using AutoMapper;

using System;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        var config = new MapperConfiguration(cfg => cfg.AddProfile<MyProfile>());

        var mapper = config.CreateMapper();

​

        var obj1 = mapper.Map<IDestination>(new Source());

        Console.WriteLine("obj1.Prop should be MyStruct(10), because null value in Source should be ignored");

        Console.WriteLine(JsonSerializer.Serialize(obj1));

​

        var obj2 = mapper.Map<IDestination>(new Source() { Prop = 20 });

        Console.WriteLine("obj2.Prop should be MyStruct(20), because Prop is 20m so it is mapped");

        Console.WriteLine(JsonSerializer.Serialize(obj2));

    }

​

    public class MyProfile : Profile

    {

        public MyProfile()

        {

            CreateMap<Source, IDestination>()

                .ConvertUsing((source, dest, context) => {

                    dest ??= new Destination();

​

                    if(source.Prop.HasValue)

                        dest.Prop = source.Prop.Value;

​

                    return dest;

                });

        }

    }

    public class Source

    {

        public decimal? Prop { get; set; }

    }

​

    public class Destination : IDestination

    {

        public Destination()

        {

            Prop = new MyStruct(10);

        }

        public MyStruct Prop { get; set; }

    }

​

    public interface IDestination

    {

        MyStruct Prop { get; set; }

    }

​

    public struct MyStruct

    {

        public decimal Value { get; private set; }

        public MyStruct(decimal value)

        {

            Value = value;

        }

        public static implicit operator MyStruct(decimal val) => new MyStruct(val);

    }

}