Map Enum to Null when not matched by Name

[matthiaslischka]

I have a nullable enum property on the source side and get this property with value 0 instead of NULL.
But 0 is not a represented value in the enum so AutoMapper with ConvertUsingEnumMapping throws an exception that the enum can not be mapped. Is there a way to map it to NULL whenever the value is not represented in the source enum? Some way to prevent this runtime error?

public class SomeSourceDto
{
    public SomeSourceEnum? SomeEnum { get; set; }
}

public class SomeDestinationDto
{
    public SomeDestinationEnum? SomeEnum { get; set; }
}

public enum SomeSourceEnum
{
    First = 1
}

public enum SomeDestinationEnum
{
    First = 1
}

Mapping configuration:

CreateMap<SomeSourceDto, SomeDestinationDto>();
CreateMap<SomeSourceEnum, SomeDestinationEnum>().ConvertUsingEnumMapping(opt => opt.MapByName());

Mapping:

var someSourceDto = new SomeSourceDto { SomeEnum = 0 };
Mapper.Map<SomeDestinationDto>(someSourceDto);

Exception:

AutoMapper.AutoMapperMappingException

Error mapping types.

...

Destination Member:

SomeEnum

My current workaround is to apply this before the map:

someSourceDto.SomeEnum = someSourceDto.SomeEnum == 0 ? null : someSourceDto.SomeEnum;

Is there a better way? I can't use ConvertUsingEnumMapping when I make the enum types in the map nullable.

The problem is that this can happen for every enum property so I'm at big risk of runtime errors atm because I have no control over the source side of the data.

Example repo: https://github.com/matthiaslischka/AutoMapperNullEnumIssue/blob/main/AutoMapperNullEnumIssue/UnitTest1.cs

[lbargaoanu]

You can write that check inside a MapFrom.

[matthiaslischka]

Isn't there a cleaner way when the property is already nullable? Some fallback to null when the ConvertUsingEnumMapping does not yield a result?

I would have to do this for every nullable enum value to prevent unnecessary runtime errors. And probably not only for value but for all unmapped values.

But then I also have an enum where I can map 9 out of 10 by name and the 10th needs a manual map because there is a typo:

opt.MapByName().MapValue(SomeSourceEnum.FoBar, SomeDestinationEnum.FooBar);

Would be nice if ConvertUsingEnumMapping could be applied to nullable Enums and map to null if no map was found but that does not work atm.

[lbargaoanu]

Your case seems totally arbitrary, but I have no thoughts on ConvertUsingEnumMapping.

[matthiaslischka]

How is it arbitrary? ^^ Mapping a nullable enum property to null when the map does not yield an result instead of throwing an runtime error doesn't sound so far fetched, does it?

[lbargaoanu]

It certainly doesn't sound like code that belongs in a reusable library, but for app code, sure.

[matthiaslischka]

Dunno. I'm actually wondering if there isn't already a way to handle some default/fallback behavior in the library that I'm just overlooking.

We use ConvertUsingEnumMapping and MapByName() in order to find any missing enum mappings at unittest time with EnableEnumMappingValidation and AssertConfigurationIsValid. But this runtime error when the enum value is undefined - surely I'm not the first one running into this.

[HenkKin]

See issue: #4 (comment)