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ASP.net MVC4 Multiselect ListBox with Many-to-Many relationship
public class Employee
{
    public int EmployeeId { get; set; }
    public String FirstName { get; set; }
    public String LastName { get; set; }

    public virtual ICollection<Division> Divisions { get; set; }
}

public class Division
{
    public int DivisionId { get; set; }
    public String DivisionName { get; set; }

    public virtual ICollection<Employee> Employees { get; set; }
}

protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {

        modelBuilder.Entity<Employee>()
                .HasMany(e => e.Divisions)
                .WithMany(d => d.Employees)
                .Map(m =>
                {
                    m.ToTable("EmployeesDivisionsId");
                    m.MapLeftKey("EmployeeId");
                    m.MapRightKey("DivisionId");
                });
    }

public ActionResult Index()
    {
        return View(db.Employees).ToList());
    }

    //
    // GET: /Employees/Details/5

    public ActionResult Details(int id = 0)
    {
        Employee employee = db.Employees.Find(id);
        if (employee == null)
        {
            return HttpNotFound();
        }
        return View(employee);
    }

    //
    // GET: /Employees/Create

    public ActionResult Create()
    {
        ViewBag.Divisions = new MultiSelectList(db.Divisions, "DivisionId", "DivisionName");

        return View();
    }

    //
    // POST: /Employees/Create

    [HttpPost]
    public ActionResult Create(Employee employee)
    {
        ViewBag.Divisions = new MultiSelectList(db.Divisions, "DivisionId", "DivisionName");

        if (ModelState.IsValid)
        {
            db.Employees.Add(employee);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

        return View(employee);
    }

<div class="editor-label">
        @Html.LabelFor(model => model.Divisions)
    </div>
    <div class="editor-field">
        @Html.ListBox("Divisions")
        @Html.ValidationMessageFor(model => model.Divisions)
    </div>