Interfaces and abstracts

InterfacesAndAbstracts.cs

using Newtonsoft.Json;

public interface IInterf
{
    string PropFromInterfaceAbstract { get; set; }
    string MethFromInterfVirtual();
    T FromJson<T>(string json);
    string ToJson();
}

public abstract class Abstr : IInterf
{
    public abstract string PropFromInterfaceAbstract { get; set; }
    public virtual string MethFromInterfVirtual()
    {
        return $"calling {nameof(MethFromInterfVirtual)} from {nameof(Abstr)}";
    }

    public abstract string MethFromAbstrAbstr();
    public virtual string MethFromAbstrVirt()
    {
        return $"calling {nameof(MethFromAbstrVirt)} from {nameof(Abstr)}";
    }
    public virtual T FromJson<T>(string json)
    {
        Console.WriteLine($"calling {nameof(FromJson)} from {nameof(Abstr)}");
        return JsonConvert.DeserializeObject<T>(json);
    }
    public virtual string ToJson()
    {
        Console.WriteLine($"calling {nameof(ToJson)} from {nameof(Abstr)}");
        return JsonConvert.SerializeObject(this, Formatting.Indented);
    }
}

public class First : Abstr
{
    public override string PropFromInterfaceAbstract { get { return $"calling {nameof(PropFromInterfaceAbstract)} from {nameof(First)}"; } set { _ = value; } }
    public override string MethFromAbstrAbstr()
    {
        return $"calling {nameof(MethFromAbstrAbstr)} from {nameof(First)}";
    }
    public override string MethFromAbstrVirt()
    {
        return $"calling {nameof(MethFromAbstrVirt)} from {nameof(First)}";
    }
    public override T FromJson<T>(string json)
    {
        Console.WriteLine($"calling {nameof(FromJson)} from {nameof(First)}");
        return JsonConvert.DeserializeObject<T>(json);
    }
}

public class Second : Abstr
{
    public override string PropFromInterfaceAbstract { get { return $"calling {nameof(PropFromInterfaceAbstract)} from {nameof(Second)}"; } set { _ = value; } }
    public override string MethFromInterfVirtual()
    {
        return $"calling {nameof(MethFromInterfVirtual)} from {nameof(Second)}";
    }
    public override string MethFromAbstrAbstr()
    {
        return $"calling {nameof(MethFromAbstrAbstr)} from {nameof(Second)}";
    }
    public override string ToJson()
    {
        Console.WriteLine($"calling {nameof(ToJson)} from {nameof(Second)}");
        return JsonConvert.SerializeObject(this, Formatting.Indented);
    }
}

IInterf obj1 = new First();
IInterf obj2 = new Second();

Console.WriteLine(obj1.PropFromInterfaceAbstract);
Console.WriteLine(obj1.MethFromInterfVirtual());
Console.WriteLine(((Abstr)obj1).PropFromInterfaceAbstract);
Console.WriteLine(((Abstr)obj1).MethFromInterfVirtual());
Console.WriteLine(((Abstr)obj1).MethFromAbstrAbstr());
Console.WriteLine(((Abstr)obj1).MethFromAbstrVirt());
Console.WriteLine(((First)obj1).PropFromInterfaceAbstract);
Console.WriteLine(((First)obj1).MethFromInterfVirtual());
Console.WriteLine(((First)obj1).MethFromAbstrAbstr());
Console.WriteLine(((First)obj1).MethFromAbstrVirt());

Console.WriteLine();

Console.WriteLine(obj2.PropFromInterfaceAbstract);
Console.WriteLine(obj2.MethFromInterfVirtual());
Console.WriteLine(((Abstr)obj2).PropFromInterfaceAbstract);
Console.WriteLine(((Abstr)obj2).MethFromInterfVirtual());
Console.WriteLine(((Abstr)obj2).MethFromAbstrAbstr());
Console.WriteLine(((Abstr)obj2).MethFromAbstrVirt());
Console.WriteLine(((Second)obj2).PropFromInterfaceAbstract);
Console.WriteLine(((Second)obj2).MethFromInterfVirtual());
Console.WriteLine(((Second)obj2).MethFromAbstrAbstr());
Console.WriteLine(((Second)obj2).MethFromAbstrVirt());

var json1 = obj1.ToJson();
Console.WriteLine(json1);
var json2 = obj2.ToJson();
Console.WriteLine(json2);

IInterf obj3 = obj1.FromJson<First>(json1);
IInterf obj4 = obj2.FromJson<Second>(json2);