Generic methods and type casting

Kevin Wallis Published at Dev

Kevin Wallis

I read the following question (and I would solve it the same way as the given answer): Passing derived type as argument to abstract class

But why is it not able to find the value attribute from the derived class? even if I add a type cast it is not possible:

public abstract class baseClass
{
    public abstract float function<T>(T a, T b) where T:baseClass;
}

public class derived: baseClass
{
    public override float function<derived>(derived a, derived b)
    {
        // Here value is not found
        return a.value + b.value;
    }

    public float value;
}

Example with type cast is also not working (and the advice redundant type cast is shown):

public abstract class baseClass
{
    public abstract float function<T>(T a, T b) where T:baseClass;
}

public class derived: baseClass
{
    public override float function<derived>(derived a, derived b)
    {
        // Here value is not found even with type cast
        return ((derived)a).value + ((derived)b).value;
    }

    public float value;
}

Yuval Itzchakov

Because you're declaring the generic type parameter on the method. The compiler doesn't understand that this should be of the derived type. All it knows it that you've introduced a new generic type parameter.

What you want is called F-bound polymorphism, where the type parameter is of the implementing class, defined recursively:

public abstract class BaseClass<T> where T : BaseClass<T>
{
    public abstract float Function(T a, T b);
}

public class Derived : BaseClass<Derived>
{
    public override float Function(Derived a, Derived b)
    {
        return a.Value + b.Value;
    }

    public float Value { get; set; }
}

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