Mapping to Enumerable.Empty<T> throws in netcoreapp3.1

[natehitze]

I believe it is related to this issue: dotnet/runtime#27552

public class SourceClass
{
    public IEnumerable<string> Values { get; set; }
}
public class TargetClass
{
    public IEnumerable<string> Values { get; set; }
}
var config = new MapperConfiguration(cfg => cfg.CreateMap<SourceClass, TargetClass>());
var mapper = new Mapper(config);
var source = new SourceClass { Values = new string[0] };
var existingTarget = new TargetClass { Values = Enumerable.Empty<string>() };
// This line throws in netcoreapp3.1 but not in netcoreapp2.2
mapper.Map(source, existingTarget);

Expected behavior

Not to throw an exception.

Actual behavior

Unable to cast object of type 'System.Linq.EmptyPartition`1[System.String]' to type 'System.Collections.Generic.IList`1[System.String]'.

[lbargaoanu]

But the thing is, this mapping can't possibly succeed. You cannot map into an existing destination that's not IList. Removing the setter would make this clear.

[natehitze]

I'm a little confused on why IList is required. If the destination type is IEnumerable<T> couldn't we just map the source items to a new list and assign that to the destination? It seems like CollectionMapperExpressionFactory.MapCollectionExpression does something similar for read only collections. Or maybe call .ToList() on the destination type and use that as the destination instead of using a cast?

My use case is mapping an update request to a database object that eager loads related data. If there is not any related data objects the database mapper assigns the related data property Enumerable.Empty<T>. I'm "lucky" enough that I control how the database mapping works so I could assign something else (like Array.Empty<T>).

What would you suggest changing in the example code going forward? Should I manually .ToList() the related data collections on my destination object before mapping over it? Or change all of my related data collections to IList<T> instead of IEnumerable<T>?

[lbargaoanu]

Check the execution plan. And compare with not passing a destination object.

[lbargaoanu]

@jbogard I tend to see this as an usage error.
https://github.com/AutoMapper/AutoMapper/blob/b26599b821cdbd441a1653cb7ade53c30752db00/src/AutoMapper/Mappers/EnumerableMapper.cs#L25

[jbogard]

Oh this is very weird. That logic is incorrect in the mapper, though. We test for IEnumerable || IList but then assume IList. We shouldn't.

[natehitze]

I've been trying to work up a solution but I'm not sure what I'm missing here.

Here's my rough current version of EnumerableMapper. It seems like the .ToList() and filling the list work fine but the result never gets assigned to the destination property.
https://gist.github.com/natehitze/174b8835e7d73d3c4a65d0acbbcd9aca

[lbargaoanu]

@jbogard How else would you clear it?

[jbogard]

We don't - if it's got a setter, we replace it.