I realize this is an old question, but it took me several hours to find a good solution and thought I'd pass on what I learned here and save someone else the trouble. Try, for example,
SELECT * FROM some_table WHERE id_column = ANY(@id_list)
where @id_list is bound to an int[] parameter by way of
command.Parameters.Add("@id_list", NpgsqlDbType.Array | NpgsqlDbType.Integer).Value = my_id_list;
where command is a NpgsqlCommand (using C# and Npgsql in Visual Studio).