How to change ResourceDictionary Dynamically

Question

I need to change ResourceDictionary in App.xaml file dynamically. I have tried the following code:

ResourceDictionary newRes = new ResourceDictionary();
newRes.Source = new Uri("/PsyboInventory;component/TitleBarResource.xaml", UriKind.RelativeOrAbsolute);
this.Resources.MergedDictionaries.Clear();
this.Resources.MergedDictionaries.Add(newRes);

There is no error, but the theme not change

Answer 1

In button click you can write this code

var app = (App)Application.Current;
app.ChangeTheme(new Uri("New Uri here"));

ChangeTheme:

public partial class App : Application
    {
        public ResourceDictionary ThemeDictionary
        {
            // You could probably get it via its name with some query logic as well.
            get { return Resources.MergedDictionaries[0]; }
        }
    
        public void ChangeTheme(Uri uri)
        {
            ThemeDictionary.MergedDictionaries.Clear();
            ThemeDictionary.MergedDictionaries.Add(new ResourceDictionary() { Source = uri });
        } 
       
    }   

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary x:Name="ThemeDictionary">
                    <ResourceDictionary.MergedDictionaries>
                        <ResourceDictionary Source="/Themes/ShinyRed.xaml"/>
                    </ResourceDictionary.MergedDictionaries>
                </ResourceDictionary>
    </ResourceDictionary.MergedDictionaries>