Add project files.

UI/WPF/Markups/BindingProxy.cs

@@ -0,0 +1,21 @@
+using System.Windows;
+
+namespace TheXamlGuy.UI.WPF;
+
+public class BindingProxy : Freezable
+{
+    public static readonly DependencyProperty DataContextProperty =
+        DependencyProperty.Register(nameof(DataContext),
+            typeof(object), typeof(BindingProxy));
+
+    public object DataContext
+    {
+        get => GetValue(DataContextProperty);
+        set => SetValue(DataContextProperty, value);
+    }
+
+    protected override Freezable CreateInstanceCore()
+    {
+        return new BindingProxy();
+    }
+}