Allow automapper to set an array property to an empty array when the source array is null on a single configured map

Question

This is a follow up to a previous question: Automapper sets an array property to a zero-length array rather than null

    private class Fiz
    {
        public string Str { get; set; }

        public string[] StrArr { get; set; }

        public object[] ObjArr { get; set; }
    }

    private class Baz
    {
        public string Str { get; set; }

        public string[] StrArr { get; set; }

        public object[] ObjArr { get; set; }
    }

    [TestMethod]
    public void ShouldAllowMapArrayToNull()
    {
        var config = new MapperConfiguration(cfg =>
        {
            cfg.AllowNullDestinationValues = true;
            cfg.CreateMap<Fiz, Baz>().ForMember(b => b.Str, opt => opt.Ignore())
                //.ForMember(b => b.StrArr, opt => opt.MapFrom(f => f.StrArr))
                //.ForMember(b => b.ObjArr, opt => opt.MapFrom(f => f.ObjArr))
                .ForMember(b => b.StrArr, opt => opt.ResolveUsing(f => f.StrArr))
                .ForMember(b => b.ObjArr, opt => opt.ResolveUsing(f => f.ObjArr))

        });
        var mapper = config.CreateMapper();
        var fiz = new Fiz
        {
            Str = null,
            StrArr = null,
            ObjArr = null
        };
        var baz = mapper.Map<Fiz, Baz>(fiz);
        Assert.AreEqual(null, baz.Str,"string mapping to null");
        Assert.AreEqual(null, baz.StrArr,"string arr mapping to null");
        Assert.AreEqual(null, baz.ObjArr,"object arr mapping to null");
    }

In the example above, baz is as follows: baz.Str == null, baz.StrArr == string[0], baz.ObjArr == object[0]

Despite using the suggested solutions in the original question, there seems to be no way of getting a mapping of the destination array to null, despite the source arrays being null themselves.

Is there a way to solve this (i.e. allow actual mapping of dest parameters to null), or is this a known limitation?

Edit: cfg.AllowNullCollections = true; Solves this problem (thanks to MosheG), however this will apply to ALL the maps created under the mapper configuration. Is there a way to allow null instead of an empty array on a SINGLE created map?

Answer 1

It's by design. You can change the default if you wish:

var configuration = new MapperConfiguration(cfg => {
    cfg.AllowNullCollections = true;
    cfg.CreateMap<Source, Destination>();
});

Or change it for one mapping (though, I didn't try this one).

https://docs.automapper.org/en/stable/Lists-and-arrays.html#handling-null-collections

Answer 2

To defined null mapping per field there is a configuration method AllowNull

Example usage:

        var config = new MapperConfiguration(cfg =>
                {
                    cfg.CreateMap<Fiz, Baz>()
                        .ForMember(b => b.StrArr, opt => opt.AllowNull())                      
                        .ForMember(b => b.ObjArr, opt => opt.AllowNull());
                });

Since version 10.0.0 it overrides parameters which are set globally via AllowNullDestinationValues/Collections