How to wrap the ASP.net session state

Update:

• The Sourcecode is now “copy & pastable”.
• I wrote an newer version which has more dependencies, but is suitable for UnitTesting (How to make ASP.net session state testable)

After reading Why Should You Wrap Your ASP.NET Session Object by Shahar A I thought the Community is ready for another step of encapsulation – and this is what I want to share.

We all know about the problems storing data in untyped dictionaries using some key, so I spare you.

This is how I want to store and retrieve session variables:

public class MySessionVariables
{
  // just an example, pls use Asp.Net Membership instead…
  public static readonly SessionVariable<string[]> UserGroups
    = new SessionVariable<string[]>(“UserGroups”);
}

// Fill the UserGroups after Login, for example

public void DeleteDatabase()
{
  if (MySessionVariables.UserGroups.HasValue &&
      MySessionVariables.UserGroups.Value.Contains(“Admin”))
    MyApplication.DeleteDatabase();
}

I would really like to not have a key to store the value, but I couldn't avoid. I thought about creating a new Guid per class instance. But this won't work when you run multiple Applications using some network session state. If someone has an idea on this, please share!

Initializer

Sometimes I need a dictionary or a List<T> to cache some values. So in order to avoid checking wether my SessionVariable has any value before accessing it I would like it to be initialized automatically on first use.

public class MySessionVariables
{
  public static readonly SessionVariable<List<string>> MarkedArticles
    = new SessionVariable<List<string>>(“MarkedArticles”, () => new List<string>());
}

If you're not using C# 3 yet, you could easily add a delegate instead of using Func<T>.

   1: public delegate T CreateInitialValueHandler();

 

Additionally I added a property called ValueOrDefault that gives back default(T) if no value is assigned yet, while Value throws an exception if neither a value nor an initializer is available.

Free Source Code

/// <summary>
/// Wrapper class for <see cref=”HttpContext.Session”/>.
/// Author: Lars Corneliussen
/// Source: http://startbigthinksmall.wordpress.com/2008/05/14/how-to-wrap-the-aspnet-session-state/
/// </summary>
/// <typeparam key=”T”>
/// The type of the value to be stored.
/// </typeparam>
public class SessionVariable<T>
{
  private readonly string key;
  private readonly Func<T> initializer;

  /// <summary>
  /// Initializes a new session variable.
  /// </summary>
  /// <param name=”key”>
  /// The key to use for storing the value in the session.
  /// </param>
  public SessionVariable(string key)
  {
    if (string.IsNullOrEmpty(key))
      throw new ArgumentNullException("key");

    this.key = GetType() + key;
  }

  /// <summary>
  /// Initializes a new session variable with a initializer.
  /// </summary>
  /// <param name=”key”>
  /// The key to use for storing the value in the session.
  /// </param>
  /// <param name=”initializer”>
  /// A function that is called in order to create a
  /// default value per session.
  /// </param>
  public SessionVariable(string key, Func<T> initializer) : this(key)
  {
    if (initializer == null)
      throw new ArgumentNullException("initializer");

    this.initializer = initializer;
  }


  private object GetInternalValue(bool initializeIfNessesary)
  {
    HttpSessionState session = CurrentSession;

    var value = session[key];

    if (value == null && initializeIfNessesary
      && initializer != null)
      session[key] = value = initializer();

    return value;
  }

  private static HttpSessionState CurrentSession
  {
    get
    {
      var current = HttpContext.Current;

      if (current == null)
        throw new InvalidOperationException(
          "No HttpContext is not available.");

      var session = current.Session;
      if (session == null)
        throw new InvalidOperationException(
          "No Session available on current HttpContext.");
      return session;
    }
  }

  /// <summary>
  /// Indicates wether there is a value present or not.
  /// </summary>
  public bool HasValue
  {
    get { return GetInternalValue(false) != null; }
  }

  /// <summary>
  /// Sets or gets the value in the current session.
  /// </summary>
  /// <exception cref=”InvalidOperationException”>
  /// If you try to get a value while none is set.
  /// Use <see cref=”ValueOrDefault”/> for safe access.
  /// </exception>
  public T Value
  {
    get
    {
      object v = GetInternalValue(true);

      if (v == null)
        throw new InvalidOperationException(
          "The session does not contain any value for ‘"
          + key + "‘.");

      return (T) v;
    }
    set { CurrentSession[key] = value; }
  }

  /// <summary>
  /// Gets the value in the current session or if
  /// none is available <c>default(T)</c>.
  /// </summary>
  public T ValueOrDefault
  {
    get
    {
      object v = GetInternalValue(true);

      if (v == null)
        return default(T);

      return (T)v;
    }
  }

  /// <summary>
  /// Clears the value in the current session.
  /// </summary>
  public void Clear()
  {
    CurrentSession.Remove(key);
  }
}

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