WPF ThemeInfo Configuration

Configuring the ThemeInfo attribute for automatic Generic.xaml loading in WPF Custom Control Library projects.

Overview

WPF uses the ThemeInfo attribute to locate theme-specific and generic resource dictionaries at runtime. Without this attribute, Themes/Generic.xaml will not be loaded automatically, and CustomControl styles will not be applied.

1. AssemblyInfo.cs Configuration

Required Code

// Properties/AssemblyInfo.cs using System.Windows;

[assembly: ThemeInfo( ResourceDictionaryLocation.None, // Theme-specific resources ResourceDictionaryLocation.SourceAssembly)] // Generic resources (Themes/Generic.xaml)

File Location

YourProject/ ├── Properties/ │ └── AssemblyInfo.cs ← ThemeInfo attribute here ├── Themes/ │ └── Generic.xaml ← Auto-loaded by ThemeInfo └── CustomButton.cs

2. ResourceDictionaryLocation Options

Value Description Use Case

None

No resource dictionary for this category Theme-specific: when no OS theme customization needed

SourceAssembly

Resources in the current assembly Generic: load from Themes/Generic.xaml in this assembly

ExternalAssembly

Resources in a separate assembly When themes are packaged in YourAssembly.ThemeName.dll

Parameter Meanings

[assembly: ThemeInfo( themeDictionaryLocation, // 1st: Theme-specific (e.g., Aero, Luna) genericDictionaryLocation)] // 2nd: Generic fallback (Themes/Generic.xaml)

• 1st parameter (themeDictionaryLocation ): OS theme-specific styles (rarely used)

• 2nd parameter (genericDictionaryLocation ): Generic.xaml location (must be SourceAssembly )

3. Anti-Pattern: App.xaml MergedDictionaries

Bad Example

<!-- App.xaml - DO NOT DO THIS for CustomControl styles --> <Application.Resources> <ResourceDictionary> <ResourceDictionary.MergedDictionaries> <ResourceDictionary Source="/MyControls;component/Themes/Generic.xaml" /> </ResourceDictionary.MergedDictionaries> </ResourceDictionary> </Application.Resources>

Why This Is Wrong

Issue Description

Not standard pattern CustomControl styles should be loaded via ThemeInfo, not App.xaml

Cross-assembly failure When another assembly references your control library, styles won't load (the consuming app doesn't have your MergedDictionary)

Breaks encapsulation The control library should be self-contained; consumers shouldn't need to configure resource loading

Correct Approach

• Add ThemeInfo attribute to AssemblyInfo.cs (see Section 1)

• Place styles in Themes/Generic.xaml

• Control styles load automatically in any consuming application

4. DefaultStyleKeyProperty Connection

Every CustomControl must set its default style key in the static constructor. This works together with ThemeInfo to locate the correct style:

public class CustomButton : Button { static CustomButton() { DefaultStyleKeyProperty.OverrideMetadata( typeof(CustomButton), new FrameworkPropertyMetadata(typeof(CustomButton))); } }

How it works:

• ThemeInfo tells WPF where to find Generic.xaml

• DefaultStyleKeyProperty tells WPF which TargetType style to apply

• WPF matches the style in Generic.xaml by TargetType="{x:Type local:CustomButton}"

5. Troubleshooting

Symptom Cause Fix

Style not applied Missing ThemeInfo attribute Add ThemeInfo to AssemblyInfo.cs

Style not applied Wrong generic parameter Set 2nd parameter to SourceAssembly

Style not applied Missing DefaultStyleKeyProperty Add OverrideMetadata in static constructor

Style not applied in other assembly Using App.xaml MergedDictionaries instead of ThemeInfo Switch to ThemeInfo attribute

Generic.xaml not found Wrong file location Must be in Themes/Generic.xaml (exact path)