Question

I have a simple WPF application I’m using for experimenting.

I have two themes defined in seperate xaml files, changing the xaml to point to them worked fine. By the way, in the xaml I’m using a straight ResourceDictionary element, not a ResourceDictionary.MergedDictionaries one.

I want to let the user select which theme to use, so I’m reseting the source property in code behind – but whilst the debugger tells me I’ve successfully set the value the applications appearance doesn’t change.

So, how do you successfully apply a theme at runtime?

EDIT: This is how I’m declaring my “style” in the xaml:

<Window x:Class="WpfUI.winMain">
  <Window.Resources>
    <ResourceDictionary Source="Themes\Blah.xaml"></ResourceDictionary>
  </Window.Resources>

  // The windows grid and other controls...

</Window>

Answer 1

The simple answer is, you need to clear the applications merged resource dictionaries. Here is some code to get you started

ResourceDictionary dictionary = GetThemeResourceDictionary(yourTheme) 

if (dictionary != null)
{
     App.Current.Resources.MergedDictionaries.Clear();
     App.Current.Resources.MergedDictionaries.Add(dictionary);
}

public ResourceDictionary GetThemeResourceDictionary(string theme)
{
    if (theme != null)
    {
        Assembly assembly = Assembly.LoadFrom("WPF.Themes.dll");
        string packUri = String.Format(YourThemeFolder/{0}.xaml", theme);
        return Application.LoadComponent(new Uri(packUri, UriKind.Relative)) as ResourceDictionary;
    }
    return null;
}

If you want a really nice packaged solution, i would reccommend WPF themes. It introduces a ThemeManager class and a set of built in themes which are really incredible. If you have any difficulty installing it or adding a new theme, contact me 🙂