ForAllMembers expression Condition incorrectly passes Nullable<T> source member as T

[xnoreq]

Source/destination types

    public class Destination
    {
        public bool Toggle { get; set; }
    }

    public class Source
    {
        public bool? Toggle { get; set; }
    }

Mapping configuration

    public class TestProfile : Profile
    {
        public TestProfile()
        {
            AllowNullCollections = true;
            AllowNullDestinationValues = true;

            CreateMap<Source, Destination>()
                .ForAllMembers(expression => expression.Condition((src, dst, srcMember) => srcMember != null));
        }
    }

Version: 11.0.1

11.0.1

Expected behavior

In the ForAllMembers condition the srcMember should be of type bool? (for the Source.Toggle property) and the condition should evaluate to false (given Toggle = null as in the example program below). The destination thing should therefore stay unmodified.

Actual behavior

The srcMember is of type bool, so the information that the input was null gets lost. The ForAllMembers condition evaluates to true changing the destination thingy.

Steps to reproduce

    public class Program
    {
        static void Main(string[] args)
        {
            var config = new MapperConfiguration(cfg => cfg.AddProfile<TestProfile>());
            var mapper = config.CreateMapper();

            var thing = new Destination()
            {
                Toggle = true
            };

            var update = new Source()
            {
                Toggle = null
            };

            Console.WriteLine(thing.Toggle); // True
            mapper.Map(update, thing);
            Console.WriteLine(thing.Toggle); // False
        }
    }

[jbogard]

Reopening to track this — it is a real bug, not just nullable semantics. The third Condition argument is documented as the source member but we were passing the value after conversion to the destination member type, so a null Nullable<T> had already collapsed to default(T) before the condition ran. Fix in #4655.

[jbogard]

Fixed by #4655, merged as 6e8697b — shipping in 16.3.

Condition now receives the value resolved from the source object before conversion to the destination member type when the condition's member parameter can hold it, so a ForAllMembers condition sees a null Nullable<T> instead of default(T). Conditional-mapping.md gained a "Nullable source members" section covering that, the PreCondition route for typed ForMember conditions, UseDestinationValue for the keep-the-destination case, and the ?? default(T) rule when there is no condition.