akaJB Posts - Answer Overflow

akaJB

•Created by akaJB on 7/30/2023 in #help

✅ Confusion over defining a default implementation on an interface

I have an interface I'll call IWidget, and a class which implements it called Widget. I created a property IWidget with a default implementation. However, I cannot access that property on a Widget instance. My expectation was that any class/interface which implements IWidget would get its default implementation. When looking up some of the docs online I see this is expected behavior and has something to do with preventing breaking changes. What I'm not finding is answers to two questions: - What's the benefit of providing an implementation to begin with if it's not direclty accessable? - What's the best pattern to follow if there's shared behavior across all implementations of IWidget? I would assume an extension for IWidget but would like to confirm Fuller example:

using System;

namespace Test
{
    public interface IWidget
    {
        public string GetName { get; }
        
        public int GetNumber => 1;    
    }
    
    public class Widget : IWidget
    {
        public string GetName => "Widget";
    }

    public static class Program
    {
        public static void Main(string[] args)
        {
            Widget widget = new Widget();
            IWidget iwidget = widget;

            // both work
            Console.WriteLine(widget.GetName);
            Console.WriteLine(iwidget.GetName);
            
            // widget has an error
            // iwidget works
            Console.WriteLine(widget.GetNumber);
            Console.WriteLine(iwidget.GetNumber);
        }
    }
}

using System;

namespace Test
{
    public interface IWidget
    {
        public string GetName { get; }
        
        public int GetNumber => 1;    
    }
    
    public class Widget : IWidget
    {
        public string GetName => "Widget";
    }

    public static class Program
    {
        public static void Main(string[] args)
        {
            Widget widget = new Widget();
            IWidget iwidget = widget;

            // both work
            Console.WriteLine(widget.GetName);
            Console.WriteLine(iwidget.GetName);
            
            // widget has an error
            // iwidget works
            Console.WriteLine(widget.GetNumber);
            Console.WriteLine(iwidget.GetNumber);
        }
    }
}

6 replies

CC#

•Created by akaJB on 7/29/2023 in #help

✅ How do I deal with generic covariance and collections types?

Let's say I have osmething like the following:

public interface IBoxCollection<out T> : IBox
    where T : IBox
{
    public Dictionary<string, T> Collection { get; }
}

public interface IBoxCollection<out T> : IBox
    where T : IBox
{
    public Dictionary<string, T> Collection { get; }
}

I get the following error: Invalid variance: The type parameter 'T' must be invariantly valid on 'IBoxCollection<T>.Collection'. 'T' is covariant I've looked around on how to resolve this issue but haven't found anything. Is there an easy solution to this problem, or is there a pattern which will accomplish what I'm looking for?

8 replies

CC#

•Created by akaJB on 7/28/2023 in #help

❔ Attempting to create a Result<T, E> type, running into issues with subclasses

I want to create a Result<T, E> and I'm running into trouble with the following: let's say I have a class Foo and a subclass Bar : Foo. If I create an instance of Result<Bar, _> I had expected it to be assignable to Result<Foo, _> but that doesn't seem to be the case. For a fuller example see: https://dotnetfiddle.net/UEgC3P Is there a way that allows such an assignment? Else how can I ergonomically work with the compiler? I could of course create a new Result instance (like the below), but it's very verbose.

return result.IsSuccess
    ? Result<Foo, int>.AsOk(result.Value)
    : Result<Foo, int>.AsError(result.Error);

return result.IsSuccess
    ? Result<Foo, int>.AsOk(result.Value)
    : Result<Foo, int>.AsError(result.Error);

9 replies

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