Application styles are only applied if there is more than one style when StartupUri is not used

[jnm2]

• .NET Core Version: 5.0.202
• Windows version: Windows 10 20H2
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

The application style has no effect, but it should have an effect. The style starts taking effect as soon as a second unrelated style is added before or after it.

This causes a long search for ways to fix the problem and is not beginner-friendly.

Full repro created from VS template

Download: Repro.zip

<Application x:Class="WpfRepro.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Application.Resources>
        <Style TargetType="TextBox">
            <Setter Property="Background" Value="Red"/>
        </Style>
        <!-- Uncomment to see the style above start taking effect -->
        <!-- <Style x:Key="Workaround" /> -->
    </Application.Resources>
</Application>

<Window x:Class="WpfRepro.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfRepro"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <TextBox />
</Window>

using System.Windows;

namespace WpfRepro
{
    public partial class App : Application
    {
        protected override void OnStartup(StartupEventArgs e)
        {
            new MainWindow(startupUriIsNotAnOption: 42).Show();
        }
    }
}

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>net5.0-windows</TargetFramework>
    <UseWPF>true</UseWPF>
  </PropertyGroup>

</Project>

[daisyTian0517]

You need to add a ResourceDictionary to contain your Style and make it to the Application.Current.Resources.Source in App.xaml.cs, eg, You can put your style into a ResourceDictionary named a Dictionary1 , and add below code to your App.xaml.cs

 public App()
        {
            Application.Current.Resources.MergedDictionaries.Add(Application.LoadComponent(new Uri("/ApplicationStyles;Component/Dictionary1.xaml",UriKind.Relative)) as ResourceDictionary);
        }

[jnm2]

Why is this needed if it's not needed when StartupUri is used and it's not needed if there is more than one style?

[ThomasGoulet73]

This bug looks really weird. It's from the code generated by PresentationBuildTasks.dll.

See the difference between the generated code in App.g.i.cs:

Without