I really wish that were true. DBNull is in third-party libraries. And first- and second- too. Check out this code. Dot net really wants me to use DBNull. No database in play at all.

  var dt = new DataTable();
  dt.Columns.Add("name", typeof(string));
  //dt.Columns.Add("count", typeof(int?)); // NotSupportedException: DataSet does not support System.Nullable<>.
  dt.Columns.Add("count", typeof(int));
  var row=dt.NewRow();
  row["name"] = null;
  //row["count"] = null; // ArgumentException: Cannot set Column 'count' to be null. Please use DBNull instead.
  row["count"] = DBNull.Value;
  dt.Rows.Add(row);