使用开放泛型自动化并在ForMember语句中包含源

Question

I've recently upgraded from Automapper 4.2.1 to 5.1.1 and am having issues with a previously valid mapping involving open generics.

Previously, within the automapper configuration, I had the following open generic mapping configuration

CreateMap(typeof(IPager<>), typeof(ModelPager<>))
    .ForMember("Items", e => e.MapFrom(o => (IEnumerable) o));

This works in Automapper 4 but fails in 5 with a InvalidOperaionException when attempting to map via IMapper.Map<TDestination>(source) . It appears to fail when executing the mapping of the Items ForMember operation with an exception message of " Sequence contains no matching element "

As reflected in the example implementation code below IPager<TSource> implements IEnumerable<TSource> , and the Items property of ModelPager<TDestination> is an IEnumerable<TDestination> so the cast should be valid. and there exist a valid mapping for each TSource to TDestination

CreateMap<TSource, TDestination>();

IPager interface

public interface IPager<out TItem> : IEnumerable<TItem>
{
    int CurrentPage { get; }

    int PageCount { get; }

    int PageSize { get; }

    int TotalItems { get; }
}

IPager implementation

public class Pager<TItem> : IPager<TItem>
{
    private readonly IEnumerable<TItem> _items;

    public Pager(IEnumerable<TItem> items,
                 int currentPage,
                 int pageSize,
                 int totalItems)
    {
        /// ... logic ... 
        this._items = items ?? Enumerable.Empty<TItem>();
        this.CurrentPage = currentPage;
        this.PageSize = pageSize;
        this.TotalItems = totalItems;
    }

    public int CurrentPage { get; }

    public int PageCount => (this.TotalItems + this.PageSize - 1) / this.PageSize;

    public int PageSize { get; }

    public int TotalItems { get; }

    IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();

    public IEnumerator<TItem> GetEnumerator() => this._items.GetEnumerator();
}

ModelPager

public class ModelPager<TItem>
{
    public int CurrentPage { get; set; }

    public IEnumerable<TItem> Items { get; set; }

    public int PageCount { get; set; }

    public int PageSize { get; set; }

    public int TotalItems { get; set; }
}

What is the proper way to map this in Automapper 5 without either abandoning open generics by explicitly mapping each possible mapping, or by using a custom open generic type converter that would require me to manually map all properties and use reflection to resolve the open types for assignment?

Answer 1

Given this looks to be a bug ( AutoMapper #1624 ), a work around can be done with a custom open generic TypeConverter that does not require reflection.

The mapping should be changed to something along the lines of

CreateMap(typeof(IPager<>), typeof(ModelPager<>))
    .ConvertUsing(typeof(PagerToModelPagerConverter<,>));

with a custom ITypeConverter

public class PagerToModelPagerConverter<TSource, TDestination> : ITypeConverter<IPager<TSource>, ModelPager<TDestination>>
{
    public ModelPager<TDestination> Convert(IPager<TSource> source,
                                            ModelPager<TDestination> destination,
                                            ResolutionContext context)
    {
        var list = source.ToList(); // avoid redundant iterations
        var itemMapping = context.Mapper.Map<IEnumerable<TSource>, IEnumerable<TDestination>>(list);

        var modelPager = new ModelPager<TDestination>
                         {
                             CurrentPage = source.CurrentPage,
                             Items = itemMapping,
                             PageCount = source.PageCount,
                             PageSize = source.PageSize,
                             TotalItems = source.TotalItems
                         };

        return modelPager;
    }