Automapper 和处理空属性

Question

I've set my mapping up as follows:

CreateMap<SourceClass, DestinationClass>().ForMember(destinationMember => destinationMember.Provider,
                memberOptions => memberOptions.MapFrom(src => src.Providers.FirstOrDefault()));

Where I'm mapping from a List in my SourceClass to a string in my destination class.

My question is, how can I handle the case where "Providers" is null?

I've tried using:

src?.Providers?.FirstOrDefault()

but I get an error saying I can't use null propagators in a lambda.

I've been reading up on Automapper and am still unsure if AM automatically handles the null case or not. I tried to build the expression tree, but was not able to see any information that provided additional informations.

If it helps, I'm using automapper v 6.1.1.

Answer 1

You could try using a ValueConverter with AutoMapper. That might look something like this

public class ListFormatter : IValueConverter<string, List<string>>
{
  public List<string> Convert(string source)
  {
    if (source != null)
    {
      return new List<string> { source };
    }
    return new List<string>();
  }
}

And then you can use it like this

CreateMap<SourceClass, DestinationClass>()
  .ForMember(destinationMember => destinationMember.Provider,
             memberOptions => memberOptions.ConvertUsing(new ListFormatter()));

This would allow you to change your value converter in the future if you need to switch logic or do something more complex.

Edit

Since you are using an older version, you could use a private/static/extension method to do the same thing. So something like

List<string> ConvertStringToList(string source)
{
  if (source != null)
  {
    return new List<string> { source };
  }
  return new List<string>();
}

and then call it like so

CreateMap<SourceClass, DestinationClass>()
  .ForMember(destinationMember => destinationMember.Provider,
             memberOptions => memberOptions.MapFrom(src => ConvertStringToList(src.Provider)));

I generally prefer this to doing something inline as things get more complex, for the sake of readability

Answer 2

尝试使用 AutoMapper 中的 NullSubstitution 选项，您可以在此处阅读