In response to an .NET Runtime API request (https://github.com/dotnet/runtime/issues/62112), The proposal was for a way to use _either_ the default values in a list, **or** the values bound from configuration. This simple gist shows an alternative, that is simply a custom `ICollection`

1_Program.cs

using System.Collections;
using Microsoft.Extensions.Configuration;

var builder = new ConfigurationBuilder()
    .SetBasePath(Directory.GetCurrentDirectory())
    .AddJsonFile("appsettings.json", optional: false);

var options = new MyImageProcessingOptions();

var config = builder.Build();
config.Bind("MyImageProcessingOptions", options);

Console.WriteLine(string.Join(",", options.ResizeWidths));

public class MyImageProcessingOptions
{
    public DefaultableCollection<int> ResizeWidths { get; } = new(100, 200, 400, 800);
}

2_DefaultableCollection.cs

public class DefaultableCollection<T> : ICollection<T>
{
    private readonly List<T> _items;
    private bool _overridden;

    public DefaultableCollection(params T[] defaults) => _items = defaults.ToList();

    public void Add(T value)
    {
        if (!_overridden)
        {
            _overridden = true;
            _items.Clear();
        }
        
        _items.Add(value);
    }

    public void Clear() => _items.Clear();

    public bool Contains(T item) => _items.Contains(item);

    public void CopyTo(T[] array, int arrayIndex) => _items.CopyTo(array, arrayIndex);

    public bool Remove(T item) => _items.Remove(item);

    public int Count => _items.Count;
    
    public bool IsReadOnly => false;

    public IEnumerator<T> GetEnumerator() => _items.GetEnumerator();

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

3_appsettings.json

{
  "MyImageProcessingOptions": {
    "ResizeWidths":  [
      "42",
      "69",
      "666"
    ]
  } 
}