public static class DatabaseLayer
  {
    public static string Dbloc { get; set; } = "Data Source=test.db";
    public static ObservableCollection<Bills> GetAllBillsFromDB()
    {
      try
      {
        using (SQLiteDataAdapter sda = new SQLiteDataAdapter("SELECT * FROM billdata", Dbloc))
        {
          DataTable dt = new DataTable();
          sda.Fill(dt);
          var Bill = new ObservableCollection<Bills>();

          foreach (DataRow row in dt.Rows)
          {

            var p = (row["PaidOn"] == DBNull.Value) ? String.Empty : (string)(row["PaidOn"]);
            var q = (row["Remarks"] == DBNull.Value) ? String.Empty : (string)(row["Remarks"]);

            var obj = new Bills()
            {
              Id = Convert.ToInt32(row["Id"]),
              Party = (string)row["Party"],
              BillNo = (string)row["BillNo"],
              BillDt = (string)(row["BillDt"]),
              Amt = (string)(row["Amt"]),
              DueDt = (string)(row["DueDt"]),
              PaidOn = p,
              Remarks = q

            };
            Bill.Add(obj);
          }
        return Bill;
        }
      }
      catch (Exception ex)
      {
        throw ex;
      }
    }
...