WPF ThemeInfo Configuration

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.

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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

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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)

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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

1. Add ThemeInfo attribute to AssemblyInfo.cs (see Section 1)
2. Place styles in Themes/Generic.xaml
3. Control styles load automatically in any consuming application

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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:

1. ThemeInfo tells WPF where to find Generic.xaml
2. DefaultStyleKeyProperty tells WPF which TargetType style to apply
3. WPF matches the style in Generic.xaml by TargetType="{x:Type local:CustomButton}"

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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)