wpf resource dictionary does not exist in namespace? - c#

I'm using .net 4.5.2
I do not have a clue why my project continues to say the resource is missing. I've paced my mainWindow.xaml inside a folder and repathed the app to target its location. What is really odd is the fact that it displays correctly in visual studio but when i try to compile it errors out saying the 'themes' namespace does not exist in the project.
Severity Code Description Project File Line Error CS0234 The type or
namespace name 'Themes' does not exist in the namespace
'WpfApplication1' (are you missing an assembly
reference?) WpfApplication1 C:\Users\jmartini\Projects\wpf_Styling_4.5.2\WpfApplication1\WpfApplication1\obj\Debug\View\MainWindow.g.i.cs 33
Here is my code...
MainWindow.xaml
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfApplication1"
mc:Ignorable="d"
Title="MainWindow" Height="200" Width="250"
WindowStartupLocation="CenterScreen">
<DockPanel>
<Button Content="Push It!" Width="70" VerticalAlignment="Center" HorizontalAlignment="Center"/>
</DockPanel>
</Window>
JMStyles.xaml
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1.Themes">
<Style TargetType="{x:Type Button}">
<Setter Property="Background" Value="Red" />
<Setter Property="Foreground" Value="White" />
<Setter Property="FontSize" Value="15" />
<Setter Property="SnapsToDevicePixels" Value="True" />
</Style>
</ResourceDictionary>
App.xaml
<Application x:Class="WpfApplication1.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1"
StartupUri="View/MainWindow.xaml">
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="Themes/JMStyles.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
</Application>
Solution to get it working...
I removed this line from the style page:
xmlns:local="clr-namespace:WpfApplication1.Themes"
original
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Style TargetType="{x:Type Button}">
<Setter Property="Background" Value="Red" />
<Setter Property="Foreground" Value="White" />
<Setter Property="FontSize" Value="15" />
<Setter Property="SnapsToDevicePixels" Value="True" />
</Style>
</ResourceDictionary>


I removed this line of code
xmlns:local="clr-namespace:WpfApplication1.Themes"
from the style page file contents below...
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1.Themes">
<Style TargetType="{x:Type Button}">
<Setter Property="Background" Value="Red" />
<Setter Property="Foreground" Value="White" />
<Setter Property="FontSize" Value="15" />
<Setter Property="SnapsToDevicePixels" Value="True" />
</Style>
</ResourceDictionary>


Microsoft.Windows.Themes is found in the theme-specific PresentationFramework assemblies. You'll need to add a reference to one of the following depending on which one is referenced in your XAML:
PresentationFramework.Aero.dll
PresentationFramework.AeroLite.dll
PresentationFramework.Classic.dll
PresentationFramework.Luna.dll
PresentationFramework.Royale.dll


For some reason Visual Studio doesn't recognize namespaces of folders with only Resource Dictionary files. If you add another file in that folder like a User Control you would notice that the namespace would be recognized. The solution is to remove the line of
"local" namespace:
xmlns:local="clr-namespace:WpfApplication1.Themes
because we will mention the resourse with
<ResourceDictionary Source="Themes/...
Without needing the namespace, in difference when we create User Controls that we do need the namespace in the code of the file where they're used

Related

Why is Visual Studio showing an error saying Windows.Resources is not found in the type Window?

I am a beginner in WPF applications. I have created a WPF application and I want to use a Main Page in my application. Inside the Window scope, I want to declare Window.Resources but it returns two errors:
1- The member Resources is not recognized or accessible.
2- The attachable property Resources wasn't found in the type Window.
What could be missing in this case?
Here's my XAML code:
<Window x:Class="ArmsPosition.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:ArmsPosition"
mc:Ignorable="d"
Loaded="Window_Loaded"
Closing="Window_Closing"
WindowState="Normal"
Title="Arms Positions" Height="560" Width="800" MinHeight="560" MinWidth="800" MaxHeight="560" MaxWidth="800">
<Grid>
<Window.Resources>
<Style TargetType="{x:Type TextBlock}">
<Setter Property="FontFamily" Value="Segoe UI"/>
<Setter Property="FontSize" Value="20"/>
<Setter Property="Foreground" Value="#FF999999"/>
</Style>
</Window.Resources>
</Grid>
</Window>

It's because you are using Windows.Resource within the Grid element. Resource is a direct child property of the Window element.
<Window x:Class="ArmsPosition.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:ArmsPosition"
mc:Ignorable="d"
Loaded="Window_Loaded"
Closing="Window_Closing"
WindowState="Normal"
Title="Arms Positions" Height="560" Width="800" MinHeight="560" MinWidth="800" MaxHeight="560" MaxWidth="800">
<Window.Resources>
<Style TargetType="{x:Type TextBlock}">
<Setter Property="FontFamily" Value="Segoe UI"/>
<Setter Property="FontSize" Value="20"/>
<Setter Property="Foreground" Value="#FF999999"/>
</Style>
</Window.Resources>
<Grid>
<!--SOME CONTROLS HERE-->
</Grid>
</Window>

