C# Inheritance, new modifier and generics

Question

I'm having a hard trying to find to correct approach to this :

My data structures :

public abstract class Flow
{
    public virtual double Value { get; set; }
    public virtual DateTime Time { get; set; }
}

public class InboundFlow : Flow
{
}

public class OutboundFlow : Flow
{
}

My business objects containing collections of these data structures

public abstract class Fluent
{
    public virtual IList<Flow> FlowCollection { get; set; }
    public virtual double InitialBaseflow { get; set; }
}

public class Affluent : Fluent
{
    public new virtual IList<InboundFlow> FlowCollection { get; set; }
}

public class Effluent : Fluent
{
    public new virtual IList<OutboundFlow> FlowCollection { get; set; }
}

The generic method I'm trying to use :

private static void FindInitialBaseflow<T>(ref T fluent) where T : Fluent
    {
        var linqFluent = fluent;

        var flows = linqFluent.FlowCollection.ToList().FindAll(
                    flow =>
                    flow.Time >= SOME_DATE &&
                    flow.Time < SOME_OTHER_DATE);
        var initialBaseflow = flows.Average(flow => flow.Value);
        fluent.InitialBaseflow = Math.Round(initialBaseflow, 5);  
    }

My problem is that calling "linqfluent.FlowCollection" in the linq method calls for the base class Fluent's FlowCollection, which is null.

How can I force the use of the child's property instead? Thanks!

Answer 1

You need to make the collection within Fluent generic so that the classes that inherit from it can specify the type:

public class Fluent<T>
    where T : Flow
{
    public IList<T> FlowCollection { get; set; }
    public double InitialBaseflow { get; set; }
}

Once you have that you don't even need sub classes of Flow, you can just make it concrete.

Your use of it would be easily modified to fit this model:

private static void FindInitialBaseflow<T>(Fluent<T> fluent) 
    where T : Flow
{
    var linqFluent = fluent;

    var flows = linqFluent.FlowCollection.Where(
                flow =>
                flow.Time >= SOME_DATE &&
                flow.Time < SOME_OTHER_DATE);
    var initialBaseflow = flows.Average(flow => flow.Value);
    fluent.InitialBaseflow = Math.Round(initialBaseflow, 5);
}

Also note that since you're not setting fluent in this method, there is no need to pass it by reference. It's already a class, so it is itself a reference; mutations of the referenced object will be observed by the caller.

Answer 2

Generics are the wrong tool. You should using polymorphism to ensure the correct implementation is called based on the type.

For example:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;

namespace ConsoleApp
{
    public abstract class Flow
    {
        public virtual double Value { get { return new Random().Next() ; } }//these values are just for demonstration purposes
        public virtual DateTime Time
        {
            get
            {
                return DateTime.MinValue.AddYears(1);
            }
        }
    }

    public class InboundFlow : Flow
    {
    }

    public class OutboundFlow : Flow
    {
    }

    public abstract class Fluent
    {
        IList<Flow> _flowCollection;
        public virtual IList<Flow> FlowCollection
        {
            get { return _flowCollection; }
            set { _flowCollection = value; }
        }

        private double _initialBaseflow;
        public virtual double InitialBaseflow
        {
            get { return _initialBaseflow; }
            set { _initialBaseflow = value; }
        }

        public Fluent()
        {
            FlowCollection = new List<Flow>();
        }
    }

    public class Affluent : Fluent
    {
        //public new virtual IList<InboundFlow> FlowCollection { get; set; }//Keep the property polymorphic

        public Affluent()
        {
            FlowCollection = new List<Flow>();
        }
    }

    public class Effluent : Fluent
    {
        //public new virtual IList<OutboundFlow> FlowCollection { get; set; }

        public Effluent()
        {
            FlowCollection = new List<Flow>();
        }
    }

    class Program
    {
        public static DateTime SOME_DATE { get { return DateTime.MinValue; } }
        public static DateTime SOME_OTHER_DATE { get { return DateTime.Now; } }

        static void Main(string[] args)
        {
            var inbound = new InboundFlow();
            var inbound2 = new InboundFlow();
            var outbound = new OutboundFlow();
            var a = new Affluent();            
            a.FlowCollection.Add(inbound);
            a.FlowCollection.Add(inbound2);
            FindInitialBaseflow(a);
        }

        private static void FindInitialBaseflow(Fluent fluent)
        {
            var linqFluent = fluent;

            var flows = linqFluent.FlowCollection.ToList().FindAll(
                        flow =>
                        flow.Time >= SOME_DATE &&
                        flow.Time < SOME_OTHER_DATE);
            var initialBaseflow = flows.Average(flow => flow.Value);
            fluent.InitialBaseflow = Math.Round(initialBaseflow, 5);
        }
    }
}