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 <Style x:Key="Workaround" />

#pragma checksum "..\..\..\App.xaml" "{ff1816ec-aa5e-4d10-87f7-6f4963833460}" "5B5BE5C8C8262B68A118BFB34A8694FD962B7EA1"
//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated by a tool.
//     Runtime Version:4.0.30319.42000
//
//     Changes to this file may cause incorrect behavior and will be lost if
//     the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------

using System;
using System.Diagnostics;
using System.Windows;
using System.Windows.Automation;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Controls.Ribbon;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Markup;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Media.Effects;
using System.Windows.Media.Imaging;
using System.Windows.Media.Media3D;
using System.Windows.Media.TextFormatting;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Windows.Shell;


namespace WpfRepro {
    
    
    /// <summary>
    /// App
    /// </summary>
    public partial class App : System.Windows.Application {
        
        /// <summary>
        /// Application Entry Point.
        /// </summary>
        [System.STAThreadAttribute()]
        [System.Diagnostics.DebuggerNonUserCodeAttribute()]
        [System.CodeDom.Compiler.GeneratedCodeAttribute("PresentationBuildTasks", "5.0.5.0")]
        public static void Main() {
            WpfRepro.App app = new WpfRepro.App();
            app.Run();
        }
    }
}

With <Style x:Key="Workaround" />

#pragma checksum "..\..\..\App.xaml" "{ff1816ec-aa5e-4d10-87f7-6f4963833460}" "D8DA150CB61153CE667AE1E7D93E9C88B025B8F8"
//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated by a tool.
//     Runtime Version:4.0.30319.42000
//
//     Changes to this file may cause incorrect behavior and will be lost if
//     the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------

using System;
using System.Diagnostics;
using System.Windows;
using System.Windows.Automation;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Controls.Ribbon;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Markup;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Media.Effects;
using System.Windows.Media.Imaging;
using System.Windows.Media.Media3D;
using System.Windows.Media.TextFormatting;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Windows.Shell;


namespace WpfRepro {
    
    
    /// <summary>
    /// App
    /// </summary>
    public partial class App : System.Windows.Application {
        
        private bool _contentLoaded;
        
        /// <summary>
        /// InitializeComponent
        /// </summary>
        [System.Diagnostics.DebuggerNonUserCodeAttribute()]
        [System.CodeDom.Compiler.GeneratedCodeAttribute("PresentationBuildTasks", "5.0.5.0")]
        public void InitializeComponent() {
            if (_contentLoaded) {
                return;
            }
            _contentLoaded = true;
            System.Uri resourceLocater = new System.Uri("/WpfRepro;V1.0.0.0;component/app.xaml", System.UriKind.Relative);
            
            #line 1 "..\..\..\App.xaml"
            System.Windows.Application.LoadComponent(this, resourceLocater);
            
            #line default
            #line hidden
        }
        
        /// <summary>
        /// Application Entry Point.
        /// </summary>
        [System.STAThreadAttribute()]
        [System.Diagnostics.DebuggerNonUserCodeAttribute()]
        [System.CodeDom.Compiler.GeneratedCodeAttribute("PresentationBuildTasks", "5.0.5.0")]
        public static void Main() {
            WpfRepro.App app = new WpfRepro.App();
            app.InitializeComponent();
            app.Run();
        }
    }
}

It does not seem to generate the InitializeComponent when there is only one item in <Resources>.

My guess would be that there is a wrong condition here that prevents the generation of InitializeComponent:
https://github.com/dotnet/wpf/blob/main/src/Microsoft.DotNet.Wpf/src/PresentationBuildTasks/MS/Internal/MarkupCompiler/MarkupCompiler.cs#L3083

[ThomasGoulet73]

This bug should be fixed by #8853. For now, you can workaround this issue by adding a dummy item in <Application.Resources>.