Load a Dropdownlist based on an enum - Asp.net core

You can create a static class and manipulate your Enum to popular your SelectListItem

public static class ExtensaoDeEnumerador
{
    public static string GetEnumDescription<T>(string value)
    {
        Type type = typeof(T);
        var name = Enum.GetNames(type).Where(f => f.Equals(value, StringComparison.CurrentCultureIgnoreCase)).Select(d => d).FirstOrDefault();

        if (name == null)
        {
            return string.Empty;
        }

        var field = type.GetField(name);
        var customAttribute = field.GetCustomAttributes(typeof(DescriptionAttribute), false);
        return customAttribute.Length > 0 ? ((DescriptionAttribute)customAttribute[0]).Description : name;
    }

    public static IEnumerable<SelectListItem> EnumToSelectList<T>(string tipoCase = null)
    {
        return (Enum.GetValues(typeof(T)).Cast<T>().Select(
            e => new SelectListItem()
            {
                Text = (tipoCase == null ? GetEnumDescription<T>(e.ToString()) : (tipoCase.ToUpper() == "U" ? GetEnumDescription<T>(e.ToString()).ToUpper() : GetEnumDescription<T>(e.ToString()).ToLower())),
                Value = e.ToString()
            })).ToList();
    }
}

View the call.

pessoaViewModel.PessoasNaturezas = ExtensaoDeEnumerador.EnumToSelectList<PessoaNatureza>("U").OrderBy(x => x.Text);

There is another option, though I prefer Marconcilio's approach to solving the functionality as a whole and not just this timely issue. But I also do not much agree with the structure presented in the question.

You can add a constructor to your ViewModel where you populate the List<SelectListItem> PessoasNaturezas with Enum values and mark the populated value in PessoaNaturezaId as selected.

public class PessoaViewModel
{
    [Key]
    [DisplayName("Código")]
    public int Id { get; set; }

    [DisplayName("Natureza")]
    [Display(Name = "Natureza")]
    [RegularExpression(@"^(\{){0,1}[0-9a-fA-F]{8}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{12}(\}){0,1}$")]
    [Required(ErrorMessage = "Escolha uma Natureza de Pessoa")]
    public int PessoaNaturezaId { get; set; }
    public List<SelectListItem> PessoasNaturezas { get; set; }

    public PessoaViewModel()
    {
        var listaPessoaNatureza = from PessoaNatureza pn in Enum.GetValues(typeof(PessoaNatureza))
                                  select new SelectListItem
                                  {
                                      Value = ((int)pn).ToString(),
                                      Text = Enum.GetName(typeof(PessoaNatureza), pn),
                                      Selected = PessoaNaturezaId == (int)pn ? true : false
                                  };
        PessoasNaturezas = listaPessoaNatureza.ToList();
    }
}

And in your view you can keep the default helpers using <select asp-for..

<div class="form-group">
    <label asp-for="PessoaNaturezaId" class="col-md-2 control-label"></label>
    <div class="col-md-2">
        <select asp-for="PessoaNaturezaId" asp-items="Model.PessoasNaturezas">
            <option value="">--Selecione--</option>
        </select>
        <span asp-validation-for="PessoaNaturezaId" class="text-danger"></span>
    </div>
</div>

This "solves" your problem, but I recommend you opt for the solution presented by your colleague. I only recommend my proposal if you have difficulties understanding or implementing the other suggestion.