gistfile1.cs

public interface IStatefulComponent
{
    StateBag ViewState { get; }
}

public class ViewStateField
{
    public static ViewStateField<T> Create<T>(StateBag viewState, string key)
    {
        return new ViewStateField<T>(() => viewState, key);
    }

    public static ViewStateField<T> Create<T>(Func<StateBag> viewState, string key)
    {
        return new ViewStateField<T>(viewState, key);
    }

    public static ViewStateField<T> Create<T>(IStatefulComponent holder, string key)
    {
        return new ViewStateField<T>(() => holder.ViewState, key);
    }

    public static ViewStateField<T> Create<T>(StateBag viewState, string key, T @default)
    {
        return new ViewStateField<T>(() => viewState, key, @default);
    }

    public static ViewStateField<T> Create<T>(Func<StateBag> viewState, string key, T @default)
    {
        return new ViewStateField<T>(viewState, key, @default);
    }

    public static ViewStateField<T> Create<T>(IStatefulComponent holder, string key, T @default)
    {
        return new ViewStateField<T>(() => holder.ViewState, key, @default);
    }
}

public class ViewStateField<T> : ViewStateField
{
    private readonly Func<StateBag> _viewStateAccessor;
    private readonly string _key;
    private readonly T _default;

    public ViewStateField(Func<StateBag> viewStateAccessor, string key)
    {
        _key = key;
        _viewStateAccessor = viewStateAccessor;
    }

    public ViewStateField(Func<StateBag> viewStateAccessor, string key, T @default)
    {
        _key = key;
        _default = @default;
        _viewStateAccessor = viewStateAccessor;
    }

    public T Value
    {
        get
        {
            var o = ViewState[_key];
            if (ReferenceEquals(o, null))
                return _default;
            return (T) o;
        }
        set { ViewState[_key] = value; }
    }

    public bool IsDirty
    {
        get { return ViewState.IsItemDirty(_key); }
        set { ViewState.SetItemDirty(_key, value); }
    }

    private StateBag ViewState
    {
        get { return _viewStateAccessor(); }
    }
}