Conditional mapping to existing nested object changed between 4.11 and 10.1.1

[JoeWindowsNoQ]

We have just upgraded from 4.1.1 to 10.1.1 and found the following inconsistency. I'm happy to narrow down the version change if necessary. I cannot find this change in the release notes.

Source/destination types

public class Source
{
    public string Field { get; set; }
    public bool Cond => false;
    public Source Nested { get; set; }
}

public class Destination
{
    public string Field { get; set; }
    public Destination Nested { get; set; }
}

Mapping configuration

config.CreateMap<Source, Destination>()
    .ForMember(d => d.Nested, o => o.Condition(s => s.Cond));  // Always false for this issue

Version: 10.1.1

Expected behavior

In v4.1.1 the nested object was not updated.

Actual behavior

In v10.1.1 the nested object was updated.

Steps to reproduce

Xunit Test

// v 4.1.1
private readonly MappingEngine mapper;

public AutoMapperTest()
{
    var config = new ConfigurationStore(new TypeMapFactory(), MapperRegistry.Mappers);
    config.CreateMap<Source, Destination>()
        .ForMember(d => d.Nested, o => o.Condition(s => s.Cond));
    mapper = new MappingEngine(config);
}

// v 10.1.1
private readonly IMapper mapper;

public AutoMapperTest()
{
    var config = new MapperConfiguration(cfg =>
        cfg.CreateMap<Source, Destination>()
            .ForMember(d => d.Nested, o => o.Condition(s => s.Cond))
        );
    mapper = new Mapper(config);
}

[Fact]
public void TestConditionalNesting()
{
    // Regular mapping
    var source = new Source
    {
        Nested = new Source
        {
            Field = "ignore!"
        }
    };
    var destination = mapper.Map<Destination>(source);
    Assert.Null(destination.Nested); //  Not mapped in either version
    
    // Mapping to existing object with null destination property
    mapper.Map(source, destination);
    Assert.Null(destination.Nested); // Not mapped in either version

    // Mapping to existing object with non-null destination property - here is the problem
    destination.Nested = new Destination
    {
        Field = "keep"
    };
    mapper.Map(source, destination);
    Assert.Equal("keep", destination.Nested.Field); // Not mapped in v4.1.1, but is mapped in 10.1.1 (value = "ignore!")
}

[lbargaoanu]

Use a PreCondition instead.
The difference is the automatic setting of PreserveReferences.

[JoeWindowsNoQ]

Thanks, that has resolved our issue.

However, I don't understand how the two things are related! Could you point me at any doco that explains it?

Cheers

[lbargaoanu]

Check the execution plan.