为什么AutoMapperExtension的行为与直接实现不同？(Why would an AutoMapperExtension behave differently from a direct implementation?)

 我在AutoMapperConfig RegisterMappings()中定义了以下映射： 
 
AutoMapper.Mapper.CreateMap<Member, PhotoViewModel>()
            .ForMember(dest => dest.Show, opt => opt.MapFrom(src => src.ShowPhoto))
            .ForMember(dest => dest.Value, opt => opt.MapFrom(src => AutoMapper.Mapper.Map<PhotoValueViewModel>(src));
AutoMapper.Mapper.CreateMap<Member, PhotoValueViewModel>();
 
 此映射映射PhotoViewModel两个属性。 首先，它根据src.ShowPhoto映射Show属性，然后将PhotoValueViewModel映射到Value属性。 
 
 映射有效！ 不幸的是，我有许多需要类似映射的对象。 所以，我尝试将一些实现抽象为AutoMapperExtension方法。 该方法如下所示： 
 
public static IMappingExpression<TSource, TDestination> ApplyPropertyMapping<TSource, TDestination>
        (this IMappingExpression<TSource, TDestination> iMappingExpression, Expression<Func<TSource, bool?>> show, Expression<Func<TSource, object>> value)
        where TDestination : PropertyViewModel
    {
        iMappingExpression
            .ForMember(dest => dest.Show, opt => opt.MapFrom(show))
            .ForMember(dest => dest.Value, opt => opt.MapFrom(value));
        return iMappingExpression;
    }
 
 我希望这个扩展方法允许我将原始映射定义为： 
 
AutoMapper.Mapper.CreateMap<Member, PhotoViewModel>()
    .ApplyPropertyMapping(src => src.ShowPhoto, src => AutoMapper.Mapper.Map<PhotoValueViewModel>(src));
 
 但它不起作用！ 现在调用Mapper.Map<Member, PhotoViewModel>(MemberObject, PhotoViewModelObject)将PhotoViewModelObject.Value属性设置为null。 
 
 造成差异的原因是什么？ 
 
 其中一些对象的定义是： 
 
public class Member
{
    /**Irrelevant Properties Not Shown**/
    public Guid? PhotoID { get; set; }
    public string PhotoFileName { get; set; }
    public bool? ShowPhoto { get; set; }
}

public class PropertyViewModel
{
    public object Value { get; set; }
    public bool? Show { get; set; }
}

public class PhotoViewModel : PropertyViewModel
{
    public PhotoValueViewModel Value { get; set; }
}

public class PhotoValueViewModel
{
    public Guid? PhotoID { get; set; }
    public string PhotoFileName { get; set; }
    public string PhotoUrl {get {return Utils.GeneratePhotoUrl(PhotoID); } }
}

I have defined the following mapping within RegisterMappings() of AutoMapperConfig:
 
AutoMapper.Mapper.CreateMap<Member, PhotoViewModel>()
            .ForMember(dest => dest.Show, opt => opt.MapFrom(src => src.ShowPhoto))
            .ForMember(dest => dest.Value, opt => opt.MapFrom(src => AutoMapper.Mapper.Map<PhotoValueViewModel>(src));
AutoMapper.Mapper.CreateMap<Member, PhotoValueViewModel>();
 
This mapping maps two properties of a PhotoViewModel. First it maps the Show property based off of src.ShowPhoto, and then it maps a PhotoValueViewModel into the Value property.
 
The mapping works! Unfortunately, I have many objects that require a similar mapping. So, I tried abstracting some of the implementation into an AutoMapperExtension method. The method looks like this:
 
public static IMappingExpression<TSource, TDestination> ApplyPropertyMapping<TSource, TDestination>
        (this IMappingExpression<TSource, TDestination> iMappingExpression, Expression<Func<TSource, bool?>> show, Expression<Func<TSource, object>> value)
        where TDestination : PropertyViewModel
    {
        iMappingExpression
            .ForMember(dest => dest.Show, opt => opt.MapFrom(show))
            .ForMember(dest => dest.Value, opt => opt.MapFrom(value));
        return iMappingExpression;
    }
 
I'd expect this extension method to allow me to define the original mapping as:
 
AutoMapper.Mapper.CreateMap<Member, PhotoViewModel>()
    .ApplyPropertyMapping(src => src.ShowPhoto, src => AutoMapper.Mapper.Map<PhotoValueViewModel>(src));
 
But it doesn't work! Now calls to Mapper.Map<Member, PhotoViewModel>(MemberObject, PhotoViewModelObject) set the PhotoViewModelObject.Value property to null.
 
What is causing the difference?
 
The definitions of some of these objects are:
 
public class Member
{
    /**Irrelevant Properties Not Shown**/
    public Guid? PhotoID { get; set; }
    public string PhotoFileName { get; set; }
    public bool? ShowPhoto { get; set; }
}

public class PropertyViewModel
{
    public object Value { get; set; }
    public bool? Show { get; set; }
}

public class PhotoViewModel : PropertyViewModel
{
    public PhotoValueViewModel Value { get; set; }
}

public class PhotoValueViewModel
{
    public Guid? PhotoID { get; set; }
    public string PhotoFileName { get; set; }
    public string PhotoUrl {get {return Utils.GeneratePhotoUrl(PhotoID); } }
}

原文：https://stackoverflow.com/questions/32915518