Question

I am using ADO.NET Entity framework for connect db and get data. I want if there is no data in object it will write “EMPTY FIELD” if it’s full than it will write to listview coloumn data from the db. I am getting ‘System.NullReferenceException’ error when there is a null object in objectcontext.Dont return a ” EMPTY ROW” string.

Here is my code:

   using (ITSEntities arama = new ITSEntities())
            {
                var sql = "SELECT VALUE s_tesis FROM ITSEntities.TB_SAGLIK_TESIS AS s_tesis WHERE s_tesis.TESIS_AD like @p1";
                ObjectQuery<TB_SAGLIK_TESIS> sorgu = new ObjectQuery<TB_SAGLIK_TESIS>(sql, arama).Include("TB_IL").Include("TB_TESIS_TIPI").Include("TB_TESIS_TURU");
                sorgu.Parameters.Add(new ObjectParameter("p1", String.Format("{0}%", btnAra.Text)));

                                   listTesis.Items.Clear();

                foreach (var item in sorgu)
                {

                    ListViewItem listitem = new ListViewItem { Text = item.KODU.ToString() };
                    listitem.SubItems.Add(item.TESIS_AD);
                    listitem.SubItems.Add(String.IsNullOrEmpty(item.TB_IL.ADI) ? "EMPTY ROW" : item.TB_IL.ADI);
                    listitem.SubItems.Add(String.IsNullOrEmpty(item.TB_TESIS_TIPI.TIP_AD) ? "EMPTY ROW" : item.TB_TESIS_TIPI.TIP_AD);
                    listitem.SubItems.Add(String.IsNullOrEmpty(item.TB_TESIS_TURU.TESIS_TURU) ? "EMPTY ROW" :item.TB_TESIS_TURU.TESIS_TURU);
                    listTesis.Items.Add(listitem);
                }
            }
        }
        catch (Exception ex)
        {

            MessageBox.Show(ex.InnerException.ToString());
        }

Answer 1

I don’t know EF, but you’re dereferencing 2 objects in the following lines:

ListViewItem listitem = new ListViewItem { Text = item.KODU.ToString() };
listitem.SubItems.Add(String.IsNullOrEmpty(item.TB_IL.ADI) ? "EMPTY ROW" : item.TB_IL.ADI);
listitem.SubItems.Add(String.IsNullOrEmpty(item.TB_TESIS_TIPI.TIP_AD) ? "EMPTY ROW" : item.TB_TESIS_TIPI.TIP_AD);
listitem.SubItems.Add(String.IsNullOrEmpty(item.TB_TESIS_TURU.TESIS_TURU

If the container object (KODU, TB_IL, TB_TESIS_TIPI. or TB_TESIS_TURU) is ever null, then you’d get a NullReferenceException.

My guess is that these are table names, and that some rows don’t have a corresponding JOIN to those tables. In any case, you’d probably need to rewrite those as:

ListViewItem listitem = new ListViewItem { Text = (item.KODU ?? "").ToString() };
listitem.SubItems.Add(
    (item.TB_TL == null || String.IsNullOrEmpty(item.TB_IL.ADI)) 
    ? "EMPTY ROW" : item.TB_IL.ADI
);

To make it a bit cleaner, a method:

string EmptyRowIfNull<T>(T o, Func<T, string> p) {
   string s;
   if (o != null) {
       s = p(o);
   }
   return string.IsNullOrEmpty(s) ? "EMPTY ROW" : s;
}

listitem.SubItems.Add(EmptyRowIfNull(item.TB_IL, t => t.ADI));
listitem.SubItems.Add(EmptyRowIfNull(item.TB_TESIS_TIPI, t => t.TIP_AD));
listitem.SubItems.Add(EmptyRowIfNull(item.TB_TESIS_TURU, t => t.TESIS_TURU));