Automapper overwrites missing source property on list with child objects

Question

I have a problem with Automapper. I set up a test windows form application and below is the code. Also look at the comments after each MessageBox:

public class FirstClass
    {
        public string FirstProp { get; set; }
        public IList<FirstClassChild> Children { get; set; }
    }

    public class FirstClassChild
    {
        public string FirstChildProp { get; set; }
    }

    public class SecondClass
    {
        public string FirstProp { get; set; }
        public string SecondProp { get; set; }
        public IList<SecondClassChild> Children { get; set; }
    }

    public class SecondClassChild
    {
        public string FirstChildProp { get; set; }
        public string SecondChildProp { get; set; }
    }

    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            AutoMapper.Mapper.CreateMap<FirstClass, SecondClass>();
            AutoMapper.Mapper.CreateMap<FirstClassChild, SecondClassChild>();

            var f = new FirstClass { FirstProp = "FirstClass" };
            f.Children = new List<FirstClassChild> { new FirstClassChild { FirstChildProp = "FirstClass" } };
            var s = new SecondClass { FirstProp = "SecondClass", SecondProp = "SecondClass" };
            s.Children = new List<SecondClassChild> { new SecondClassChild { FirstChildProp = "SecondClass", SecondChildProp = "SecondClass" } };
            AutoMapper.Mapper.Map(f, s);

            var fc = new FirstClassChild { FirstChildProp = "FirstClass" };
            var sc = new SecondClassChild { FirstChildProp = "SecondClass", SecondChildProp = "SecondClass" };
            AutoMapper.Mapper.Map(fc, sc);

            MessageBox.Show(sc.FirstChildProp);//FirstClass as expected
            MessageBox.Show(sc.SecondChildProp);//SecondClass as expected

            MessageBox.Show(s.FirstProp);//FirstClass as expected
            MessageBox.Show(s.SecondProp);//SecondClass as expected
            MessageBox.Show(s.Children.First().FirstChildProp);//FirstClass as expected
            MessageBox.Show(s.Children.First().SecondChildProp);//Empty not expected!!

        }
    }

What can I do to avoid this? Is this behavior expected? Anyway can anyone guide me how make SecondClass childs SecondChildProp to remain "SecondClass" as it is before the mapping occurs.

Answer 1

I asked a similar question here and found another similar one here .

I think @PatrickSteele makes a very good point: how is AutoMapper supposed to map a source list to a dest list of existing objects , when the dest list may not necessarily bear any resemblance to the source list? ie " But what if one list has 3 and the other list has 5? "

If you are sure that FirstClass and SecondClass have the same number of Children , and if the FirstClass 's Nth Child always corresponds to SecondClass 's Nth child, you could try something like this:

Mapper.CreateMap<FirstClass, SecondClass>()
    .ForMember(m => m.Children, o => o.Ignore())
    .AfterMap((src, dest) =>
        {
            for (var i = 0; i < dest.Children.Count; i++)
                Mapper.Map(src.Children[i], dest.Children[i]);
        });

or if FirstChildProp is some kind of unique key:

Mapper.CreateMap<FirstClass, SecondClass>()
    .ForMember(m => m.Children, o => o.Ignore())
    .AfterMap((src, dest) =>
        {
            foreach (var dChild in dest.Children)
            {
                var sChild = src.Children.Single(c => c.FirstChildProp == dChild.FirstChildProp);
                Mapper.Map(sChild, dChild);
            }
        });