Automapper with immutablelist

In Source class, I have IEnumurable<T> Property. In the dest class, constructor takes the IEnumurable<T> and converts to IImmutableList<T> and assigns to a IEnumurable<T> property.

Here is the code copied from program.cs in .Net 7.0:

// Copy and Paste to Program.cs in .Net 7 should work.
using AutoMapper;
using System.Collections.Immutable;

var mapperConfiguration = new MapperConfiguration(cfg => {
    cfg.CreateMap<ClassSource, ClassDestination>()
    .ConstructUsing(m => new ClassDestination(m.Nums));
    });

var mapper = mapperConfiguration.CreateMapper();

var classSource = new ClassSource();

var classDestination = mapper.Map<ClassDestination>(classSource);

Console.WriteLine(classDestination.Nums.Count());

public class ClassSource
{
    public IEnumerable<int> Nums { get; init; } = new List<int> { 1, 2, 3, 4, 5 };
}

public class ClassDestination
{
    public IEnumerable<int> Nums {get;}
    public ClassDestination(IEnumerable<int> ints)
    {
        //Nums = ints.ToList(); // This works
        Nums = ints.ToImmutableList(); // This throws exception
    }
}

Mapping fails with: AutoMapper.AutoMapperMappingException: 'Error mapping types.'

How can this be solved?

If you let the code run, the full error message is:

Unhandled exception. AutoMapper.AutoMapperMappingException: Error mapping types.

Mapping types:
ClassSource -> ClassDestination
ClassSource -> ClassDestination

Type Map configuration:
ClassSource -> ClassDestination
ClassSource -> ClassDestination

Destination Member:
Nums

 ---> System.NotSupportedException: Specified method is not supported.
   at System.Collections.Immutable.ImmutableList`1.System.Collections.Generic.ICollection<T>.Clear()
   at lambda_method1(Closure, Object, ClassDestination, ResolutionContext)
   --- End of inner exception stack trace ---
   at lambda_method1(Closure, Object, ClassDestination, ResolutionContext)
   at Program.<Main>$(String[] args)
Command terminated by signal 6

As you can see, it tries to call the method .Clear() of the immutable list in the target property Nums, which throws the NotSupportedException.

Because you already initialize the list through the constructor, you should explicitly ignore this member by adding the corresponding .Ignore() call to your mapping profile if you need to use ConstructUsing():

var mapperConfiguration = new MapperConfiguration(cfg =>#
{
    cfg.CreateMap<ClassSource, ClassDestination>()
        .ForMember(m => m.Nums, conf => conf.Ignore())
        .ConstructUsing(m => new ClassDestination(m.Nums));
});

If your target class is already fully initialized after calling the constructor, you should instead use .ConvertUsing():

var mapperConfiguration = new MapperConfiguration(cfg =>#
{
    cfg.CreateMap<ClassSource, ClassDestination>()
        .ConvertUsing(m => new ClassDestination(m.Nums));
});