Include this
xmlns:system="clr-namespace:System;assembly=mscorlib"
Have a resource of system:string
like this.
<Window.Resources>
<system:String x:Key="GreetingText">Hello</system:String>
</Window.Resources>
and use it in xaml as
<TextBlock Text="{StaticResource GreetingText}" />
and use it in code behind as
string s = (string)objectofMainWindow.Resources["GreetingText"];
Edit: Answer to your comment
Its this way. Resource Dictionary is inside Window.Resources
<Window
xmlns:system="clr-namespace:System;assembly=mscorlib"
Your Rest namespaces
/>
<Window.Resources>
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:ATTFamilyMap.strings">
<system:String x:Key="GreetingText">Hello</system:String>
</ResourceDictionary>
</Window.Resources>
Your Code
</Window>