The code to go with my blog https://kenbonny.net/rediscovering-implicit-casting

ImplicitConversions

using System;
using Newtonsoft.Json;
using Shouldly;
using Xunit;

namespace TestSolution.UnitTests
{
    public class ImplicitConversionTests
    {
        private const string ExampleNumber = "+32 1234 567 890";


        [Fact]
        public void String_to_telephone()
        {
            Telephone number = ExampleNumber;
            number.Number.ShouldBe(ExampleNumber);
        }

        [Fact]
        public void Telephone_to_string()
        {
            Telephone number = new(ExampleNumber);
            string original = number;
            original.ShouldBe(ExampleNumber);
        }

        [Fact]
        public void Telephone_to_number()
        {
            Telephone number = new(ExampleNumber);
            int length = number;
            length.ShouldBe(ExampleNumber.Length);
        }

        [Fact]
        public void Deserialize_from_json()
        {
            const string json = "{\"Name\": \"Ken\", \"BirthDate\": \"1987-01-01\", \"Telephone\": \"+32 1234 567 890\"}";
            var person = JsonConvert.DeserializeObject<Person>(json);
            person.ShouldNotBeNull();
            person.Name.ShouldBe("Ken");
            person.BirthDate.ShouldBe(new DateTimeOffset(new DateTime(1987, 1, 1)));
            person.Telephone.Number.ShouldBe("+32 1234 567 890");
        }

        [Fact]
        public void Deserialize_extended_from_json()
        {
            const string json = "{\"Name\": \"Ken\", \"BirthDate\": \"1987-01-01\", \"Telephone\": {\"Number\": \"+32 1234 567 890\"}}";
            var person = JsonConvert.DeserializeObject<Person>(json);
            person.ShouldNotBeNull();
            person.Name.ShouldBe("Ken");
            person.BirthDate.ShouldBe(new DateTimeOffset(new DateTime(1987, 1, 1)));
            person.Telephone.Number.ShouldBe("+32 1234 567 890");
        }

        [Fact]
        public void Serialize_to_json()
        {
            const string expected = "{\"Name\":\"Ken\",\"BirthDate\":\"1987-01-01T00:00:00+01:00\",\"Telephone\":{\"Number\":\"+32 1234 567 890\"}}";
            Person person = new()
            {
                Name = "Ken", BirthDate = new DateTime(1987, 1, 1), Telephone = "+32 1234 567 890"
            };
            var json = JsonConvert.SerializeObject(person);
            json.ShouldBe(expected);
        }

        [Fact]
        public void Serialize_with_converter_to_json()
        {
            const string expected = "{\"Name\":\"Ken\",\"BirthDate\":\"1987-01-01T00:00:00+01:00\",\"Telephone\":\"+32 1234 567 890\"}";
            Person person = new()
            {
                Name = "Ken", BirthDate = new DateTime(1987, 1, 1), Telephone = "+32 1234 567 890"
            };
            var json = JsonConvert.SerializeObject(person, new TelephoneJsonConverter());
            json.ShouldBe(expected);
        }
    }

    class Person
    {
        public string Name { get; set; }
        public DateTimeOffset BirthDate { get; set; }
        public Telephone Telephone { get; set; }
    }

    class Telephone
    {
        public string Number { get; }

        public Telephone(string number)
        {
            Number = number;
        }

        public override string ToString() => Number;

        public static implicit operator Telephone(string number) => new(number);
        public static implicit operator string(Telephone number) => number.ToString();
        public static implicit operator int(Telephone number) => number.Number.Length;
    }
    
    class TelephoneJsonConverter : JsonConverter
    {
        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
        {
            if (value is not Telephone number)
                return;
            
            writer.WriteValue(number.ToString());
        }

        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
        {
            throw new NotImplementedException();
        }

        public override bool CanConvert(Type type) => type == typeof(Telephone);
    }
}