EmitMapper's List Mapping Issue with Collections

Question

The source class:

public class Post
{
    public long ID { get; set; }

    [Column(TypeName="nvarchar")]
    [Required]
    [StringLength(250)]
    public string Name { get; set; }

    [Column(TypeName="varchar")]
    [StringLength(250)]
    public string UrlName { get; set; }

    [Column(TypeName="ntext")]
    public string Excerpt { get; set; }

    [Column(TypeName="ntext")]
    [Required]
    public string Content { get; set; }

    public DateTime PostedTime { get; set; }
    public DateTime? PublishedTime { get; set; }
    public DateTime? LastUpdatedTime { get; set; }
    public bool IsPublished { get; set; }

    public virtual List<Category> Categories { get; set; }
    public virtual List<Comment> Comments { get; set; }
    public virtual List<Tag> Tags { get; set; }
}

the destination class

public class Post : Model
{
    public long ID { get; set; }
    public string Name { get; set; }
    public string UrlName { get; set; }
    public string Excerpt { get; set; }
    public string Content { get; set; }
    public DateTime PostedTime { get; set; }
    public DateTime LastCommentedTime { get; set; }
    public bool IsPublished { get; set; }

    public List<Category> Category { get; set; }
    public List<Comment> Comments { get; set; }
    public List<Tag> Tags { get; set; }
}

I try using EmitMapper to map from each other; when mapping from source to desction, here is the code sample：

[TestMethod]
    public void ShouleMapEntityToModel()
    {
        Post eP = new Post();
        eP.ID = 2;
        eP.Comments = new List<Comment>();

        eP.Comments.Add(new Comment()
            {
                ID = 2,
                Author = "derek"
            });

        var mP = eP.Map<Post, mBlog.Core.Models.Post>();

        Assert.IsNotNull(mP);
        Assert.AreEqual(1, mP.Comments.Count());
    }

and I got an exception,

Test method mBlog.Test.EmitMapperTest.ShouleMapEntityToModel threw exception: System.Exception: Constructor for types [] not found in System.Collections.Generic.IList`1[[mBlog.Core.Models.Post, mBlog.Core, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]

Is Map an extension to Post? If so it would be useful to give it a look to see how you create the mapper — onof, Jul 18 '11 at 08:54
An easy fix is to put a default constructor and initialize the lists yourself — leon.io, Aug 20 '13 at 14:12

score 1 · Accepted Answer · answered Mar 02 '12 at 18:19

I had the same problem, but I have found the solution. Don't user Lists for your destination object. If you use simple arrays in your mBlog.Core.Models.Post object you should get a nicely filled object. So your destination class should look like:

public class Post : Model
{
    public long ID { get; set; }
    public string Name { get; set; }
    public string UrlName { get; set; }
    public string Excerpt { get; set; }
    public string Content { get; set; }
    public DateTime PostedTime { get; set; }
    public DateTime LastCommentedTime { get; set; }
    public bool IsPublished { get; set; }

    public Category[] Category { get; set; }
    public Comment[] Comments { get; set; }
    public Tag[] Tags { get; set; }
}

though it works, but it doesn't fix the problem of EmitMapper. Anyway I've turn back to AutoMapper. — fengd, Mar 05 '12 at 15:53

score 0 · Answer 2 · edited May 23 '17 at 10:32

This answer shows how to handle IEnumerable to IEnumerable: EmitMapper and List

I believe that can be applied to this case too. Take a look:

This can be done creating a custom class, implementing the interface "ICustomConverterProvider" and adding a ConvertGeneric to the "DefaultMapConfig".

Looking on the source code of EmitMapper, i found a class named "ArraysConverterProvider", which is the default generic converter from ICollections to Arrays.

Adapting the code from this class to work with IEnumerable collections:

class GenericIEnumerableConverterProvider : ICustomConverterProvider
    {
      public CustomConverterDescriptor GetCustomConverterDescr(
            Type from,
            Type to,
            MapConfigBaseImpl mappingConfig)
        {
            var tFromTypeArgs = DefaultCustomConverterProvider.GetGenericArguments(from);
            var tToTypeArgs = DefaultCustomConverterProvider.GetGenericArguments(to);
            if (tFromTypeArgs == null || tToTypeArgs == null || tFromTypeArgs.Length != 1 || tToTypeArgs.Length != 1)
            {
                return null;
            }
            var tFrom = tFromTypeArgs[0];
            var tTo = tToTypeArgs[0];
            if (tFrom == tTo && (tFrom.IsValueType || mappingConfig.GetRootMappingOperation(tFrom, tTo).ShallowCopy))
           {
               return new CustomConverterDescriptor
                {
                    ConversionMethodName = "Convert",
                    ConverterImplementation = typeof(GenericIEnumerableConverter_OneTypes<>),
                    ConverterClassTypeArguments = new[] { tFrom }
                };
            }

            return new CustomConverterDescriptor
            {
                ConversionMethodName = "Convert",
                ConverterImplementation = typeof(GenericIEnumerableConverter_DifferentTypes<,>),
                ConverterClassTypeArguments = new[] { tFrom, tTo }
            }; 
        }
    }


class GenericIEnumerableConverter_DifferentTypes<TFrom, TTo> : ICustomConverter
    {
        private Func<TFrom, TTo> _converter;
        public IEnumerable<TTo> Convert(IEnumerable<TFrom> from, object state)
        {
            if (from == null)
            {
                return null;
            } 

            TTo[] result = new TTo[from.Count()];
            int idx = 0;
            foreach (var f in from)
            {
                result[idx++] = _converter(f);
            }

            return result;
        } 

        public void Initialize(Type from, Type to, MapConfigBaseImpl mappingConfig)
        {
            var staticConverters = mappingConfig.GetStaticConvertersManager() ?? StaticConvertersManager.DefaultInstance;

            var staticConverterMethod = staticConverters.GetStaticConverter(typeof(TFrom), typeof(TTo));

           if (staticConverterMethod != null)
            {
                _converter = (Func<TFrom, TTo>)Delegate.CreateDelegate(
                    typeof(Func<TFrom, TTo>),
                    null,
                    staticConverterMethod
                );
            }
            else
            {
                _subMapper = ObjectMapperManager.DefaultInstance.GetMapperImpl(typeof(TFrom), typeof(TTo), mappingConfig);

                _converter = ConverterBySubmapper;
            }
        } 

        ObjectsMapperBaseImpl _subMapper;

        private TTo ConverterBySubmapper(TFrom from)
        {
            return (TTo)_subMapper.Map(from);
        }
    }


class GenericIEnumerableConverter_OneTypes<T>
    {

        public IEnumerable<T> Convert(IEnumerable<T> from, object state)
        {
            if (from == null)
          {
                return null;
            }

            return from;
        }
    }

This code is just a copy with a minimum of adaptation as possible and can be applyed to objects with many levels of hierarchy.

You can use the above code with the following command:

  new DefaultMapConfig().ConvertGeneric(
    typeof(IEnumerable<>),
    typeof(IEnumerable<>),
    new GenericIEnumerableConverterProvider());

This saved my day and I hope to save yours too! hehehe

EmitMapper's List Mapping Issue with Collections

2 Answers2