ASP.NET const string session key lost on PostBack

Question

My session key is a const string variable. See below.

On first Page Load, I add a string to the session using this key. I also indicate to KeepAlive on the key on the first load and every PostBack. However, on PostBack, I notice that the key is no longer in the session.

I found that to fix this, I simply have to remove "const" from the variable, and everything works fine.

Can someone explain and provide any educational resources on why this is happening.

private const string ENTITY_KEY = "c335a928-72ac-4403-b5f8-418f1e5ac1ec";

public string CurrentEntity
{
    get { WebClientSession.Current[ENTITY_KEY] as string); }
    set { WebClientSession.Current.AddTransient(ENTITY_KEY, value); }
}

protected void Page_Load(object sender, System.EventArgs e)
{
    string key = (string)Request["Id"] + "";
    CurrentEntity = Mapper.Lookup.FindById(key);

    WebClientSession.Current.KeepAlive(ENTITY_KEY);
}

private void _bindGrid()
{
    ...
    // CurrentEntity is null here on PostBack. Good on first load.
    ...
}

Answer 1

I am not sure what WebClientSession is but the HttpSessionState will work with const . There is no reason why it should not work. Here is the proof that it will work:

private const string ENTITY_KEY = "c335a928-72ac-4403-b5f8-418f1e5ac1ec";

protected void Page_Load(object sender, EventArgs e) {
      if( !this.IsPostBack ) {

      Session.Add( "ENTITY_KEY", ENTITY_KEY );
   }

}

protected void Button1_Click(object sender, EventArgs e) {
   string s = Session[ "ENTITY_KEY" ].ToString();
}

I simply added a button to my form. In the load method if the page is being requested I added a const variable's contents to the Session . In the click handler of the button, which is the form being posted, I access it from the Session and it is there.

So why is it not working for you?

There are 2 possible reasons:

Reason 1

The issue is in your WebClientSession class. I do not know the details of that class so cannot say what the issue is.

Reason 2

Session is stored in memory on the server. So, if this site is deployed on a farm, it is possible that the server which served the page initially added the ENTITY_KEY to Session . But when the page is posted back on the button click, another server serves the request. This server may not have the ENTITY_KEY in its memory since it is possible it has never served that page yet. In a web farm, you would want to use another source to store session related data such as a database or a file etc.