Question

I working within an older application (from 1.1 days) and there are many non-generic collections like:

[Serializable]
public class MyEntityList : CollectionBase
{
    private int _virtualRecordCount;

    public int VirtualRecordCount
    {
        get { return _virtualRecordCount; }
        set { _virtualRecordCount = value; }
    }

    public MyEntityList()
    {
    }

    public MyEntityList(MyEntity[] arr)
    {
        AddRange(arr);
    }

    public MyEntityList this[int index]
    {
        get { return (MyEntity)InnerList[index]; }
    }

    public void Add(MyEntity item)
    {
        InnerList.Add(item);
    }

    etc...

I’ve upgraded a layer of the application to use a generic Collection<T> for the return type. This layer is auto-generated and the class names are based upon the datasource table names. The business entity classes don’t necessary line up either, but not so in this case. In this case, they match perfectly 1:1.

I tried mapping the collections like so:

Collection<MyEntityResponse> responses = GetMyEntityResponses();

AutoMapper.Mapper.CreateMap<MyEntityResponse, MyEntity>();
myEntityList = AutoMapper.Mapper.Map<MyEntityList>(responses);

The strangest thing… I thought that it might squawk at the use of CollectionBase, but I hit F5 anyway. To my suprise, no compiler error and no exception. WOOHOO!

However, later in the app it threw an exception complaining about a type conversion from MyEntityResponse to MyEntity when it tried to perform a foreach() on MyEntityList that was returned from Mapper.Map.

What happened is a new MyEntityList collection was returned but it was filled with MyEntityResponse objects. Huh?? The custom collection overrides the Add() method and specifies that the type must be of type MyEntity. I would expect it to blow up when attempting to add the wrong type to the collection, but it didn’t seem to have any issues working with CollectionBase.

So my questions are, if the two types I’m trying to map match perfectly (property to property), and AutoMapper didn’t have an issue with CollectionBase, why was it not able to map the enties? And why didn’t it throw an exception instead of shoving the wrong type into the collection?

---

EDIT: I think I know why… because the non-generic collection doesn’t have a known type associated with it, like the generic one does.

So, new question… how do I tell it to use MyEntity instead of MyEntityResponse ?

Answer 1

I think I found my answer: http://automapper.codeplex.com/wikipage?title=Lists%20and%20Arrays

For the non-generic enumerable types, only unmapped, assignable types
are supported, as AutoMapper will be unable to “guess” what types
you’re trying to map. As shown in the example above, it’s not
necessary to explicitly configure list types, only their member types.

As of release 0.2.0, no custom destination collection types are
supported.