How to Change Theme XAML with ComboBox?

Question

How can I apply different color themes to my program using a ComboBox?

Such as ThemeBlue.xaml, ThemeRed.xaml, ThemePurple.xaml.

I need it to swap out one theme xaml file for another on selection change.

Here's the example project file:
https://drive.google.com/open?id=0BycnCTAQ1U7gSU5kUUdaNzRIZDg

---

Themes

I have a theme file ThemeBlue.xaml with colors set for controls and window.

<SolidColorBrush x:Key="TextBoxCustom.Static.Background" Color="Blue"/>
<SolidColorBrush x:Key="TextBoxCustom.Static.Border" Color="Blue"/>

<Style x:Key="TextBoxCustom" TargetType="{x:Type TextBox}">
    <Setter Property="Background" Value="{StaticResource TextBoxCustom.Static.Background}"/>
    <Setter Property="BorderBrush" Value="{StaticResource TextBoxCustom.Static.Border}"/>
...

---

ComboBox Theme Select

XAML

<ComboBox x:Name="cboTheme" Style="{StaticResource ComboBoxCustom}" HorizontalAlignment="Left" Margin="199,120,0,0" VerticalAlignment="Top" Width="75" SelectionChanged="themeSelect_SelectionChanged">
    <System:String>Blue</System:String>
    <System:String>Red</System:String>
    <System:String>Purple</System:String>
</ComboBox>

C#

This is where I need help to apply the theme file.

private void themeSelect_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    // Apply theme file to App.xaml from option selected
    // Blue
    // Red
    // Purple
}

---

App.xaml

The theme file is loaded through App.xaml at startup

<Application x:Class="MyProgram.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">

    <Application.Resources>
        <ResourceDictionary Source="ThemeBlue.xaml"/>
    </Application.Resources>

</Application>

---

MainWindow.xaml

A TextBox with the Theme Style applied:

<TextBox x:Name="textBox1" Style="{StaticResource TextBoxCustom}" HorizontalAlignment="Left" Height="22" Margin="93,43,0,0" Width="464" />

---

Saving Theme

I save and load the theme through Settings.

private void themeSelect_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    // Save Theme for next launch
    Settings.Default["Theme"] = cboTheme.SelectedItem.ToString();
    Settings.Default.Save();
    Settings.Default.Reload();
}

Answer 1

You neet to replace the resource of the App.xaml with another resource in code behind. You can take the resource by loading the component. Here is a possible solution:

private void themeSelect_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    AssemblyName assemblyName = Assembly.GetAssembly(this.GetType()).GetName();

    ResourceDictionary dictionary = (ResourceDictionary)Application.LoadComponent(
        new Uri(assemblyName.Name + @";component/" + cboTheme.SelectedItem + ".xaml", UriKind.Relative))));

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

Answer 2

Try this:

private void themeSelect_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    string theme = cboTheme.SelectedItem.ToString();

    App.Current.Resources.MergedDictionaries.Clear();
    App.Current.Resources.MergedDictionaries.Add(new ResourceDictionary() { Source = new Uri("Theme" + theme + ".xaml", UriKind.RelativeOrAbsolute) });
    // Save Theme for next launch
    Settings.Default["Theme"] = theme;
    Settings.Default.Save();
}

---

App.xaml:

<Application x:Class="MyProgram.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="ThemeBlue.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>

</Application>