If you use styles (you DO use styles, right?) you may find that you cannot set the style of a Window or User Control to a static resource in XAML. Consider the following resource dictionary which contains the GreenWindowStyle style.
<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Style TargetType="Window" x:Key="GreenWindowStyle">
<Setter Property="Background" Value="Lime"/>
</Style>
</ResourceDictionary>
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Style TargetType="Window" x:Key="GreenWindowStyle">
<Setter Property="Background" Value="Lime"/>
</Style>
</ResourceDictionary>
<Window x:Class="WindowStyle.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WindowStyle"
mc:Ignorable="d"
Style="{StaticResource GreenWindowStyle}"
Title="MainWindow" Height="450" Width="800">
<Window.Resources>
<ResourceDictionary Source="/Styles.xaml"/>
</Window.Resources>
<Grid>
</Grid>
</Window>
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WindowStyle"
mc:Ignorable="d"
Style="{StaticResource GreenWindowStyle}"
Title="MainWindow" Height="450" Width="800">
<Window.Resources>
<ResourceDictionary Source="/Styles.xaml"/>
</Window.Resources>
<Grid>
</Grid>
</Window>
The problem, of course, is that the reference to GreenWindowStyle is parsed before the reference to Styles.xaml. How can we put it after? Try this...
<Window x:Class="WindowStyle.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WindowStyle"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="800">
<Window.Resources>
<ResourceDictionary Source="/Styles.xaml"/>
</Window.Resources>
<Window.Style>
<Binding Source="{StaticResource GreenWindowStyle}"/>
</Window.Style>
<Grid>
</Grid>
</Window>
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WindowStyle"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="800">
<Window.Resources>
<ResourceDictionary Source="/Styles.xaml"/>
</Window.Resources>
<Window.Style>
<Binding Source="{StaticResource GreenWindowStyle}"/>
</Window.Style>
<Grid>
</Grid>
</Window>
Now the reference to GreenWindowStyle is after the reference to Styles.xaml so we get this beautiful window!