NullReferenceException when adding value to session

Question

I need to store a collection of strings in session, and I have a web method that I'm calling through a jQuery ajax call:

[WebMethod]
public string AddIdToSession(string userId)
{
    List<string> userIds = new List<string>();

    if (HttpContext.Current.Session != null && 
        HttpContext.Current.Session["userIds"] != null)
    {
        userIds = (List<string>)Session["userIds"];
        userIds.Add(userId);
        HttpContext.Current.Session["userIds"] = userIds;
    }
    else
    {
        userIds.Add(userId);
        HttpContext.Current.Session.Add("userIds", userIds);  // error here
    }

    return userId;
}

I'm getting an error when I try to add the id to session:

Object reference not set to an instance of an object.

Am I doing this wrong?

mellamokb · Accepted Answer · 2013-01-21 18:30:14Z

5

You need to explicitly enable session access using the EnableSession property:

[WebMethod(EnableSession = true)]

Note that your else logic will fire in the case HttpContext.Current.Session == null, which is likely the cause of the NullReferenceException.

answered Jan 21, 2013 at 18:30

mellamokb

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Comments

Shai Cohen · Accepted Answer · 2013-01-21 18:29:47Z

0

You need to change [WebMethod] to [WebMethod(EnableSession = true)] in order to access the current session.

Then you can access the current session like so:

HttpContext.Current.Session

answered Jan 21, 2013 at 18:29

Shai Cohen

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Comments

jon · Accepted Answer · 2013-01-21 18:31:05Z

0

You need to enable session for the web method, change the attribute to

[WebMethod(EnableSession = true)]

answered Jan 21, 2013 at 18:31

jon

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