I’ve got a problem where I am not able to implicitly cast a Dictionairy<int, class> to a Dictionairy<int, interfaceOfClass> where I feel that this should normally be possible with the rules of polymorphism. The workaround I currently have is to do an explicit cast like this class => class as interfaceOfClass.

I am not sure if I am missing something in my code to make this work or if its a limitation of C#. Below is a concrete example of my problem.

namespace ImplicitCasting;

internal class Program {
    static void Main(string[] args) {
        IDictionary<int, Person> persons = new Dictionary<int, Person> {
            { 0, new Person() { Name = "Person1", Age = 20 } },
            { 1, new Person() { Name = "Person2", Age = 30 } },
            { 2, new Person() { Name = "Person3", Age = 35 } }
        };

        /*
         This doenst work
         Gives error: CS0266    Cannot implicitly convert type
         'System.Collections.Generic.IDictionary<int, ImplicitCasting.Program.Person>' to
         'System.Collections.Generic.IDictionary<int, ImplicitCasting.Program.IPerson>'.
          An explicit conversion exists (are you missing a cast?)
        */
        //IDictionary<int, IPerson> implicitCast = persons;

        // This is what I need to do to make it work
        IDictionary<int, IPerson> explictCast = persons
            .ToDictionary(entry=>entry.Key, entry=>entry.Value as IPerson);

        foreach (var enrty in explictCast) {
            Console.WriteLine($"{enrty.Key}. Person: {enrty.Value.Name}");
        }
    }

    public interface IPerson {
        string Name { get; set; }
        void DoSomething();
    }

    public class Person : IPerson {
        public string Name { get; set; } = "Test";
        public int Age { get; set; }
        public void DoSomething() {
            Console.WriteLine($"{Name} doint something");
        }
    }
}