How to use ThemeManager in code behinde - xaml

Question

I'm new in mahapps.metro Themes . when i used ThemeManager it works just once and then in second use it rise an null reference exception . for example first i select Blue Theme it works but When after that i select green theme it not work and rise a null reference Exception .
here is a sample code :
ThemeManager.ChangeAppStyle(Application.Current,
ThemeManager.GetAccent("Blue"),
ThemeManager.GetAppTheme("BaseDark"));
ThemeManager.ChangeAppStyle(Application.Current,
ThemeManager.GetAccent("Green"),
ThemeManager.GetAppTheme("BaseDark"));
what is wrong ?

Answer 1

This lines should work at this point in App.xaml code behind or at your main Window constructor.
using System.Windows;
using MahApps.Metro;
namespace MetroDemo
{
/// <summary>
/// Interaction logic for App.xaml
/// </summary>
public partial class App : Application
{
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
ThemeManager.ChangeAppStyle(Application.Current,
ThemeManager.GetAccent("Blue"),
ThemeManager.GetAppTheme("BaseDark"));
ThemeManager.ChangeAppStyle(Application.Current,
ThemeManager.GetAccent("Green"),
ThemeManager.GetAppTheme("BaseDark"));
}
}
}
Make sure you are adding the necessary resources to the App.xaml, cause you want to change the resources of your App!
<Application x:Class="MetroDemo.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>
<!-- MahApps.Metro resource dictionaries. Make sure that all file names are Case Sensitive! -->
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Controls.xaml" />
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Fonts.xaml" />
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Colors.xaml" />
<!-- Accent and AppTheme setting -->
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/Blue.xaml" />
<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/BaseLight.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
</Application>
Hope this helps.