Webservice - Sending objects via Httpget

Question

I’m developing a service Get data with feedback in JSON, following the model below:

Survey:

{
[SchSISQST].[Qst]. QstAltDat
[SchSISQST].[Qst]. QstNom
[SchSISQST].[Qst]. QstAtvSit
[SchSISQST].[QstGrp]. QstGrpNom
[SchSISQST].[QstSbo]. QstSboNom
}

I already have following structures of my Entity Framework Core:

Classes:

Questionario.Cs

 public partial class Questionario
{
    public short QstCod { get; set; }
    public string QstNom { get; set; }
    public bool QstAtvSit { get; set; }
    public decimal QstCpfAltNum { get; set; }
    public DateTime QstAltDat { get; set; }
}

Grupoquestionario.Cs

public partial class GrupoQuestionario
{
    public short QstGrpCod { get; set; }
    public string QstGrpNom { get; set; }
    public decimal QstGrpCpfAltNum { get; set; }
    public DateTime QstGrpAltDat { get; set; }
}

Subgrupoquestionario.Cs

public partial class SubGrupoQuestionario
{
    public short QstSboCod { get; set; }
    public short QstGrpCod { get; set; }
    public string QstSboNom { get; set; }
    public decimal QstSboCpfAltNum { get; set; }
    public DateTime QstSboAltDat { get; set; }
}

Questionsubgroup.Cs

 public partial class QuestionarioSubGrupo
{
    public short QstCod { get; set; }
    public short QstSboCod { get; set; }
    public short QstGrpCod { get; set; }
}

Listarquestionariosrepository.Cs

public class ListarQuestionariosRepository
{
    public void ListarQuestionario()
    {
        using(SisquestDataContext Db = new SisquestDataContext())
        {
            //return Db.Set<Questionario>().ToList();

            var finalQuest = (from qst in Db.Questionario
                    join qstSubGrp in Db.QuestionarioSubGrupo on qst.QstCod equals qstSubGrp.QstCod
                    join qstGrup in Db.GrupoQuestionario on qstSubGrp.QstGrpCod equals qstGrup.QstGrpCod
                    join qstSubGrpQuest in Db.SubGrupoQuestionario on qstSubGrp.QstSboCod equals qstSubGrpQuest.QstSboCod
                    select new { qst.QstAltDat, qst.QstNom, qst.QstAtvSit, qstGrup.QstGrpNom, qstSubGrpQuest.QstSboNom }).ToList();

        }
    }

}

Equivalent to my query that brings the 5 fields I need to capture in format JSON, down:

   SELECT a.QstAltDat, a.QstNom, a.QstAtvSit, c.QstGrpNom, d.QstSboNom FROM SchSISQST.Qst a INNER JOIN SchSISQST.QstSboAsa b ON b.QstCod = a.QstCod INNER JOIN SchSISQST.QstGrp c ON c.QstGrpCod = b.QstGrpCod INNER JOIN SchSISQST.QstSbo d ON d.QstSboCod = b.QstSboCod

Ilistarquestionariosrepository.Cs

public interface IListarQuestionariosRepository
{
    IEnumerable<Questionario> ListarQuestionario { get; }
    //IEnumerable<GrupoQuestionario> GrupoQuestionario { get; }
    //IEnumerable<QuestionarioSubGrupo> QuestionarioSubGrupo { get; }
}

Listarquestionariosmodel.Cs

[Serializable]
public class ListarQuestionariosModel
{
    public DateTime QstAltDat { get; set; }
    public string QstNom { get; set; }
    public bool QstAtvSit { get; set; }
    public string QstGrpNom { get; set; }
    public string QstSboNom { get; set; }
}

Sisquestcontroller.Cs

[HttpGet]
    public ActionResult ListarQuestionarios()
    {
        //return _contextSisquest.Questionario.ToList();
        ListarQuestionariosModel listarQuestionariosModel = new ListarQuestionariosModel();
        try
        {


            //var listarQuestionariosRepository = new ListarQuestionariosRepository();
            //listarQuestionariosRepository.ListarQuestionario();
            foreach (var item in collection)
            {

            }
        }
        catch (Exception ex)
        {
            return BadRequest(ex);
        }
    }

I’m making a mistake in ActionResult ListarQuestionarios called 'not all code paths Return value' and would like to try to understand this, another point is to call the 5 members as return in my foreach following the ListarQuestionariosModel that I created, because I have here a question of taking two fields from other tables, that I had not yet done that are QstGrpNom and QstSboNom, when it is only 1 table I can mount the get service, there is some way using good practices to be able to return the fields based on the LINQ what have I put together and other parts of the rules? Thank you personally.