Question

I have this code

if (PreviousPage.IsPostBack)
    {
        if (Request.Form["username"] == ConfigurationManager.AppSettings["username"] && Request.Form["password"] == ConfigurationManager.AppSettings["password"])
        {
            Session["username"] = Request.Form["username"];
            using (var context = new mallEntities())
            {
                var countProducts = (from p in context.Products
                                     select p).Count();

                var countStores = (from p in context.Stores
                                   select p).Count();

                var countCategories = (from p in context.Categories
                                       select p).Count();

                Label3.Text = countProducts.ToString();
                Label2.Text = countStores.ToString();
                Label1.Text = countCategories.ToString();
            }
        }
        else
        {
            Response.Redirect("Default.aspx?invaild=true");
        }
    } else if(Session["username"] == null)
    {
        Response.Redirect("Default.aspx?session=false");
    }

and I got this error message:

Object reference not set to an instance of an object

at PreviousPage.IsPostBack

Why?

What is the problem?

Answer 1

I’m not sure why you’re using PreviousPage.IsPosback so I’ll guess this is a mistake. You should use Page.IsPostback

In case you indeed want to use PreviousPage (I would be stumped) take into account that it can be null if you access the page direclty.

When you use the Transfer method or cross-page posting to transfer processing from one ASP.NET page to another, the originating page contains request information that might be required for the destination page. You use the PreviousPage property to access that information

The PreviousPage property is a null reference (Nothing in Visual Basic) when the user requests that page directly from the server.