Collection is empty after mapping elements to an existing collection if existing target instance is returned by the resolver

[sk-helana]

Source/destination types

    public class SourceType
    {
        public SourceType(List<string> list) => Strings = list;
        public List<string> Strings { get; set; }
    }

    public class TargetType
    {
        public TargetType(List<string> list) => Strings = list;
        public List<string> Strings { get; set; }
    }

Mapping configuration

        var conf = new MapperConfiguration(conf =>
            conf.CreateMap<SourceType, TargetType>()
                .ForMember(
                    t => t.Strings, opt => opt.MapFrom(
                        (src, tgt, obj) => { obj.AddRange(src.Strings); return obj; })
            ));

For reference, configuration working as expected:

        var conf = new MapperConfiguration(conf =>
            conf.CreateMap<SourceType, TargetType>()
                .ForMember(
                    t => t.Strings, opt => opt.MapFrom(
                        (src, tgt, obj) => { obj.AddRange(src.Strings); return obj.ToList(); }) //Passes
            ));

Version: 12.0.1-alpha.0.2, 12.0.0, 11.0.1

Expected behavior

When mapping elements from a source collection to a target collection, the target collection should contain any elements returned in the MapFrom.

Actual behavior

Target collections becomes null or empty if the same instance is returned by the resolver. Works as expected if a shallow copy is returned instead.

Steps to reproduce

Complete NUnit test below. Test1 fails, Test2 passes, difference being a .ToList() in Test2's MapFrom.
Edit: Capacity of the resulting target collection is correct (4) in Test1 below. Maybe the target is being cleared before being assigned the new value.

using NUnit.Framework;
using AutoMapper;
using System.Collections.Generic;
using System.Linq;

namespace MapToCollection;

public class MapToCollectionTests
{
    public class SourceType
    {
        public SourceType(List<string> list) => Strings = list;
        public List<string> Strings { get; set; }
    }

    public class TargetType
    {
        public TargetType(List<string> list) => Strings = list;
        public List<string> Strings { get; set; }
    }

    [SetUp]
    public void Setup()
    {
    }

    [Test]
    public void Test1()
    {
        var conf = new MapperConfiguration(conf =>
            conf.CreateMap<SourceType, TargetType>()
                .ForMember(
                    t => t.Strings, opt => opt.MapFrom(
                        (src, tgt, obj) => { obj.AddRange(src.Strings); return obj; }) //Fails, tgt.Strings become empty
            ));
        var mapper = conf.CreateMapper();

        var src = new SourceType(new() { "one", "two" });
        var tgt = new TargetType(new() { "three", "four" });

        mapper.Map(src, tgt);

        Assert.NotNull(tgt.Strings);
        Assert.Contains("one", tgt.Strings);
        Assert.Contains("two", tgt.Strings);
        Assert.Contains("three", tgt.Strings);
        Assert.Contains("four", tgt.Strings);
    }

    [Test]
    public void Test2()
    {
        var conf = new MapperConfiguration(conf =>
            conf.CreateMap<SourceType, TargetType>()
                .ForMember(
                    t => t.Strings, opt => opt.MapFrom(
                        (src, tgt, obj) => { obj.AddRange(src.Strings); return obj.ToList(); }) //Passes
            ));
        var mapper = conf.CreateMapper();

        var src = new SourceType(new() { "one", "two" });
        var tgt = new TargetType(new() { "three", "four" });

        mapper.Map(src, tgt);

        Assert.NotNull(tgt.Strings);
        Assert.Contains("one", tgt.Strings);
        Assert.Contains("two", tgt.Strings);
        Assert.Contains("three", tgt.Strings);
        Assert.Contains("four", tgt.Strings);
    }
}

[lbargaoanu]

I think this is better suited for StackOverflow.

[sk-helana]

Sorry, found that this behaviour is documented at https://docs.automapper.org/en/stable/Lists-and-arrays.html: "When mapping to an existing collection, the destination collection is cleared first.". Might have overlooked since our original problem was that the target became null instead of empty.
Thanks

[sk-helana]

For anyone finding their way here, issue #1801 discusses why the destination is cleared.