Mapping a list to a dictionary

问题

I have this class:

class ClassFrom
{
    public int Id { get; set; }
    public string Foo { get; set; }
    public string Bar { get; set; }
}

I want to map it into this, with the Id property becoming the dictionary's key:

class ClassTo
{
    public string Foo { get; set; }
    public string Bar { get; set; }
}

Dictionary<int, ClassTo> toDict
    = mapper.Map<List<ClassFrom>, Dictionary<int, ClassTo>>(fromList);

Is there a recommended way to accomplish this?

The best way I've found to do this using Automapper has a slight code smell to me. I am essentially double-mapping the object, first to ClassTo and then to KeyValuePair through its constructor:

var cfg = new MapperConfiguration(c =>
{
    c.CreateMap<ClassFrom, ClassTo>();

    c.CreateMap<ClassFrom, KeyValuePair<int, ClassTo>>()
        .ForCtorParam("key", paramOptions => paramOptions.MapFrom(from => from.Id))
        .ForCtorParam("value", paramOptions => paramOptions.MapFrom(from => from));
});

IMapper mapper = new AutoMapper.Mapper(cfg);

List<ClassFrom> fromList = new List<ClassFrom>
{
    new ClassFrom { Id = 1, Foo = "foo1", Bar = "Bar1" },
    new ClassFrom { Id = 2, Foo = "foo2", Bar = "Bar2" }
};

Dictionary<int, ClassTo> toDict
    = mapper.Map<List<ClassFrom>, Dictionary<int, ClassTo>>(fromList);

回答1:

You can just use the ToDictionary extension method in the System.Linq namespace.

//using System.Linq;
var toDict = fromList.ToDictionary
(
    //Define key
    element => element.Id,   

    //Define value
    element => new ClassTo { Foo = element.Foo, Bar = element.Bar } 
);

回答2:

You can use ConstructUsing instead of ForCtorParam. If you change the mapper configuration like below, it will work correctly.

var cfg = new MapperConfiguration(c =>
        {
            c.CreateMap<ClassFrom, ClassTo>();

            c.CreateMap<ClassFrom, KeyValuePair<int, ClassTo>>()
                .ConstructUsing(x => new KeyValuePair<int, ClassTo>(x.Id, new ClassTo { Bar = x.Bar, Foo = x.Foo }));
        });

来源：https://stackoverflow.com/questions/51109761/mapping-a-list-to-a-dictionary

标签

.net

automapper