using System;

using System.Collections.Generic;

using AutoMapper;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var config = new MapperConfiguration(cfg =>

        {                   

            cfg.CreateMap<InputModel, DomainModel>()

                .ForMember(dest => dest.UserCustomerMapping, opt => new List<UserCustomerMapping>())

                .ForMember(dest => dest.UserCustomerMapping, opt => { opt.PreCondition(src => (src.C != null));

                           opt.MapFrom(src => new List<UserCustomerMapping>() { new UserCustomerMapping { Role = new Role(new Guid((src.C))) }});

                            });

        });

        IMapper mapper = config.CreateMapper();

        var domainModel = new DomainModel 

        {

            A = "Before ChangeA",

            B = "Before ChangeB",

            UserCustomerMapping = new List<UserCustomerMapping>() 

            {

                { new UserCustomerMapping { Role = new Role { Id = new Guid("00000000-0000-0000-0000-000000000001"), Name = "BeforeChangeC" } } }

            }

        };

        InputModel src = new InputModel 

        { 

            A = "Value A",

            B = "Value B",

            C = "00000000-0000-0000-0000-000000000002"

        };          

​

        mapper.Map(src, domainModel);

        Console.WriteLine(JsonConvert.SerializeObject(domainModel, new JsonSerializerSettings { Formatting = Formatting.Indented }));

    }

}

​

class InputModel

{

  public string A {get; set;}

  public string B {get; set;}

  public string C {get; set;}

}

​

class DomainModel

{

  public string A {get; set;}

  public string B {get; set;}

  public List<UserCustomerMapping> UserCustomerMapping {get; set;} 

}

​

class UserCustomerMapping

{

  public Role Role {get; set;}

}

​

class Role

{

  public Guid Id {get; set;}

  public string Name {get; set;}

  public Role() {  }

  public Role(Guid id)

  {

      Id = id;

      Name = "SetInConstructor";

  }

}