Return List<T> by Json to JS from c# with class containing another class

The problem arises from a Recursion between the Provinces class that contains a List of Localities and the Localities class that Contains an attribute of the Province type The strange thing is that the error jumps in the ajax method and not in Visual Studio, which makes it difficult for me to find the error , because ajax was only telling me Error 500 Internal server error and I don't have much experience in JS. Removing either of the two attributes (the list of localities in province, or the province in localities) the code works correctly.

 public class Localidad
        {
            public int Id { get; set; }
            public string Descripcion { get; set; }

            [Display(Name ="Provincia")]
            public int IdProvincia { get; set; }

            **public Provincia Provincia
            {
                get { return new Provincia(this.IdProvincia); }
                set { }
            }**

            public void set_Provincia(int idProvincia)
            {
                this.Provincia = new Provincia(1);
            }
        }

  public class Provincia
    {
        // public int Id { get; set; }
        public int Id { get; set; }
        public string Descripcion { get; set; }
        public string Gobernador { get; set; }
        **public List<Localidad> Localidades { get; set; }**
    }

From what I understand, the error you're getting would be Cannot implicitly convert type X to 'System.Collections.Generic.IEnumerable<Locality>' .

This would be because the type returned by db.Localities cannot be converted directly to a list of Localities .

You could try the following:

localidadList = db.Localidades.Where(x => x.IdProvincia == id).Select(
    l => new Localidad
    {
        Id = l.Id,
        Descripcion = l.Descripcion,
        IdProvincia = l.IdProvincia
    }).ToList();

A problem I see is that you are using Int32.Parseto convert the string IdProvincia, but if it becomes null it will send you a ArgumentNullException, this generates an exception and therefore the 500 error makes sense, so the ideal would be to use the Convert.ToInt32.

Ideally, you'll want to change your Controller code to the following:

public ActionResult GetSecondValues(int IdProvincia)
{
    IEnumerable<Localidad> localidadList = db.Localidades.Where(x => x.IdProvincia == IdProvincia).ToList();
    return Json(localidadList, JsonRequestBehavior.AllowGet);
}

But, seeing the result you use it to fill a drop-down list, I would recommend generating the HTML from the Controller:

public ActionResult GetSecondValues(int IdProvincia)
{
    IEnumerable<Localidad> localidadList = db.Localidades.Where(x => x.IdProvincia == IdProvincia).ToList();

    StringBuilder sb = new StringBuilder();
    foreach (var localidad in localidadList)
    {
        sb.Append("<option value='" + localidad.Id + "'>" + localidad.Descripcion + "</option>");
    }

    ViewBag.Localidades = sb.ToString();

    return View();
}

Finally, in the Ajax call, it would only be necessary to render that HTML in the desired control:

$.ajax({
    cache: false,
    type: "POST",
    url: "@(Url.Action(" GetSecondValues "))",
    data: {
        "IdProvincia": selectedItem
    },
    success: function (data) {
        ddlStates.html('');
        ddlStates.append(@Html.Raw(ViewBag.Localidades));
        statesProgress.hide();
    },
    error: function (xhr, ajaxOptions, thrownError) {
        alert('Failed to retrieve states.');
        statesProgress.hide();
    }
});