Sample content to try and load content of an attach property outside the UI thread

gistfile1.txt

// Class to be inside the WPF class 
public static class CustomClass
{
    public static readonly DependencyProperty TextProperty = DependencyProperty.RegisterAttached(
        "Text",
        typeof(string),
        typeof(CustomClass),
        new UIPropertyMetadata(default(string)));

    public static string GetText(DependencyObject obj)
    {
        return (string)obj.GetValue(TextProperty);
    }

    public static void SetText(DependencyObject obj, string text)
    {
        obj.SetValue(TextProperty, text);
    }
}


// XAML file inside the WPF project 

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:system="clr-namespace:System;assembly=mscorlib"
    x:Class="ComplexWpfApplication.TestingAttachedProperties"
    Width="500"
    Title="Atached Properties">
        <TextBlock CustomClass.Text="I should be able to read the text value"/>
</Window>


// Method use to load assembly up to reading the content of the Text property defined in the AttachProperty 

var assembly = Assembly.LoadFrom(Path.Combine(assemblyLocation, "SomeAssembly" + ".dll"));
foreach(var uiElementType in assembly.assembly.GetTypes().Where(x => typeof(UIElement).IsAssignableFrom(x)))
{
   BindingFlags bindingFlags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic;
   ConstructorInfo[] constructors = typeToInstantiate.GetConstructors(bindingFlags);

   // Some checks before instanciation

   var uiElement = (UIElement)Activator.CreateInstance(typeToInstantiate, true);

   foreach(var child in LogicalTreeHelper.GetChildren(uiElement)<UiElement>())
   {
       if(uiElemetInstance is TextBlock tBlock && !string.IsNullOrEmpty(CustomClass.GetText(tBlock))
       {
           //Great... I can read the attach property content 
       }
   }
}