Define a string as a static resource

You can define it as an application resource:

 <Application x:Class="xxxxxx"
                 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                 xmlns:clr="clr-namespace:System;assembly=mscorlib"
                 StartupUri="MainWindow.xaml">
        <Application.Resources>
            <clr:String x:Key="MyConstString">My string</clr:String>
        </Application.Resources>
    </Application>