问题:

Background: I'm working on a webservice that I want to allow input that has a null field to mean, "don't do an update". The input object is very similar but not identical to the database model, so we're using automapper to do the transforms.

So in the case of an update, I'd like to be able to take the existing values, use them to override any of the null fields in the input, and then save that to do the whole update. So is there a way to make automapper only put values into the destination if the destination field is null?

回答1:

Yes, it can, but you probably wouldn't want to go through the hassle. To do this, you're going to need to have a custom map handler for every field on the object that you want to do this (You may be able to share the custom handler among properties of the same type, but I'm not 100% sure without looking at some of my old code).

回答2:

I recently solved this on my own problem using a PreCondition with Automapper 5.2.0.

CreateMap<Foo, Foo>()
  .ForAllMembers(opt => opt.Precondition(
    new Func<Foo, bool>( foo =>
      if (opt.DestinationMember == null) return true;
      return false;
    )
  ));

This looks at all destination members, and first looks to see if the destination member is null before even looking at the source member. (If it is not null, then it never looks at the source.)