Question

Edited after initial suggestions about using IEnumerable instead of List.

I am trying to do this:

public interface IKPIDetails // Previously: interface IKPIDetails
{ 
    string name {get; set;}
    //...
}


public abstract class BaseReport
{
    public List<IKPIDetails> m_KPIDetails;  // Previously: list<IKPIDetails> m_KPIDetails;

    public BaseReport()
    {
        m_KPIDetails = MakeReportDataStructure();
        //...
    }
    public abstract List<IKPIDetails> MakeReportDataStructure(); // Previously: public abstract IKPIDetails MakeReportDataStructure();

    //...
}


public class AirTemp_KPIDetails : NetVisSolution.Reports.IKPIDetails // Previously: protected class AirTemp_KPIDetails : IKPIDetails
{
    #region IKPIDetails implementation
    private string _Name;
    public string Name
    {
        get { return _Name; }
        set { _Name = value; }
    }
    #endregion
    ...
}

public class SSAirTempSummaryReport : BaseReport
{
    public SSAirTempSummaryReport() : base ()
    {
        // Do any extra initialisation here
        m_reportName = "Air Temperature Summary Report";
    }

    //It now complains on the declaration below at the word
    // MakeReportDataStructure()
    public override IEnumerable<IKPIDetails> MakeReportDataStructure() // Previously: public override List<IKPIDetails> MakeReportDataStructure()
    {
        List<AirTemp_KPIDetails> d = new List<AirTemp_KPIDetails>();
        return  d;  // <-- this is where it used to break
    }

    //...
}

So I have a report data object interface and potentially multiple report data classes implementing this interface and adding their specific extra bits. And I have an abstract report class implementing generic report functionality and multiple derived classes that each use their own report data class.

Unfortunately, when I try to create a list of my report detail structures in the derived class and pass it back to the base class as a list of the interface, it complains:

Cannot implicitly convert type 
'System.Collections.Generic.List<NetVisSolution.Reports.AirTemp_KPIDetails>' to 
'System.Collections.Generic.List<NetVisSolution.Reports.IKPIDetails>'

It won’t let me do it implicitely or explicitly. Is there any way that I can do this?

Thanks in advance,

— Alistair.

Answer 1

you shouldn’t cast a List of one type to another because, then either the insert operation would fail, or the getting.

example: type A implements(extends) type B

List<A> listA;
List<B> listB;


((List<B>) listA).add(new B()); 

if this would legal, then you break the listA invariant by putting something more general than A.

A a = ((List<A>) listB).get(); //like get first, or something similar

this would break the fact that only A objects, could be assigned to the A reference.