AutoMapper - 如何映射到嵌套类型中的值 - 可能为null？

Question

This is probably a simple answer, but i'm just starting to get the hand of AutoMapper.

Anyway, i have a domain object like this:

public class User
{
   public string Name { get; set; }
   public FacebookUser FacebookUser { get; set; }
}

And a ViewModel like this:

public class UserViewModel
{
   public string Name { get; set; }
   public long FacebookUniqueId { get; set; }
}

Here's what i have in my AutoMapper configuration:

Mapper.CreateMap<User,UserViewModel>()
   .ForMember(dest => dest.FacebookUniqueId, opt => opt.MapFrom(src => src.FacebookUser.FacebookUniqueId))

But it throws an exception when the FacebookUser object is null, which is to be expected.

How do i tell AutoMapper:

Map UserViewModel.FacebookUniqueId to User.FacebookUser.FacebookUniqueId, except for when it's null, then use 0 .

Any ideas?

Answer 1

opt => opt.MapFrom(src => src.FacebookUser == null ? 0 : src.FacebookUser.FacebookUniqueId)

如果我刚刚检查过第一个想法是否有效， MapFrom认为MapFrom方法只采用了一个指向属性的表达式来解决它...

Answer 2

The code you provided works on the latest Automapper. I investigated the sources a little and found the following code:

try
{
    var result = _method((TSource)source.Value);

    return source.New(result, MemberType);
}
catch (NullReferenceException)
{
    return source.New(null, MemberType);
}

As you can see now Automapper catches all the Null exceptions while resolving chain mappings. The following test is green:

[TestFixture]
public class AutomapperChainingMappingTest
{
    [Test]
    public void ChainMapping_NullProperty_DefaultValueSet()
    {
        AutoMapper.Mapper.CreateMap<User, UserViewModel>()
                  .ForMember(x => x.FacebookUniqueId, o => o.MapFrom(x => x.FacebookUser.FacebookUniqueId));

        var source = new User();
        var model = AutoMapper.Mapper.Map<UserViewModel>(source);

        Assert.AreEqual(model.FacebookUniqueId, default(long));
    }
}

public class User
{
    public FacebookUser FacebookUser { get; set; }
}

public class UserViewModel
{
    public long FacebookUniqueId { get; set; } 
}

public class FacebookUser
{
    public long? FacebookUniqueId { get; set; }
}