Repository Pattern For Accessing Text File - c#-4.0

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.