Consider how would you implement Style object in the HTML DOM?

These are some characteristics of that object:

• It has a long list of properties, e.g. in IE 11 there are more than 300 properties over a style object.
• Any specific instance usually have only several properties assigned.
• Reads of properties are much more frequent than writes. In fact style often stays unchanged after initialization.
• DOM contains many style instances (often thousands).
• The number of distinct instances in terms of values of properties is moderate (usually dozens).

Here is how would we approached to such an object.

1. Styles are sparse objects, thus there is no point to implement plain class with all those properties, as it's wasteful.

We would rather use two techniques to keep style's state:

• A dictionary of properties with their values;
• An aggregation of objects, where all properies are grouped into families, each group is defined by a separate type, and a style's state is an aggregation of that groups.

A current style of an element is an aggregation of styles of ancestor element. It can either by dynamic or be fused into a single style instance.

2. Make style's state immutable, and share all these states among all style instances.

In this implementation property write turns into a state transition operation: state = set(state, property, value). Thus no state is modified but replaced with other state that corresponds to a required change.

If state is seen as a dictionary then API may look like this :

public class State<K, V>
{
  // Gets shared dictionary for an input dictionary.
  public IDictionary<K, V> Get(IDictionary<K, V> dictionary);

  // Gets a shared dictionary for an input dictionary with key set to a value.
  public IDictionary<K, V> Set(IDictionary<K, V> dictionary, K key, V value);

  // Gets a shared dictionary for an input dictionary.
  public IDictionary<K, V> Remove(IDictionary<K, V> dictionary, K key);

  // Gets typed value.
  public T Get<T>(IDictionary<K, V> dictionary, K key)
    where T: V
  {
    V value;

    if ((dictionary == null) || !dictionary.TryGetValue(key, out value))
    {
      return default(T);
    }

    return (T)value;
  }

  // Sets or removes a typed value.
  // dictionary can be null.
  // null returned if output dictionary would be empty.
  public IDictionary<K, V> Set<T>(IDictionary<K, V> dictionary, K key, T value)
    where T : V
  {
    return value == null ? Remove(dictionary, key) :
      Set(dictionary, key, (V)value);
  }
}

States can be cached. Provided the cache keeps states in a weak way, no unsued state will be stored for a long time. We may use weak table of dictionary to dictionary WeakTable<Dictionary<K, V>, Dictionary<K, V>> as a storage for such a cache. All required API is described in the WeakTable and Hash Code of Dictionary posts.

3. Style can be implemented as a structure with shared state as a storage. Here is a scetch:

[Serializable]
public struct Style
{
  // All properties.
  public enum Property
  {
    Background,
    BorderColor,
    BorderStyle,
    Color,
    FontFamily,
    FontSize,
    // ...
  }

  public int? Background
  {
    get { return states.Get<int?>(state, Property.Background); }
    set { state = states.Set(state, Property.Background, value); }
  }

  public int? BorderColor
  {
    get { return states.Get<int?>(state, Property.BorderColor); }
    set { state = states.Set(state, Property.BorderColor, value); }
  }

  public string BorderStyle
  {
    get { return states.Get<string>(state, Property.BorderStyle); }
    set { state = states.Set(state, Property.BorderStyle, value); }
  }

  public int? Color
  {
    get { return states.Get<int?>(state, Property.Color); }
    set { state = states.Set(state, Property.Color, value); }
  }

  public string FontFamily
  {
    get { return states.Get<string>(state, Property.FontFamily); }
    set { state = states.Set(state, Property.FontFamily, value); }
  }

  public double? FontSize
  {
    get { return states.Get<double?>(state, Property.FontSize); }
    set { state = states.Set(state, Property.FontSize, value); }
  }

  // ...

  [OnDeserialized]
  private void OnDeserialized(StreamingContext context)
  {
    state = states.Get(state);
  }

  // A state.
  private IDictionary<Property, object> state;

  // A states cache.
  private static readonly State<Property, object> states =
    new State<Property, object>();
}

Note that:

•  default state is a null dictionary;
• states are application wide shared.

The following link is our implementation of State<K, V> class: State.cs.

Here we have outlined the idea of shared state object, and how it can be applied to sparse mostly immutable objects. We used HTML style as an example of such an object. Shared state object may work in many other areas, but for it to shine its use case should fit to the task.