HttpSession.cs

public abstract class HttpSession : IHttpSession
{
  protected IKeyValueStore storage;
  
  public virtual int UserId
  {
    get { return Get<int>("user_id"); }
    set { storage["user_id"] = value; }
  }
  
  public virtual void ResetSession()
  {
    UserId = 0;
  }
  
  protected virtual T Get<T>(string key)
  {
    try
    {
      var obj = storage[key];
      return (T)obj;
    }
    catch (Exception)
    {
      // This should really catch KeyNotFoundException, InvalidCastException, etc., 
      // but I am being lazy for demo purposes.
      return default(T);
    }
  }
}

HttpSessionKeyValueStore.cs

public class HttpSessionKeyValueStore : IKeyValueStore<string, object>
{
  private readonly HttpSessionStateBase session;
  
  public HttpSessionKeyValueStore(HttpSessionStateBase session)
  {
    if (session == null)
      throw new ArgumentNullException("session");
    
    this.session = session;
  }
  
  public IEnumerable<string> Keys
  {
    get
    {
      foreach (var key in session.Keys)
        yield return key.ToString();
    }
  }
  
  public object this[string key]
  {
    get { return session[key]; }
    set { session[key] = value; }
  }
}

IKeyValueStore.cs

public interface IKeyValueStore<TKey, TValue>
{
  IEnumerable<TKey> Keys { get; }
  TValue this[TKey key] { get; set; }
}

InMemoryHttpSession

public class InMemoryHttpSession : HttpSession
{
  public InMemoryHttpSession()
  {
    this.storage = new InMemoryKeyValueStore();
  }
}

InMemoryKeyValueStore.cs

public class InMemoryKeyValueStore : IKeyValueStore<string, object>
{
  private readonly Dictionary<string, object> dictionary = new Dictionary<string, object>();
  
  public IEnumerable<string> Keys
  {
    get { return dictionary.Keys; }
  }
  
  public object this[string key]
  {
    get { return dictionary[key]; }
    set { dictionary[key] = value; }
  }
}

WebHttpSession

public class WebHttpSession : HttpSession
{
  private readonly HttpSessionStateBase session;

  public WebHttpSession(HttpSessionStateBase session)
  {
    if (session == null)
      throw new ArgumentNullException("session");

    this.session = session;
    this.storage = new HttpSessionKeyValueStore(session);
  }

  public override void ResetSession()
  {
    session.Clear();
    base.ResetSession();
  }
}