A utility for importing separate xaml files while previewing it in xaml editor

FlatDictionary.cs

using System;
using System.ComponentModel;
using System.IO;
using System.Windows;
using System.Windows.Markup;

/// <summary>
/// A utility for importing separate xaml files while previewing it in xaml editor.
/// </summary>
// <example>
// <Window.Resources>
//   <ResourceDictionary>
//     <ResourceDictionary.MergedDictionaries>
//       <local:FlatDictionary Key = "Import" DesignSource="pack://application,,,/res/Import.xaml"/>
//     </ResourceDictionary.MergedDictionaries>
//   </ResourceDictionary>
// </Window.Resources>
public class FlatDictionary : ResourceDictionary, ISupportInitialize {

  // TODO: It seems to be able to support autocomplete by EndInit() and
  // LoadComponent(object, Uri), but have limitation from
  // Application.LoadComponent that cannot reference other assembly.

  private class Proxy { }

  private string _Key;
  public string Key {
    get { return _Key; }
    set {
      _Key = value;
      if (DesignSource != null) {
        Register();
      }
    }
  }

  private Uri _DesignSource;
  public Uri DesignSource {
    get { return _DesignSource; }
    set {
      _DesignSource = value;
      if (Key != null) {
        Register();
      }
    }
  }

  private void Register() {
    this[Key] = new Proxy();
  }

  protected override void OnGettingValue(object key, ref object value, out bool canCache) {
    if (value is Proxy) {
      value = LoadComponent(DesignSource);
      canCache = false;
    }
    else {
      base.OnGettingValue(key, ref value, out canCache);
    }
  }

  private object LoadComponent(Uri uri) {
    if (uri == null) {
      return null;
    }

    if (uri.OriginalString.Length >= 2 && uri.OriginalString[1] == ':') {
      using (var f = new FileStream(uri.OriginalString, FileMode.Open)) {
        return XamlReader.Load(f);
      }
    }

    Uri relUri;
    if (uri.OriginalString.StartsWith("pack:")) {
      relUri = new Uri($";component{uri.AbsolutePath}", UriKind.Relative);
    }
    else {
      relUri = uri;
    }
    return Application.LoadComponent(relUri);
  }
}