AutoMapper cannot convert enum to nullable int?

Question

I got AutoMapperMappingException exception

Exception of type 'AutoMapper.AutoMapperMappingException' was thrown. ---> System.InvalidCastException: Invalid cast from 'DummyTypes' to 'System.Nullable`1[[System.Int32, ...

when

public enum DummyTypes : int
{
    Foo = 1,
    Bar = 2
}

public class DummySource
{
    public DummyTypes Dummy { get; set; }
}

public class DummyDestination
{
    public int? Dummy { get; set; }
}

[TestMethod]
public void MapDummy()
{
    Mapper.CreateMap<DummySource, DummyDestination>();
    Mapper.AssertConfigurationIsValid();
    DummySource src = new DummySource()
    {
        Dummy = DummyTypes.Bar
    };
    Mapper.Map<DummySource, DummyDestination>(src);
}

Should not AutoMapper map this implicitly without any extra explicit rule?

P.S. I cannot change the definition of DummyDestination.Dummy to enum. I have to deal with such interfaces.

Answer 1

It looks like no, it won't take care of this automatically for you. Interestingly, it will map an enum to a regular int.

Looking at AutoMapper's source, I think the problematic line is:

Convert.ChangeType(context.SourceValue, context.DestinationType, null);

Assuming context.SourceValue = DummyTypes.Foo and context.DestinationType is int?, you would end up with:

Convert.ChangeType(DummyTypes.Foo, typeof(int?), null)

which throws a similar exception:

Invalid cast from 'UserQuery+DummyTypes' to 'System.Nullable`1[[System.Int32, mscorlib, Version=4.0.0.0

So I think really the question is why can't we cast a variable of type enum to int? That question has already been asked here.

This seems like a bug in AutoMapper. Anyway the workaround is to map the property manually:

Mapper.CreateMap<DummySource, DummyDestination>()
    .ForMember(dest => dest.Dummy, opt => opt.MapFrom(src => (int?)src.Dummy));