How to switch themes in WPF with click of button

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I am developing a WPF MVVM-based application using c#.
I am trying to create 2 themes light & dark.
There will be a button on all windows in either the header or the footer to switch between light & dark themes.
I have created a folder hierarchy like this
AppName/
>Resources/
Light.xaml
Dark.xaml

all appropriate styles & solid colour brushes are defined in each XAML file.
I want to know how to switch the theme with a button click.

What I have tried:

ResourceDictionary resX = new ResourceDictionary();
resX.Source = new Uri("/Resources/Light.xaml", UriKind.RelativeOrAbsolute);
this.Resources.MergedDictionaries.Clear();
this.Resources.MergedDictionaries.Add(resX);
This code executes on button click.

Posted 3-May-24 0:15am

Christopher Fernandes

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Richard Deeming 3-May-24 6:54am

Aside from wrapping the update in a BeginInit/EndInit block, that code looks like it should work.

this.Resources.BeginInit();
try
{
    this.Resources.MergedDictionaries.Clear();
    this.Resources.MergedDictionaries.Add(resX);
}
finally
{
    this.Resources.EndInit();
}

So what's the problem?

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Pete O'Hanlon · Accepted Answer · 2024-05-03T00:51:00

The ability to switch a theme is largely going to depend on whether or not you have declared a particular resource as a StaticResource or a DynamicResource. Suppose you have defined a brush colour as a StaticResource. When the application starts up, that resource is going to be baked in for the duration of the application lifetime, and can't be switched. If it's a DynamicResource, then reloading it results in the brush colour changing.

While this[^] is an old article, it should still hold true today.