public class SqlReservationsRepository : IReservationsRepository
{
    public SqlReservationsRepository( string connectionString)
    {
        ConnectionString = connectionString;
    }

    public string ConnectionString {  get; }

    public async Task Create(Reservation reservation)
    {
        if (reservation  is null)
            throw new ArgumentNullException(nameof(reservation));

        using var conn =  new SqlConnection(ConnectionString);
        using var cmd =  new SqlCommand(createReservationSql, conn);
        cmd.Parameters.Add( new SqlParameter("@At", reservation.At));
        cmd.Parameters.Add( new SqlParameter("@Name", reservation.Name));
        cmd.Parameters.Add( new SqlParameter("@Email", reservation.Email));
        cmd.Parameters.Add(
            new SqlParameter("@Quantity", reservation.Quantity));