ASP / C＃会话变量 - 未将对象引用设置为对象的实例

Question

I'm somewhat new to ASP / C# and I'm having an issue (probably simple) with sessions variables. My project has a Site.Master in which session variables are set under the Page_Load method like so:

    protected void Page_Load(object sender, EventArgs e)
    {

        if ((Session)["UserID"]==null || (Session)["UserID"].ToString() == "")
        {
            (Session)["UserID"] = HttpContext.Current.User.Identity.Name.ToString();
            SqlDataReader dr = Sprocs.GetPermissionGroups();

            string groupList = "";

            while (dr.Read())
            {
            if (groupList != "")
                    {
                        groupList = groupList + "|" + dr["WG_Group"].ToString();
                    }
                    else
                    {
                        groupList = dr["WG_Group"].ToString();
                    }
            }
            dr.Close();

            if (groupList != "")
            {
                (Session)["UserGroups"] = groupList;
            }
        }

This does work. If I dump out the session variable 'UserGroups' to a label or something within this method, it does display the variable contents correctly.

So, my problem lies within another page (say default.aspx) when I try to access that same session variable. In the Page_Load method of the other page I attempt to do this:

    protected void Page_Load(object sender, EventArgs e)
    {
            string GroupList = HttpContext.Current.Session["UserGroups"].ToString();
            //some code with the variables here
    }

This always fails with an "Object reference not set to an instance of an object." error. Am I trying to get the Session variable wrong? I've tried

string GroupList = Session["UserGroups"].ToString(); 

this also errors with the same error.

string GroupList = (string)(Session["UserGroups"]); 

This always returns an empty string.

What am I doing wrong?

Thanks!

Answer 1

The syntax you are using in your Page_Load method I wouldn't have even expected to compile. Regardless, the issue is that you have not set the Session with that key, so it will return null . When you call ToString() on that null, you get the exception. In your second example:

string groupList = (string)(Session["UserGroups"])

That is doing a conversion of null to string , which results in a null string (thus not causing the exception).

You should be able to rewrite your Page_Load implementation like so:

string sessionUserId = Session["UserId"] as string;

if(string.IsNullOrEmpty(sessionUserId))
{
    Session["UserId"] = HttpContext.Current.User.Identity.Name.ToString();

    SqlDataReader dr = Sprocs.GetPermissionGroups();

    string groupList = "";

    while (dr.Read())
    {
        if (groupList != "")
        {
            groupList = groupList + "|" + dr["WG_Group"].ToString();
        }
        else
        {
            groupList = dr["WG_Group"].ToString();
        }
    }
    dr.Close();

    Session["UserGroups"] = groupList;
}

Then, when accessing the session variable later, do it like so:

 string userGroup = Session["UserGroups"] as string;

This is a safe way to attempt the conversion of whatever bucket that is in session into a string. If the key doesnt exist, or the value is not a string, you will get null . Otherwise, you will get the string from that hash.