Question

I have a number of classes that reflect tables in a database. I would like to have a base class that has some basic functionality (say, it would have a “isDirty” flag), and a static array of strings with the column names as they appear in the database. The following code doesn’t work but illustrates what I would like to do:

public class BaseRecord {  
  public bool isDirty;  
  public object [] itemArray;  
  public static string [] columnNames;    
}

public class PeopleRec : BaseRecord {  
}

public class OrderRec : BaseRecord {  
}

public static void Main() {
  PeopleRec.columnNames = new string[2];
  PeopleRec.columnNames[0]="FIRST_NAME";
  PeopleRec.columnNames[1]="LAST_NAME";

  OrderRec.columnNames = new string[4];
  OrderRec.columnNames[0] = "ORDER_ID";
  OrderRec.columnNames[1] = "LINE";
  OrderRec.columnNames[2] = "PART_NO";
  OrderRec.columnNames[3] = "QTY";
}

public class DoWork<T> where T : BaseRecord {
  public void DisplayColumnNames() {
    foreach(string s in T.columnNames)
      Console.Write("{0}", s);
    }
  public void DisplayItem(T t) {
    for (int i=0; i<itemValues.Length; i++) {
      Console.Write("{0}: {1}",t.columnNames[i],t.itemValues[i])
    }
  }
}

I would like each derived class to have it’s own static array of strings of database column names, and I would like the generic class to access this static member without the need for an instance.

But it doesn’t work:
(A) columnNames is the identical array in BaseRec, PeopleRec and OrderRec. I cannot have columnNames be different. BaseRec.columnNames.Length would be 3 because the columnNames in OrderRec is initialized last.
(B) The notation T.columnNames does not compile.

Any ideas on how to fix this?

Answer 1

The issue is you that you want to associate some data with the types, not with instances of the types. I’m not sure that there’s a neat way of doing this in C#, but one possibility is using a static Dictionary<Type, string[]> on BaseRecord. An example is below, you could neaten this up by adding some generic static members on BaseRecord for initializing/accessing the record names (and add some error checking…):

using System;
using System.Collections.Generic;

namespace Records
{
    public class BaseRecord
    {
        public bool isDirty;
        public object[] itemArray;

        public static Dictionary<Type, string[]> columnNames = new Dictionary<Type, string[]>();
    }

    public class PeopleRec : BaseRecord
    {
        static PeopleRec()
        {
            string[] names = new string[2];
            names[0] = "FIRST_NAME";
            names[1] = "LAST_NAME";
            BaseRecord.columnNames[typeof(PeopleRec)] = names;
        }
    }

    public class DoWork<T> where T : BaseRecord
    {
        public void DisplayColumnNames()
        {
            foreach (string s in BaseRecord.columnNames[typeof(T)])
                Console.WriteLine("{0}", s);
        }

        public void DisplayItem(T t)
        {
            for (int i = 0; i < t.itemArray.Length; i++)
            {
                Console.WriteLine("{0}: {1}", BaseRecord.columnNames[typeof(T)][i], t.itemArray[i]);
            }
        }
    }

    class Program
    {
        public static void Main()
        {
            PeopleRec p = new PeopleRec
            {
                itemArray = new object[] { "Joe", "Random" }
            };

            DoWork<PeopleRec> w = new DoWork<PeopleRec>();
            w.DisplayColumnNames();
            w.DisplayItem(p);
        }
    }
}