using System;

using AutoMapper;

public class Program

{

    public static void Main()

    {

        var configuration = new MapperConfiguration(cfg => 

                                                    cfg.CreateMap<Order, OrderDto>()

                                                       .ForMember(d => d.A, opt => opt.MapFrom(src => src.B))

                                                       .ForMember(d => d.B, opt => opt.MapFrom(src => src.B))

                                                   );

        configuration.AssertConfigurationIsValid();

        var mapper = configuration.CreateMapper();

        var order = new Order

        {

            A = "A",

            B = "B",

        };

        var orderDto = mapper.Map<OrderDto>(order);

        Console.WriteLine(orderDto.A);

        Console.WriteLine(orderDto.B);

    }

}

​

public class Order

{

    public string A { get; set; }

    public string B { get; set; }

}

​

public class OrderDto

{

    public string A { get; set; }

    public string B { get; set; }

    public string C { get; set; }

}