Question

I’m a beginner at ASP.net, and want to create a class file that contains common used methods across my application. Like the one here that hides certain ButtonLink‘s in the login.aspx and registration.aspx pages.

But when i start either one of these pages i get this error :

Object reference not set to an instance of an object.

Here’s my code :

Helper.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace WebApplication1
{
    public class Helper : System.Web.UI.Page
    {
        public void hideLinks(){
            // error is produced at the following line at the start of
            // login.aspx or registration.aspx pages.
            LinkButton profile = (LinkButton)Master.FindControl("LinkButton1");
            LinkButton logout = (LinkButton)Master.FindControl("LinkButton2");

            profile.Visible = false;
            logout.Visible = false;
        }        
    }
}

login.aspx.cs & registration.aspx.cs:

void Page_PreInit(object sender, EventArgs e)
{
   //LinkButton profile = (LinkButton)Master.FindControl("LinkButton1");
   //LinkButton logout = (LinkButton)Master.FindControl("LinkButton2");

   //profile.Visible = false;
   //logout.Visible = false;
   Helper master_helper = new Helper();
   master_helper.hideLinks();
}

Answer 1

Instantiating a new Helper (that is, a new System.Web.UI.Page) doesn’t exist as part of the full page request so it doesn’t have a reference to the same Master (if it even has one at all). Instead, redesign your Helper to take a Master (or the Page) in:

public class Helper
{
    private Page AssociatedPage;

    public Helper(Page page)
    {
        this.AssociatedPage = page;
    }

    public void hideLinks(){
        // error is produced at the following line at the start of
        // login.aspx or registration.aspx pages.
        LinkButton profile = (LinkButton)AssociatedPage.Master.FindControl("LinkButton1");
        LinkButton logout = (LinkButton)AssociatedPage.Master.FindControl("LinkButton2");

        profile.Visible = false;
        logout.Visible = false;
    }        
}

Then your usage might look like:

void Page_PreInit(object sender, EventArgs e)
{
   Helper master_helper = new Helper(this);
   master_helper.hideLinks();
}

You could also redesign the method to be static and simply pass in the Page (or Master) reference in with the hideLinks method as an argument, but this is up to you.