Question

Coming from Java or Python I am used to making such constructs and normally it “just works”. Been discovering C# lately and, other than a few frustrations, find it quite pleasant to work with.

One such frustration is how much I have to hold it’s hands through polymorphism constructs (override, virtual, etc).

Here is one such case I have not yet found how to “just make work”.

class Polymorphism
{
    public interface IPerson
    {
        String Name { get; set; }
    }

    public class Doctor : IPerson
    {
        public String Name { get; set; }
    }

    public class DoctorGroup
    {
        public IEnumerable<IPerson> Members
        {
            get
            {
                return DoctorMembers;
            }
        }

        public List<Doctor> DoctorMembers { get; set; }
        public DoctorGroup()
        {
            DoctorMembers = new List<Doctor>();
        }
    }
}

Here the DoctorMembers are IPersons, thus I should be able to just return a list of doctors when asked for an IEnumerable of IPerson.

public IEnumerable<IPerson> Members
{
    get
    {
        return DoctorMembers;
    }
}

Yet… the compiler complains… why? What am I missing? Semantically there is nothing wrong with this, most OOP languages I have worked with thus far can digest this without syntactic Pepto-Bismol. Am I missing a keyword to make it obvious to the compiler?

The obvious shortcut here is to LINQ convert all the doctors to IPerson but I fail to see why this is necessary or even desirable.

Anybody can light my lantern?

Answer 1

This code does compile with C# 4 targeting .NET 4, but wouldn’t have compiled in C# 3, which didn’t have generic variance.

Basically you want an implicit conversion from IEnumerable<Doctor> to IEnumerable<IPerson>. That’s safe, because you can only get values “out” of an IEnumerable<T>… which is why in .NET 4, it’s declared as IEnumerable<out T>.

Read more on generic variance for details, and remember that this is only available in C# 4 and upwards.

(If you have to use C# 3 or .NET 3.5, you can use the solution shown by CSharpie… but ideally, upgrade to a more recent version 🙂