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.