Question

I can’t understand why happens this. I have a simple application in WPF. This application have a window, and in the App.xaml have defined one style, that changes the style of all the buttons:

<Application x:Class="PruebasDesk.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="Window1.xaml">
    <Application.Resources>
        <Style TargetType="{x:Type Button}">
            <Setter Property="Height" Value="23"></Setter>
            <Setter Property="Width" Value="75"></Setter>
            <Setter Property="Background" Value="DarkCyan"></Setter>
        </Style>
    </Application.Resources>
</Application>

This works fine, all the buttons get the style. Now, here is the problem. If instead of using the StartupUri attribute to start the application, I start it with using the OnStartup method:

public partial class App : Application
    {
        protected override void OnStartup(StartupEventArgs e)
        {
            Window1 win1 = new Window1();
            win1.Show();
        }
    }

The buttons of the application don’t get applied the button style defined at App.xaml. But… if I add another style to the App.xaml, like this:

<Application x:Class="PruebasDesk.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             >
    <Application.Resources>
        <Style TargetType="{x:Type Button}">
            <Setter Property="Height" Value="23"></Setter>
            <Setter Property="Width" Value="75"></Setter>
            <Setter Property="Background" Value="DarkCyan"></Setter>
        </Style>
        <Style TargetType="{x:Type TextBox}">
            <Setter Property="Height" Value="23"></Setter>
            <Setter Property="Width" Value="180"></Setter>
            <Setter Property="Background" Value="Azure"></Setter>
        </Style>
    </Application.Resources>
</Application>

Then the buttons get the style applied!!! This seems really weird to me. Does anyone know if I am missing something?

Answer 1

I can’t tell you with certainty why that behavior occurs but I can tell you that best practices would have steered you away from using App.xaml in that fashion. What I believe to be a better practice is to only merge in your resource dictionaries in app.xaml. Storing the actual styles is a good way to create an unmanageable project.

Create a new ResourceDictionary file and add those styles. Merge the dictionaries in as you add more of them.

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

Alternatively, you could hook into the event Startup (app.xaml: Startup="App_Startup") and not override OnStartup. This will work with the App.xaml resource setup you have defined. It is likely a timing issue.