Style>

How can I resolve this kind of problem?

Answer 1

If a resource in TaskListTheme.xaml has a dependency on a resource that is defined in another resource dictionary, you should merge the latter into TaskListTheme.xaml:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="Colors.xaml"/>
    </ResourceDictionary.MergedDictionaries>
</ResourceDictionary>

So move your *Color brush resources to Colors.xaml and then either merge Colors.xaml into the other resource dictionaries or into App.xaml:

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="Colors.xaml"/>
            <ResourceDictionary Source="./Themes/CheckboxTheme.xaml"/>
            <ResourceDictionary Source="./Themes/TasksTheme.xaml"/>
            <ResourceDictionary Source="./Themes/TaskListTheme.xaml"/>
            <ResourceDictionary Source="./Themes/MenuButtonTheme.xaml"/>
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>