ASP.NET

public static class SequentialNumbers
  {
    private static Object obj = new Object();
    private static UInt16 lastNumber = UInt16.MinValue;

    public static UInt16 GetNext()
    {
      lock (obj)
      {
        return ++lastNumber;
      }
    }
  }

/* Global.asax */
void Application_Start(object sender, EventArgs e)
{
    int id;
    int.TryParse(System.IO.File.ReadAllText(Server.MapPath("~/nextid.txt")), out id);
    Application["ID"] = id;
}

void Application_End(object sender, EventArgs e)
{
    // Delete the old file
    System.IO.File.Delete(Server.MapPath("~/nextid.txt"));
    // Write out the new one
    System.IO.File.WriteAllLines(Server.MapPath("~/nextid.txt"), new string[] { Application["ID"].ToString() });
}

/* Some publicly accessible class */
public int GetNextID()
{
    // Increase the id
    Application["ID"] = Convert.ToInt32(Application["ID"]) + 1;
    return (int)Application["ID"];
}

objDA.Fill(ds);
            DataColumn c1 = ds.Tables["category"].Columns["categoryID"];
            DataColumn c2 = ds.Tables["Item"].Columns["catID"];
            dRel = new DataRelation("categoryItem", c1, c2);
            ds.Relations.Add(dRel);
            Repeater1.DataSource = ds;
            Repeater1.DataBind();

 Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 1222:            objDA.Fill(ds);
Line 1223:
Line 1224:              DataColumn c1 = ds.Tables["category"].Columns["categoryID"];
Line 1225:           DataColumn c2 = ds.Tables["Item"].Columns["catID"];
Line 1226:            dRel = new DataRelation("categoryItem", c1, c2);

Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 1222:            objDA.Fill(ds);
Line 1223:
Line 1224:            DataColumn c1 = ds.Tables["category"].Columns["ID"];
Line 1225:            DataColumn c2 = ds.Tables["item"].Columns["catID"];
Line 1226:            dRel = new DataRelation("categoryItem", c1, c2);

void Repeater1_ItemDataBound(object sender, RepeaterItemEventArgs e)
       {
           RepeaterItem item = e.Item;
           if ((item.ItemType == ListItemType.Item) ||
               (item.ItemType == ListItemType.AlternatingItem))
           {

               Repeater2 = (Repeater)e.Item.FindControl("Repeater2");
               DataRowView drv = (DataRowView)item.DataItem;
               Repeater2.DataSource = drv.CreateChildView("categoryItem");
               Repeater2.DataBind();
           }
       }

objDA.Fill(ds);
            DataColumn c1 = ds.Tables["category"].Columns["ID"];
            DataColumn c2 = ds.Tables["Item"].Columns["catID"];
            dRel = new DataRelation("categoryItem", c1, c2);
            ds.Relations.Add(dRel);
            Repeater1.DataSource = ds;
            Repeater1.DataBind();

if(  $('#obj').is(":visible") == true )
{
  alert('div is visible');
}
else
{
  alert('div is hidden');
}