Cannot access resource defined in app.xaml

I am using Visual Studio 2010 RC1.

I define a resource "Brush2" in app.xaml_:

<Application x:Class="VideoThumbnails.App"
             xmlns="http://schemas.microsof开发者_开发技巧t.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">
    <Application.Resources>

        <RadialGradientBrush x:Key="Brush2" RadiusX="1" RadiusY="1" GradientOrigin="0.3,0.3">
            <GradientStop Color="White" Offset="0"/>
            <GradientStop Color="#ffc0c0" Offset="1"/>
        </RadialGradientBrush>

    </Application.Resources>
</Application>

In my Mainwindow I am trying to use that resource:

...
<Border Margin="4,2" BorderBrush="Black" BorderThickness="2" CornerRadius="4"
        ToolTip="{Binding Path=FullPath}" HorizontalAlignment="Stretch"
        Background="{StaticResource Brush2}">
...

No matter what I do it always raises an exception at runtime (Resource not found). I changed build action without success.

How can I use resources defined in app.xaml?

If you've set the Startup Object to a custom class you need to create the custom Application class and also call its InitializeComponent method, like this:

App app = new App();
app.InitializeComponent();

Update: As @qqww2 suggested the InitializeComponent call can be moved inside the App class constructor.

I start my application not with a StartupUri in App.xaml, but with a event handler in App.xaml.cs.

When using override Startup() resources from App.xaml are not loaded:

public partial class App
{
    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);
        ...
    }
}

But when using event Startup the resources are loaded just fine:

<Application x:Class="OxyplotTest.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             Startup="OnStartup">

And code behind:

public partial class App
{
    private void OnStartup(object sender, StartupEventArgs e)
    {
        ...
    }
}

Nothing you have done is incorrect. You either have 1) screwed up the project build somehow while randomly doing things to try to get it to work or 2) something else is going on here and we'll never know without the exception details.

I would highly suggest you try to repro this in a fresh brand new WPF project. Do the following steps (and ONLY the following steps):

Create a new WPF project, add the exact same brush to app.xaml, then open Window1 and bind the window's background to the resource. Run the app.

It should work as expected. If not, come back with the exception details. If it does, compare this new project with your current one to see what you are doing differently.

I know there is an already accepted answer, but I thought I would add my solution as well. I had code that was working, but some configuration change broke the resource references in the designer. In executing code, it worked fine.

After some initial research, I determined that the BuildAction property for App.xaml should be set to ApplicationDefinition. My was set to Page. However, that causes some issues with multiple entry points. Main() was already defined in App.xaml.cs. The compile error was indicating another entry point in App.g.cs (which is an autogenerated file).

I ended up using the approach #3 described at http://www.infosysblogs.com/microsoft/2008/09/how_to_write_custom_main_metho.html. The basic idea is that you create a new class that is only responsible for startup. In my case, I named it Startup.cs. It should have code that is similar to this:

using System.Threading;

namespace MyNamespace
{
    public class Startup
    {
        [System.STAThreadAttribute()]
        private static void Main()
        {   
            var app = new App();
            app.InitializeComponent();
            app.Run();
        }
    }
}

Then in the project settings, change the Application -> Startup object so that your new class is selected.

I had a similar problem and solved it, so I figured I may as well post my solution. I kept getting the Resource not found error only at runtime as described above. In my Windows 8.1 c# App, I was using a style I had defined, and it showed up fine in Blend and the designer view, but didn't work at runtime. I was trying to use this style in a SettingsFlyout I had created following these instructions. After getting that to work, I set up a field in App.xaml to hold onto my flyouts (Preferences and ColorSettings) so I wouldn't be making a new one every time.

public static Preferences preferences;
public static ColorSettings colorsettings;

public App()
{
    this.InitializeComponent();
    this.Suspending += OnSuspending;
    preferences = new Preferences();
    colorsettings = new ColorSettings();
}

After poking around and googling for about an hour, I figured out that I was creating the flyouts too early, and when they were created they couldn't access the application's resources. So I moved their creation down to App.OnLaunched() and that solved the problem.

I'm not sure if that's the best way to go about things, but it worked. So, try to pinpoint where you're trying to access the resources you want, and if you're maybe trying too early. Sorry for the vagueness and maybe incorrectness, I'm really new to WPF.

I can concur that resources get messed up easily if you have something in App constructor. Move initialization of your own global objects into OnStartup method:

protected override void OnStartup(StartupEventArgs e)
{
}