Why do I get "Object reference not set to an instance of an object." when changing pages in asp.net

I have a master page that set ups some variables that I want to use across the site..

protected void Page_Load(object sender, EventArgs e)
{
    //Get users name from AD
    str_DomainName = HttpContext.Current.User.Identity.Name;
    str_CurrentLogin = str_DomainName.Substring(5);

    //Display current user information
    DirectorySearcher search = new DirectorySearcher("LDAP://DCHS");
    search.Filter = String.Format("(SAMAccountName={0})", str_CurrentLogin);
    SearchResult result = search.FindOne();
    DirectoryEntry entry = result.GetDirectoryEntry();
    lbl_CurrentUser.Text = result.Properties["givenName"][0].ToString() + ' ' + result.Properties["sn"][0].ToString();

    // Get SID
    IntPtr logonToken = WindowsIdentity.GetCurrent().Token;
    WindowsIdentity windowsId = new WindowsIdentity(logonToken);

    //Set session variabls
    this.CurrentFirstName = result.Properties["givenName"][0].ToString();
    //this.CurrentEmail = result.Properties["mail"][0].ToString();
    //this.CurrentSID = windowsId.User.ToString();
    //this.CurrentUserName = str_CurrentLogin;
    //this.CurrentFullName = lbl_CurrentUser.Text;
    //this.CurrentDomain = str_DomainName;
    this.Session.Add("currentEmail", result.Properties["mail"][0].ToString(开发者_StackOverflow));
}

public String CurrentFirstName
        {
            get { return (String)ViewState["currentFirstName"]; }
            set { ViewState["currentFirstName"] = value; }
        }

I then call them in my defalut.aspx page as follows:

protected void Page_PreRender(object sender, EventArgs e)
        {
            //try
            //{
                lbl_FullName.Text = Master.CurrentFullName;
                lbl_SID.Text = Master.CurrentSID;
                testLabel.Text = Master.CurrentEmail;
            //}
            //catch (Exception ex)
            //{ }
        }

This works fine.. If I however navigate away from the default page, then I get the following error..

Object reference not set to an instance of an object.

One the lbl_FullName.Text = Master.CurrentFullName; line

If I uncomment the try catch then it works fine, but I don't believe that this is the correct method of avoiding the fault..

I'm only new to ASP, so be nice..

EDIT:

The variabes are being set in Master.cs as follows.

public String CurrentUserName
        {
            get { return (String)ViewState["currentUserName"]; }
            set { ViewState["currentUserName"] = value; }
        }

---

A few questions:

• Which side of the expression is generating the error: this.lbl_FullName or this.Master?
• If it's the former, then there's something funky going on in your page.aspx.designer.cs file. Make sure the label control has been declared in there (this file is automatically generated by visual studio, but can sometimes not update properly). You should see a single line like this:

protected global::System.Web.UI.WebControls.Label lbl_FullName;

• If it's the this.Master property then your page is obviously not referencing your masterpage. Check the page directive (the top line of your .aspx file) and make sure that the DynamicMasterPage value is set and that the path is correct.

---

On a design note, the masterpage isn't the best place to "store variables". The masterpage should literally just be used to initialise the common parts of the page across your site.

If you need to gather information from a user and use it across several pages, use session variables. This is a method of storing objects "in memory" for as long as the user's browser is open.

Items can be added to the session as follows:

this.Session.Add("fullName", fullName);

Items can then be retrieved later from any other page / usercontrol in your site as follows:

string fullName = (string)this.Session["fullName"];

---

ViewState is per-page. When you navigate to the new page, it isn't set.

Your master page should put these into Session instead.