Style template for addin in WPF

I am designing a revit addin.I'm using WPF and I want to design style templates which can then be inherited by all WPF pages.I have read that I need to put the styles in app.xaml but there is no app.xaml as this is an addin.
As of now,I'm creating style templates but one template can only be used by the elements of that particular page.
I want to create a style template which every page can inherit without app.xaml.How do I do that?

If all you need is to share styles and maybe some resources you can do it without an App.xaml. In your project, create a xaml file to contain the shared styles and call it something like "MyCommonResources.xaml". It should simply contain a resource dictionary that holds the shared styles/resources. For example:
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:MyAppNS">
<SolidColorBrush x:Key="Window.Background" Color="#FF202020"/>
<SolidColorBrush x:Key="MyTitleBar.Background" Color="#FF5A5A5A"/>
<SolidColorBrush x:Key="MyTitleBar.Foreground" Color="WhiteSmoke"/>
<SolidColorBrush x:Key="MyTitleBar.Disabled.Background" Color="Black"/>
<SolidColorBrush x:Key="MyTitleBar.Disabled.Foreground" Color="Gray"/>
<Style x:Key="CommandButtonStyle" TargetType="Button">
<Setter Property="MinHeight" Value="40"/>
</Style>
<Style x:Key="MyTitleBarStyle" TargetType="Label">
<Setter Property="Background" Value="{StaticResource MyTitleBar.Background}"/>
<Setter Property="Foreground" Value="{StaticResource MyTitleBar.Foreground}"/>
<Setter Property="HorizontalContentAlignment" Value="Center"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="MinHeight" Value="34"/>
<Style.Triggers>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Background" Value="{StaticResource MyTitleBar.Disabled.Background}"/>
<Setter Property="Foreground" Value="{StaticResource MyTitleBar.Disabled.Foreground}"/>
</Trigger>
</Style.Triggers>
</Style>
</ResourceDictionary>
now in your xaml for the Window/Page/UserControl add a resource section and merge this dictionary in with the local...
<Window x:Class="MyAppNS.MyWPFWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:MyAppNS"
mc:Ignorable="d">
<Window.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="MyCommonResources.xaml"/>
</ResourceDictionary.MergedDictionaries>
<Style x:Key="MyLocalStyle" TargetType="Button">
<Setter Property="VerticalContentAlignment" Value="Left"/>
</Style>
</ResourceDictionary>
</Window.Resources>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Label Grid.Row="0" Content="This is the Title"
Style="{StaticResource MyTitleBarStyle}"/>
<Button x:Name="btnOK" Grid.Row="1" Content="OK"
Style="{StaticResource CommandButtonStyle}"
Click="btnOK_Click"/>
</Grid>
</Window>
Excuse me for any typos in the above, cut & pasted it from a Winforms app I have but quickly edited it down so it wouldn't be so large and changed some names to make better sense.

Do you have a WPF project or just a WPF view in a Forms application?
Maybe this contribution will help you ->
App.XAML where are you?

How to make a Style generic so I can use it in my entire application?

I've search around here on SO and what not able to find a clear answer explaining how to setup a 'style resource'. In my case my dialog controls several buttons and lists, and other various controls which I want to set a generic theme/style for. Similar to how you would do using a CSS file in HTML.
For the sake of simplicity in this example I have a style i want to use across the board on all my buttons. However I would prefer not to contain all these style resources in the xaml of my UI layout. I would like to move the styles to a general xaml resource file which would contain just the styles, that way i could also easily reference them into other wpf dialogs throughout the tool.
How do I set this up to make use of a general resource file containing styles for the various controls in my tool? Then be able to reference and use that style in my xaml UI's.
Thank you
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="200" Width="200">
<Window.Resources>
<Style TargetType="Button" x:Key="CoolButton" >
<Setter Property="Margin" Value="1,2,1,2"/>
<Setter Property="Background" Value="LightBlue"/>
<Setter Property="HorizontalAlignment" Value="Left"/>
<Setter Property="FontSize" Value="12" />
<Setter Property="FontFamily" Value="Calibri" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Border Background="{TemplateBinding Background}"
BorderBrush="Lavender" BorderThickness="5" CornerRadius="6,0,6,0" x:Name="bd">
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" RecognizesAccessKey="True" />
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter TargetName="bd" Property="Background" Value="LightGray"/>
<Setter Property="Foreground" Value="White" />
<Setter Property="Cursor" Value="Hand" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>
<StackPanel>
<Button Style="{StaticResource CoolButton}" Content="Button" Margin="2" HorizontalAlignment="Left" VerticalAlignment="Top" Width="75"/>
<Button Style="{StaticResource CoolButton}" Content="Button" Margin="2" HorizontalAlignment="Left" VerticalAlignment="Top" Width="75"/>
<CheckBox Margin="2" Content="Great"></CheckBox>
</StackPanel>
</Window>
On a side note why does this not work to use variables for colors in the resource style xaml?
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<!--COLORS-->
<Color x:Key="AccentColor">#CC4021</Color>
<Style TargetType="Button">
<Setter Property="Foreground" Value="{StaticResource AccentColor}"/>
</Style>
</ResourceDictionary>

