Override abstract readonly property to read/write property

Question

I would like to only force the implementation of a C# getter on a given property from a base abstract class. Derived classes might, if they want, also provide a setter for that property for public use of the statically bound type.

Given the following abstract class:

public abstract class Base
{
    public abstract int Property { get; }
}

If I want a derived class that also implements a setter, I could naively try:

public class Derived : Base
{
    public override int Property
    {
        get { return field; }
        set { field = value; } // Error : Nothing to override.
    } 

    private int field;
}

But then I get a syntax error since I try to override the non existing setter. I tried some other way such as declaring the base setter private and such and I still stumble upon all kind of errors preventing me from doing that. There must be a way to do that as it doesn't break any base class contract.

Incidentaly, it can be done with interfaces, but I really need that default implementation.

I stumbled into that situation so often, I was wondering if there was a hidden C# syntax trick to do that, else I will just live with it and implement a manual SetProperty() method.

Solution 1

You can't do it directly, since you can't new and override with the same signature on the same type; there are two options - if you control the base class, add a second property:

public abstract class Base
{
    public int Property { get { return PropertyImpl; } }
    protected abstract int PropertyImpl {get;}
}
public class Derived : Base
{
    public new int Property {get;set;}
    protected override int PropertyImpl
    {
        get { return Property; }
    }
}

Else you can introduce an extra level in the class hierarchy:

public abstract class Base
{
    public abstract int Property { get; }
}
public abstract class SecondBase : Base
{
    public sealed override int Property
    {
        get { return PropertyImpl; }
    }
    protected abstract int PropertyImpl { get; }
}
public class Derived : SecondBase
{
    public new int Property { get; set; }

    protected override int PropertyImpl
    {
        get { return Property; }
    }
}

Solution 2

Would this suit your needs?

public abstract class TheBase
{
    public int Value
    {
        get;
        protected set;
    }
}
public class TheDerived : TheBase
{
    public new int Value
    {
        get { return base.Value; }
        set { base.Value = value; }
    }
}

The virtual was removed, but the base value is still the only storage for the value. So this should show '5'. And the compiler should fuss about b.Value = 4;

TheDerived d = new TheDerived();
d.Value = 5;
TheBase b = d;
//b.Value = 4;    // uncomment for compiler error
cout << "b.Value == " << b.Value << endl;

-Jesse

Solution 3

What about something like:

public abstract class Base
{
    public virtual int Property
    {
        get { return this.GetProperty(); }
        set { }
    }

    protected abstract int GetProperty();
}

Solution 4

I had a similar requirement where I needed an interface to be able to share common sorting functionality between two loosely related classes. One of them had a read-only Order property and the other had a read-write Order property, but I needed a way to read the property the same way from both classes.

It turns out that this can be done by hiding the read-only value in a derived interface. Here is how I did it.

interface ISortable
{
    int Order { get; }
}

interface ISortableClass2
    : ISortable
{
    // This hides the read-only member of ISortable but still satisfies the contract
    new int Order { get; set; }
}

class SortableClass1
    : ISortable
{
    private readonly int order;

    public SortableClass1(int order)
    {
        this.order = order;
    }

    #region ISortable Members

    public int Order
    {
        get { return this.order; }
    }

    #endregion
}

class SortableClass2
    : ISortableClass2
{
    #region ISortableClass2 Members

        public int Order { get; set; } 

    #endregion
}

class RunSorting
{
    public static void Run()
    {
        // Test SortableClass1
        var list1 = new List<SortableClass1>();

        list1.Add(new SortableClass1(6));
        list1.Add(new SortableClass1(1));
        list1.Add(new SortableClass1(5));
        list1.Add(new SortableClass1(2));
        list1.Add(new SortableClass1(4));
        list1.Add(new SortableClass1(3));

        var sorted1 = SortObjects(list1);

        foreach (var item in sorted1)
        {
            Console.WriteLine("SortableClass1 order " + item.Order);
        }

        // Test SortableClass2
        var list2 = new List<SortableClass2>();

        list2.Add(new SortableClass2() { Order = 6 });
        list2.Add(new SortableClass2() { Order = 2 });
        list2.Add(new SortableClass2() { Order = 5 });
        list2.Add(new SortableClass2() { Order = 1 });
        list2.Add(new SortableClass2() { Order = 4 });
        list2.Add(new SortableClass2() { Order = 3 });

        var sorted2 = SortObjects(list2);

        foreach (var item in sorted2)
        {
            Console.WriteLine("SortableClass2 order " + item.Order);
        }
    }

    private static IEnumerable<T> SortObjects<T>(IList<T> objectsToSort) where T : ISortable
    {
        if (objectsToSort.Any(x => x.Order != 0))
        {
            return objectsToSort.OrderBy(x => x.Order);
        }

        return objectsToSort;
    }
}

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