DropDownList Asp.NET C # MVC

There are many ways to do it, the easiest is through a ViewBag

Suppose that your Group table is composed as follows.

On your Controller.

ViewBag.Grupos= context.Grupos.Select(x => new { IdGrupo = x.Id, Grupo = x.Grupo}).ToList();

In your view you assign it like that.

@Html.DropDownListFor(m => m.SelectedGrupoId, new SelectList(ViewBag.Grupos, "IdGrupo", "Grupo"), "- Selecciona uno -", new { @class = "form-control" })

I hope it serves you.

For this it is advisable to use viewmodels that allow us to send a customized model to the view.

ViewModel

// Este lo puedes usar para crear y etitar
public class AumnoEditViewModel
{
    public int GrupoId { get; set; }
    public IEnumerable<Grupo> Grupos{ get; set; }

    // Incluir propiedades del alumno
    public AlumnoId { get; set; }
    public Nombre { get; set; } // Etc
}

Driver

public class AlumnoController : Controller
{
    MyContext db = new MyContext();

    [HttpGet]
    public ActionResult AddOrEdit()
    {
       var model = new AlumnoEditViewModel();
       model.Grupos = db.Grupos.ToList();
       return View(model);
    }

    [HttpPost]
    public ActionResult AddOrEdit(AlumnoEditViewModel model)
    {
       var nuevoAlumno = new Alumno()
       {
           Nombre = model.Nombre,
           GrupoId = model.GrupoId // El Id del grupo seleccionado
       };
       db.Alumnos.Add(nuevoAlumno);
       db.SaveChanges();
       return RedirectToAction("Index");
    }
}

Vista

@model TuProyecto.Models.AumnoEditViewModel

@using (Html.BeginForm("AddOrEdit", "Alumnos", FormMethod.Post))
{
    @Html.AntiForgeryToken()
    <div class="form-group">
        <input type="text" name="Nombre">
    </div>
    <div class="form-group">
        <select name="GrupoId">
            // Llenar las opciones del select
            @foreach (var grupo in Model.Grupos)
            {
               <option value="@grupo.Id">@grupo.Nombre</option>
            }
        </select>
    </div>
}

I hope you serve