SqlParameterCollection only accepts non-null SqlParameter type objects, not DBNull objects
You aren't adding your new SqlParameter. p is the result of new SqlParameter("ParentId", SqlDbType.Int).Value = parentId ?? (object) DBNull.Value. In other words, p itself is DBNull.Value.
Split the statement in two, like so:
var p = new SqlParameter("ParentId", SqlDbType.Int);p.Value = parentId ?? (object) DBNull.Value;cmd.Parameters.Add(p);Alternatively,
var p = new SqlParameter("ParentId", SqlDbType.Int) { Value = parentId ?? (object) DBNull.Value };cmd.Parameters.Add(p);Either would make sure p is the parameter, not the parameter's value.