Question

If i use MergedDictionaries multiple times to define styles, it doesn’t work at runtime, but in WPF Designer of VS2010 it works. It also work if load MergedDictionaries using code at runtime.

Why this is happening ? Is it my problem only or ? And how to solve it ?

I am using WPF4 and loading themes/styles from an assembly at application level.

Not working

<!--Application.xaml-->
 <Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="pack://application:,,,/Lib;component/Themes/Theme.xaml" />
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>


<!--Theme.xaml-->
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<ResourceDictionary.MergedDictionaries>
    <ResourceDictionary Source="Theme/Shared.xaml" />
    <ResourceDictionary Source="Theme/Button.xaml" />
</ResourceDictionary.MergedDictionaries>

Working

<Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="pack://application:,,,/Lib;component/Themes/Theme/Shared.xaml" />
                <ResourceDictionary Source="pack://application:,,,/Lib;component/Themes/Theme/Button.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>

Answer 1

See the answer to this question. I have a feeling you’re having the same problem

This is an optimization bug, see this link

On the creation of every object in
XAML, if a default style is present
(i.e. style w/ a key of Type) that
style should be applied. As you can
imagine there are several performance
optimizations to make that (implied)
lookup a light weight as possible. One
of them is that we don’t look inside
Resource Dictionaries unless they are
flagged as “containing default
Styles”. There is a bug: if all your
default styles are nested in merged
dictionaries three levels deep (or
deeper) the top dictionary does not
get flagged so the search skips it.
The work around is to put a default
Style to something, anything, in the
root Dictionary.

So adding a dummy style to the root dictionary fixes this. Example

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="pack://application:,,,/Lib;component/Themes/Theme.xaml" />
        </ResourceDictionary.MergedDictionaries>
        <!-- Dummy Style, anything you won't use goes -->
        <Style TargetType="{x:Type Rectangle}" />
    </ResourceDictionary>
</Application.Resources>