#Mapping with classes.cs

public class CustomerToFaithfulCustomerDto : Mapping<FaithfulOrderDto, Order>
{
	public override void Fill(FaithfulOrderDto target, Order source)
	{
		target.Total = source.Total;
		target.Customer = With<CustomerToCustomerDto>().Map(source.Customer);
	}
}

public class CustomerToCustomerDto : Mapping<CustomerDto, Customer>
{
	public override void Fill(CustomerDto target, Customer source)
	{
		target.Name = source.Name;
	}
}

public class OrderToFlatOrderDto : Mapping<FlatOrderDto, Order>
{
	public override void Fill(FlatOrderDto target, Order source)
	{
		target.Total = source.Total;
		target.CustomerName = source.Customer?.Name;
	}
}

2_Infrastructure.cs

public abstract class Mapping<TDestination, TSource>
	where TDestination : new()
{
	public abstract void Fill(TDestination target, TSource source);

	public TMapping With<TMapping>()
		where TMapping : new()
	{
		return new TMapping();
	}

	public TDestination Map(TSource source)
	{
		var target = new TDestination();
		Fill(target, source);
		return target;
	}
}