Exception: Cannot find resource named '*'. Resource names are case sensitive - wpf

Question

But why. I see to be correct how the file works:
Error in IFocus.xaml but the converter is defined before.
I don't understant what is wrong.
Referance: Modern.xaml
Is a referance from another project. and i like this.
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:c="clr-namespace:Modern.Converters">
<SolidColorBrush x:Key="C_FocusBush" Color="Red"/>
<c:ThicknessConverter x:Key="ThicknessConverter"/>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="Interfaces/IFocus/IFocus.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
The IFocus.xaml
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:i="clr-namespace:Modern.Interfaces">
<Style TargetType="{x:Type i:IFocus}" x:Key="{x:Type i:IFocus}">
<Setter Property="BorderBrush" Value="{DynamicResource C_FocusBush}"/>
<Setter Property="BorderThickness" Value="2"/>
<Setter Property="Padding" Value="2"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type i:IFocus}">
<Grid Margin="{TemplateBinding Padding}">
<Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness, Converter={StaticResource ThicknessConverter}"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ResourceDictionary>
The main app to include all resources:
<Application x:Class="*.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
StartupUri="MainWindow.xaml">
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/Modern;component/Modern.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
</Application>
The brush works fine but the Converter not, why not?

Answer 1

I encountered an error format the same like this after I changed my Visual Studio theme to custom (mine's OneMonokai). I just reverted it back to the default theme and surprisingly it was resolved. It may seem far from the question but you could try this fix if you want.

Answer 2

Since it is the Style in IFocus.xaml that references a Brush resource in Modern.xaml, it's IFocus.xaml that should merge Modern.xaml and not the other way around:
Modern.xaml:
<ResourceDictionary ...>
<SolidColorBrush x:Key="C_FocusBush" Color="Red"/>
</ResourceDictionary>
IFocus.xaml:
<ResourceDictionary ...>
<Style ...>
<Setter Property="BorderBrush" Value="{StaticResource C_FocusBush}"/>
</Style>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="../../Modern.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
App.xaml:
<ResourceDictionary Source="pack://application:,,,/Modern;component/Interfaces/IFocus/IFocus.xaml"/>
Alternatively, you can create a separate resource dictionary with all brushes and merge this one and the one with the styles into App.xaml or another resource dictonary.
You may also want to see my answer here for more information about the order in which resource dictionaries are loaded.