There are a few steps to follow from making a instance-specific style a generic style.
Remove the Key. This will make the style to be used for every button:
<Style TargetType="Button">
Move it to a resource file, for example Default.xaml:
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>
<Style TargetType="Button">
...
</Style>
</ResourceDictionary>
Include a reference to the resource from a central point, for example the App.xaml, which will load the resources. The App.xaml will cause the styles to be used application-wide in just one go:
<Application x:Class="..."
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="Default.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
</Application>

Style "Cannot find resource 'MetroCheckBox'" in MahApps

I'm starting to use styles in WPF. I'm using MahApps styles as a base, which so far has been really good. I've been able to make specific modifications using the BasedOn property.
One of the simpler changes is to add a default margin so when adding controls they don't touch. It's worked really well so far until I tried to use the MetroCheckBox. With this specific control it throws an xaml parse exception: "Cannot find resource named 'MetroCheckBox'. Resource names are case sensitive."
I had a look in the source code, trying to track down this issue and coppied the name directly from the GitHub:
https://github.com/MahApps/MahApps.Metro/blob/master/MahApps.Metro/Styles/Controls.CheckBox.xaml
It's definately and all my other controls work fine:
<Style TargetType="{x:Type Button}" BasedOn="{StaticResource MetroButton}">
<Setter Property="Margin" Value="2"/>
</Style>
<Style TargetType="{x:Type ComboBox}" BasedOn="{StaticResource MetroComboBox}">
<Setter Property="Margin" Value="2"/>
</Style>
<Style TargetType="{x:Type CheckBox}" BasedOn="{StaticResource MetroCheckBox}">
<Setter Property="Margin" Value="2"/>
</Style>
<Style TargetType="{x:Type TextBox}" BasedOn="{StaticResource MetroTextBox}">
<Setter Property="Margin" Value="2"/>
</Style>
Any ideas on how to fix this?
Note, I'm including the reference to the styles like so:
<Application
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:extra="http://schemas.extra.com/ui"
xmlns:system="clr-namespace:System;assembly=mscorlib"
xmlns:visualizationToolkit="clr-namespace:System.Windows.Controls.DataVisualization;assembly=System.Windows.Controls.DataVisualization.Toolkit"
xmlns:chartingToolkit="clr-namespace:System.Windows.Controls.DataVisualization.Charting;assembly=System.Windows.Controls.DataVisualization.Toolkit"
xmlns:Primitives="clr-namespace:System.Windows.Controls.DataVisualization.Charting.Primitives;assembly=System.Windows.Controls.DataVisualization.Toolkit"
xmlns:Themes="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero"
xmlns:Custom="http://metro.mahapps.com/winfx/xaml/controls"
x:Class="MyApp.App"
StartupUri="/MyApp;component/GUI/Window/Window3.xaml"
>
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Controls.xaml" />
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Fonts.xaml" />
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Colors.xaml" />
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/Blue.xaml" />
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/BaseDark.xaml" />
</ResourceDictionary.MergedDictionaries>
<Style TargetType="{x:Type Button}" BasedOn="{StaticResource MetroButton}">
<Setter Property="Margin" Value="2"/>
</Style>
<Style TargetType="{x:Type ComboBox}" BasedOn="{StaticResource MetroComboBox}">
<Setter Property="Margin" Value="2"/>
</Style>
<!--<Style TargetType="{x:Type CheckBox}" BasedOn="{StaticResource MetroCheckBox}">
<Setter Property="Margin" Value="2"/>
</Style>-->
<Style TargetType="{x:Type TextBox}" BasedOn="{StaticResource MetroTextBox}">
<Setter Property="Margin" Value="2"/>
</Style>
<!--Chart Style-->
<Style TargetType="{x:Type chartingToolkit:Chart}
...
</Style>
<!--Top Tab Item-->
<Style x:Key="TopTabItemStyle" TargetType="{x:Type TabItem}" BasedOn="{StaticResource {x:Type TabItem}}">
...
</Style>
<!--Tab Item-->
<Style TargetType="{x:Type TabItem}" BasedOn="{StaticResource MetroTabItem}">
...
</Style>
<!-- Group Box-->
<Style TargetType="{x:Type GroupBox}" BasedOn="{StaticResource MetroGroupBox}">
<!--<Setter Property="Margin" Value="5"/>
<Setter Property="Padding" Value="5"/>
<Setter Property="Foreground" Value="{DynamicResource BlackBrush}"/>
<Setter Property="Background" Value="{DynamicResource AccentColorBrush}"/>
<Setter Property="BorderBrush" Value="{DynamicResource AccentColorBrush}"/>
<Setter Property="Custom:ControlsHelper.HeaderFontSize" Value="{DynamicResource ContentFontSize}"/>
<Setter Property="Custom:GroupBoxHelper.HeaderForeground" Value="{x:Null}"/>-->
<Setter Property="Custom:ControlsHelper.HeaderFontWeight" Value="SemiBold"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type GroupBox}">
<Grid x:Name="GroupBoxRoot">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="0,0,0,2" Background="Transparent" Grid.Row="0">
<ContentPresenter ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" ContentStringFormat="{TemplateBinding HeaderStringFormat}" ContentSource="Header" TextElement.FontWeight="{TemplateBinding Custom:ControlsHelper.HeaderFontWeight}" TextElement.FontStretch="{TemplateBinding Custom:ControlsHelper.HeaderFontStretch}" TextElement.FontSize="{TemplateBinding Custom:ControlsHelper.HeaderFontSize}" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True"/>
</Border>
<Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="0" Background="Transparent" Grid.Row="1">
<ContentPresenter Cursor="{TemplateBinding Cursor}" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Margin="{TemplateBinding Padding}"/>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ResourceDictionary>
</Application.Resources>

