Question

In C# I’m in a scenario with the following types provided by the environment:

public interface IFoo {
}

public abstract class Base {
}

public class Derived : Base, IFoo {
}

public class Arbitrary {
    public Base GetBase() { }
}

Here’s what I’ve written in addition to this. Note that I can guarantee in my code that Arbitrary.GetBase() will always return an instance of Derived.

public class Arbitrary2 : Arbitrary {
    public IFoo GetDerived() {
        return (IFoo)base.GetBase();
    }
}

However this code fails with the message “Cannot convert type ‘Base’ to ‘IFoo'”.

But if I do this then it works:

public class Arbitrary2 : Arbitrary {
    public IFoo GetDerived() {
        Object baseAsObject = base.GetBase();
        return (IFoo)baseAsObject ;
    }
}

Why is this upcast to Object necessary before I downcast it to IFoo? The two pieces of code are functionally identical, and the latter will reliably crash at runtime if the cast is invalid. I don’t understand why the compiler complains.

Answer 1

You don’t need to do this. Your code should work, as is. See this program for details:

using System;

public interface IFoo { }

public abstract class Base { }

public class Derived : Base, IFoo { }

public class Arbitrary {
    public Base GetBase() { return new Derived(); }
}

public class Arbitrary2 : Arbitrary {
    public IFoo GetDerived() {
        return (IFoo)base.GetBase();
    }
}

class Program
{
    static void Main(string[] args)
    {
        Arbitrary2 test = new Arbitrary2();
        IFoo check = test.GetDerived();

        Console.WriteLine(check.GetType().Name);

        Console.WriteLine("Press key to exit:");
        Console.ReadKey();
    }
}