Repository Pattern For Accessing Text File

Question

I am fairly new to the Repository Pattern and I would like to do this correctly. I am also trying to make use of Inversion of Control (also new).

I would like to make sure I am using the repository pattern correctly.

I picked this up as an example of a base interface for my repositories.

public interface IRepository<T> where T : class
{
    IEnumerable<T> Find(Expression<Func<T, bool>> where);

    IEnumerable<T> GetAll();

    void Create(T p);

    void Update(T p);
}

IPaymentRepository is intended for extensions to IRepository (although I don't see why I would need this if I have the Find method above)

public interface IPaymentRepository : IRepository<Payment>
{
}

PaymentRepository simply reads a text file and builds a POCO.

public class PaymentRepository : IPaymentRepository
{
    #region Members

    private FileInfo paymentFile;
    private StreamReader reader;
    private List<Payment> payments;

    #endregion Members

    #region Constructors

    #endregion Constructors

    /// <summary>
    /// Initializes a new instance of the <see cref="PaymentRepository"/> class.
    /// </summary>
    /// <param name="paymentFile">The payment file.</param>
    public PaymentRepository(FileInfo paymentFile)
    {
        if (!paymentFile.Exists)
            throw new FileNotFoundException("Could not find the payment file to process.");

        this.paymentFile = paymentFile;
    }

    #region Properties

    #endregion Properties

    #region Methods

    public IEnumerable<Payment> Find(Expression<Func<Payment, bool>> where)
    {
        throw new NotImplementedException();
    }

    /// <summary>
    /// Gets all payments from payment file.
    /// </summary>
    /// <returns>Collection of payment objects.</returns>
    public IEnumerable<Payment> GetAll()
    {
        this.reader = new StreamReader(this.paymentFile.FullName);
        this.payments = new List<Payment>();

        while (!reader.EndOfStream)
        {
            string line = reader.ReadLine();
            Payment payment = new Payment()
            {
                AccountNo = line.Substring(0, 11),
                Amount = double.Parse(line.Substring(11, 10))
            };

            this.payments.Add(payment);
        }

        return this.payments;
    }

    public void Create(Payment p)
    {
        throw new NotImplementedException();
    }

    public void Update(Payment p)
    {
        throw new NotImplementedException();
    }

    #endregion Methods

I would like to know how to implement the Find method. I am assuming I would call GetAll and build an internal cache to the repository. For example, I would like to find all accounts that have payments greater than $50.

Answer 1

With your current IRepository signature you would implement it like this:

public IEnumerable<Payment> Find(Expression<Func<Payment, bool>> where)
{
    this.reader = new StreamReader(this.paymentFile.FullName);
    this.payments = new List<Payment>();

    while (!reader.EndOfStream)
    {
        string line = reader.ReadLine();
        Payment payment = new Payment()
        {
            AccountNo = line.Substring(0, 11),
            Amount = double.Parse(line.Substring(11, 10))
        };
        if (where(payment) 
        {
           this.payments.Add(payment);
        }
    }

    return this.payments;
}

However, If your system memory allows it, you could keep a cached list (from GetAll()) and use Find() on the list. This should be an order of magnitude faster depending on the size of your list.