Automapper - 嵌套类型的AutoMapperMappingException和NullReferenceException(Automapper - AutoMapperMappingException and NullReferenceException for nested type)

 我有一个Tag类和一个相应的TagDto类，我想从Tag到TagDto。 
 
 由于我使用EF Core，我有一个名为MoneyItemTag的类，它代表了多对多的关系。 
 
 这是我的课程： 
 
public abstract class MoneyItemBase
{
    public int Id { get; set; }

    public string ItemText { get; set; }

    public decimal Amount { get; set; }

    public MoneyItemType MoneyItemType { get; protected set; }

    public ICollection<MoneyItemTag> MoneyItemTags { get; set; }
}

public class MoneyItemTag
{
    public int MoneyItemId { get; set; }

    public MoneyItemBase MoneyItem { get; set; }

    public int TagId { get; set; }

    public Tag Tag { get; set; }
}

public class Tag
{
    public int TagId { get; set; }

    public string TagName { get; set; }

    public ICollection<MoneyItemTag> MoneyItemTags { get; set; }
}

 [Fact]
    public void Tag_TagDto_Mapping()
    {
        Mapper.Initialize(cfg =>
        {
            //cfg.AddProfile<MappingProfile>();
            cfg.CreateMap<Tag, TagDto>()
                .ForMember(x => x.MoneyItems, opts => opts.MapFrom(src => src.MoneyItemTags.Select(x => x.MoneyItem)));
            cfg.CreateMap<MoneyItemBase, MoneyItemBaseDto>()
                .ForMember(x => x.Tags, opts => opts.MapFrom(src => src.MoneyItemTags.Select(y => y.Tag.TagName).ToList()));
        });

        MoneyItemTag mo = new MoneyItemTag();
        mo.MoneyItem = new SingleIncome() { Id = 2, ItemText = "test", Active = false, DueDate = DateTime.Now, Amount = 33 };
        mo.MoneyItemId = 2;
        var mit = new List<MoneyItemTag>() { mo };

        Tag tag = new Tag() { TagId = 2, TagName = "test", MoneyItemTags = mit };
        mo.TagId = 2;
        mo.Tag = tag;

        var dto = Mapper.Map<TagDto>(tag);

        Assert.NotNull(dto);
 
 而这种关系造成了一些麻烦。 当我运行以下测试或当我想在我的Asp.Net核心应用程序中进行映射时，出现以下异常： 
 
AutoMapper.AutoMapperMappingException : Error mapping types.

Mapping types:
Tag -> TagDto
HouseholdBook.Data.Model.Tag -> HouseholdBook.Dto.TagDto

Type Map configuration:
Tag -> TagDto
HouseholdBook.Data.Model.Tag -> HouseholdBook.Dto.TagDto

Property:
MoneyItems
---- AutoMapper.AutoMapperMappingException : Error mapping types.

Mapping types:
MoneyItemBase -> MoneyItemBaseDto
HouseholdBook.Data.Model.MoneyItemBase -> HouseholdBook.Dto.MoneyItemBaseDto

Type Map configuration:
MoneyItemBase -> MoneyItemBaseDto
HouseholdBook.Data.Model.MoneyItemBase -> HouseholdBook.Dto.MoneyItemBaseDto

Property:
Id
-------- System.NullReferenceException : Object reference not set to an 
instance of an object.
 
 这里有什么问题？ 我从异常消息中看不到我缺少的东西。 

I have a Tag class and a corresponding TagDto class which I want to map from Tag to TagDto.
 
Due to my usage of EF Core I have a Collection to a class called MoneyItemTag which represents a many-to-many relationship.
 
Here are my classes:
 
public abstract class MoneyItemBase
{
    public int Id { get; set; }

    public string ItemText { get; set; }

    public decimal Amount { get; set; }

    public MoneyItemType MoneyItemType { get; protected set; }

    public ICollection<MoneyItemTag> MoneyItemTags { get; set; }
}

public class MoneyItemTag
{
    public int MoneyItemId { get; set; }

    public MoneyItemBase MoneyItem { get; set; }

    public int TagId { get; set; }

    public Tag Tag { get; set; }
}

public class Tag
{
    public int TagId { get; set; }

    public string TagName { get; set; }

    public ICollection<MoneyItemTag> MoneyItemTags { get; set; }
}

 [Fact]
    public void Tag_TagDto_Mapping()
    {
        Mapper.Initialize(cfg =>
        {
            //cfg.AddProfile<MappingProfile>();
            cfg.CreateMap<Tag, TagDto>()
                .ForMember(x => x.MoneyItems, opts => opts.MapFrom(src => src.MoneyItemTags.Select(x => x.MoneyItem)));
            cfg.CreateMap<MoneyItemBase, MoneyItemBaseDto>()
                .ForMember(x => x.Tags, opts => opts.MapFrom(src => src.MoneyItemTags.Select(y => y.Tag.TagName).ToList()));
        });

        MoneyItemTag mo = new MoneyItemTag();
        mo.MoneyItem = new SingleIncome() { Id = 2, ItemText = "test", Active = false, DueDate = DateTime.Now, Amount = 33 };
        mo.MoneyItemId = 2;
        var mit = new List<MoneyItemTag>() { mo };

        Tag tag = new Tag() { TagId = 2, TagName = "test", MoneyItemTags = mit };
        mo.TagId = 2;
        mo.Tag = tag;

        var dto = Mapper.Map<TagDto>(tag);

        Assert.NotNull(dto);
 
And this relationship is causing some trouble. When I run the following test or when I want make the mapping in my Asp.Net core application, I get the following exception:
 
AutoMapper.AutoMapperMappingException : Error mapping types.

Mapping types:
Tag -> TagDto
HouseholdBook.Data.Model.Tag -> HouseholdBook.Dto.TagDto

Type Map configuration:
Tag -> TagDto
HouseholdBook.Data.Model.Tag -> HouseholdBook.Dto.TagDto

Property:
MoneyItems
---- AutoMapper.AutoMapperMappingException : Error mapping types.

Mapping types:
MoneyItemBase -> MoneyItemBaseDto
HouseholdBook.Data.Model.MoneyItemBase -> HouseholdBook.Dto.MoneyItemBaseDto

Type Map configuration:
MoneyItemBase -> MoneyItemBaseDto
HouseholdBook.Data.Model.MoneyItemBase -> HouseholdBook.Dto.MoneyItemBaseDto

Property:
Id
-------- System.NullReferenceException : Object reference not set to an 
instance of an object.
 
What is wrong here? I cannot see from the exception message what I am missing.

原文：https://stackoverflow.com/questions/43968442