You are missing the required ResourceDictionary to bring in the styles for the MahApps. You can do by including the ResourceDictionary definition in the XAML of either the XAML file where you are composing your view or you can add it to the APP.XAML file - the later will give you better performance in the short term.
For example: here is the MahApps controls xaml included with other MahApps resource dictionaries, the controls xaml is the one you need in this case:
<Application x:Class="MyApp"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Controls.xaml" />
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Fonts.xaml" />
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Colors.xaml" />
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/Blue.xaml" />
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/BaseLight.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
</Application>

I ran it today and it works fine:
<Style TargetType="{x:Type ComboBox}" BasedOn="{StaticResource MetroComboBox}">
<Setter Property="Margin" Value="2"/>
</Style>
As far as I can tell I've changed nothing to do with the project or references.
However, in the last few days Visual Studio had corrupted somehow with the designer throwing exceptions left right and center, solution explorer failing to display anything (was fixed after clearing Component Model Cache) and NuGet failing to work for me to check the MahApps version.
So I set off the installer on a repair install, and now the style works fine.
I have no idea if these are connected... but they may be, so I'll leave this up as an answer in case anyone else comes across this same issue.

This can be caused if you upgrade from a version of MahApps prior to 2.0 to a more recent version.
For more details, see:
mahapps.com: Migration to v2.0 - MahApps.Metro
https://mahapps.com/docs/guides/migration-to-v2.0

Using BasedOn property with a Style defined in a different dictionary

The application I'm working on has 2 ResourceDictionary, DefaultStyles.xaml and CustomStyles.xaml.
Is it possible that a style in the CustomStyles dictionary uses a base style defined in the other dictionary?
DefaultStyles.xaml:
<Style x:Key="TextBlockDefaultStyle" TargetType="TextBlock">
<Setter Property="Margin" Value="4" />
</Style>
CustomStyles.xaml:
<Style x:Key="SectionTitleStyle" TargetType="TextBlock" BasedOn="{StaticResource TextBlockDefaultStyle}">
<Setter Property="FontSize" Value="16" />
</Style>
App.xaml:
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="Assets/Styles/DefaultStyles.xaml"/>
<ResourceDictionary Source="Assets/Styles/CustomStyles.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
When the code runs, the following exception is thrown:
Cannot find a Resource with the Name/Key TextBlockDefaultStyle.
It works well if both styles are in the same file.

You need to reference the dictionary with the other style directly.
CustomStyles.xaml:
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="DefaultStyles.xaml" />
</ResourceDictionary.MergedDictionaries>
<Style x:Key="SectionTitleStyle" TargetType="TextBlock" BasedOn="{StaticResource TextBlockDefaultStyle}">
<Setter Property="FontSize" Value="16" />
</Style>

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