Unable to map property of type custom collection interface

[mingyaulee]

Source/destination types

public class Source
{
    public ICustomCollection Collection { get; set; }
}

public class Destination
{
    public ICustomCollection Collection { get; set; }
}

public interface ICustomCollection : ICollection<int>
{
}

public class CustomCollection : ICustomCollection
{
    private readonly List<int> data = new();
    public int Count => data.Count;
    public bool IsReadOnly => false;
    public void Add(int item) => data.Add(item);
    public void Clear() => data.Clear();
    public bool Contains(int item) => data.Contains(item);
    public void CopyTo(int[] array, int arrayIndex) => data.CopyTo(array, arrayIndex);
    public IEnumerator<int> GetEnumerator() => data.GetEnumerator();
    public bool Remove(int item) => data.Remove(item);
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

Mapping configuration

CreateMap<Source, Destination>();

Version: 11.0.1

Expected behavior

Property Collection is mapped successfully from Source to Destination.

Actual behavior

A mapping exception was thrown Unable to cast object of type 'System.Collections.Generic.List1[System.Int32]' to type 'Test.ICustomCollection'.`.

Steps to reproduce

public static class Program
{
    public static void Main()
    {
        var mapper = new MapperConfiguration(c => c.CreateMap<Source, Destination>()).CreateMapper();
        var source = new Source()
        {
            Collection = new CustomCollection() { 1, 2, 0 }
        };
        // the following line throws a mapping exception "Unable to cast object of type 'System.Collections.Generic.List`1[System.Int32]' to type 'Test.ICustomCollection'."
        var destination = mapper.Map<Destination>(source);
    }
}

Link to fiddle

[lbargaoanu]

Check the execution plan.

[lbargaoanu]

You're supposed to try to understand that code :) That's what AM can do with your configuration. If you want smth else, you have to actually write the code to do that.

[mingyaulee]

So the AM is supposed to try to map any IEnumerable to a List even though the type is not assignable from List? It sounds more like a bug than a "feature" if you look at this basic scenario. We can't even inherit from List from an interface so why is AM trying to cast a List to an interface?

[lbargaoanu]

If you think you can do better, a PR is welcome.