Combining ConvertUsing and IncludeMembers

[x2764tech]

I'm having an issue combining ConvertUsing, specifically when using IncludeMembers, but it also happens if I try to map the property itself.

void Main()
{
	var cfg = new AutoMapper.MapperConfiguration(cfg =>
	{
		cfg.CreateMap<B, Dto>()
			.ForMember(d => d.DT, m => m.ConvertUsing(UtcDateConverter.ToNullableDateTimeOffset, b => b.DT));
		cfg.CreateMap<A, Dto>()
			.IncludeMembers(a => a.B);
			
	});
	cfg.AssertConfigurationIsValid();
	var mapper = cfg.CreateMapper();
	mapper.ProjectTo<Dto>(new [] { new A() }.AsQueryable());
	
}

// The implementation of UtcDateConverter is not relevant here

class B { public DateTime DT { get; set; } }

class A { public B B { get; set; } }

class Dto { public DateTimeOffset? DT { get; set; } }

Running this gives

AutoMapperMappingException: Unable to create a map expression from . (A) to Nullable`1.DT (System.Nullable`1[System.DateTimeOffset])

Mapping types:
A -> Dto
A -> Dto

Type Map configuration:
A -> Dto
A -> Dto

Destination Member:
DT

using

cfg.CreateMap<A, Dto>()
			.ForMember(d => d.DT, m => m.ConvertUsing(UtcDateConverter.ToNullableDateTimeOffset, a => a.B.DT));

brings back the slight different message:

AutoMapperMappingException: Unable to create a map expression from . (A) to Nullable`1.DT (System.Nullable`1[System.DateTimeOffset])

and the only solution is to map it as:

cfg.CreateMap<A, Dto>()
			.ForMember(d => d.DT, m => m.MapFrom(a => a.B == null ? (DateTimeOffset?)null: a.B.DT.ToUniversalTime()));

However, as I reuse this logic in several places (but places I need to be explicit about), I'd really rather use a converter that I can wrap in an extension method.

🤔 Curiously, I get different errors when I use c#8 and #nullable enable - the original mapping throws "NullReferenceException: Object reference not set to an instance of an object."

[lbargaoanu]

This is the expected behavior. You don't get null checks as you do with MapFrom. But it seems to me that the right mapping here is with a type converter, not a value converter.