WPF global styles problem with .Net4

I have just changed my WPF application from .Net3.5 to .Net4. Doing this caused all my global styles to stop working. Only the styles explicitly set using a key did work. I've done some research and figured out what causes this, and reproduced it in a simple app.

I have a simple WPF app containing only a button with text - no style or anything else. I define a style for all buttons in the ResourceDictionary of App.Xaml:

<Style TargetType="{x:Type Button}">
    <Setter Property="Background" Value="Red"></Setter>
</Style>

My button is now Red - all fine. I now move this to a separate ResourceDictionary in a separate project. This is where I want to hold all my shared styles. The button is still red, and my reference from App.xaml to SharedStyl开发者_Go百科es.xaml looks like this:

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>              
           <ResourceDictionary Source="/StyleLib;component/SharedStyles.xaml"/> 
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>

Now - I want SharedStyles.xaml of StyleLib to hold all specific style definitions, so I create a new file in the same project called ButtonStyles.xaml, and I add the resource there. Actually - I add another style too with a key to be used explicitly (technically I added this later, so this does not have anything to do with the problem that occurs).

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Style TargetType="{x:Type Button}">
        <Setter Property="Background" Value="Red"></Setter>
    </Style>

    <Style x:Key="Explicit" TargetType="{x:Type Button}">
        <Setter Property="Background" Value="Blue"></Setter>
    </Style>    
</ResourceDictionary>

ButtonStyles.xaml is referenced from SharedStyles.xaml:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="/StyleLib;component/ButtonStyles.xaml"/>
    </ResourceDictionary.MergedDictionaries>   
</ResourceDictionary>

Now - my button control is not styled any more. Actually - it is still shown as styled in the preview-window in VS2010, but when I run the application they aren't styled. If I explicitly reference the style with key "Explicit" they get this style - so the file is successfully included.

Another funny thing is that if I now add another style in SharedStyle.xaml - e.g. a global style for StackPanel (which was what I tried) - then the global style inside ButtonStyle.xaml magically starts working..!

My question now is if I'm doing something wrong, or if this sounds like a bug in .Net4? Sounds like a bug to me.. This did work just fine in .Net3.5!

I find this very odd. This did work just fine in .Net3.5, and I can see no reason why it shouldn't work now!

I reported this as a bug to Microsoft. You can view the bug report here. When they confirm the file the source code for the solution with the isolated problem should also be available there.