http://stackoverflow.com/questions/32185447/how-do-you-support-selecting-multiple-enum-values-for-an-episerver-property

I ran into this issue today, and my problem was also the underlying Value type of the SelectItem. It looks like the Value needs to be a string.

Given you have a property that is a SelectMany:

    [SelectMany(SelectionFactoryType = typeof(ChildPagesSelectionFactory))]
    public virtual string ChildPages { get; set; }

A common custom ISelectionFactory could be to return a list of child pages:

    public class ChildPagesSelectionFactory : ISelectionFactory
    {
        private Injected<IContentLoader> _contentLoader;
        public IEnumerable<ISelectItem> GetSelections(ExtendedMetadata metadata)
        {
            var contents = _contentLoader.Service.GetChildren<IContent>(ContentReference.StartPage);
            return contents.Select(content => new SelectItem { Text = content.Name, Value = content.ContentLink.ID });
        }
    }

This code would cause the issue you described, since the Value is of type int.

To solve this, cast the Value to a string:

    public class ChildPagesSelectionFactory : ISelectionFactory
    {
        private Injected<IContentLoader> _contentLoader;
        public IEnumerable<ISelectItem> GetSelections(ExtendedMetadata metadata)
        {
            var contents = _contentLoader.Service.GetChildren<IContent>(ContentReference.StartPage);
            return contents.Select(content => new SelectItem { Text = content.Name, Value = content.ContentLink.ID.ToString() });
        }
    }

This issue was reproduced and tested in Alloy MVC running CMS 9.8.3.

This is not an issue if you are using a SelectOne property.

I have a support ticket opened for this. Hopefully it'll be marked as a bug and fixed.