Getting Multiple Selected Values in Html.DropDownlistFor

Use a ListBoxFor instead of DropDownListFor:

@Html.ListBoxFor(m => m.branch, CommonMethod.getBranch("", Model.branch), "--Select--")

@Html.ListBoxFor(m => m.division, CommonMethod.getDivision(Model.branch, Model.division), "--Select--")

The branch and division properties must obviously be collections that will contain the selected values.

And a full example of the proper way to build a multiple select dropdown using a view model:

public class MyViewModel
{
    public int[] SelectedValues { get; set; }
    public IEnumerable<SelectListItem> Values { get; set; }
}

that would be populated in the controller:

public ActionResult Index()
{
    var model = new MyViewModel();

    // preselect items with values 2 and 4
    model.SelectedValues = new[] { 2, 4 };

    // the list of available values
    model.Values = new[]
    {
        new SelectListItem { Value = "1", Text = "item 1" },
        new SelectListItem { Value = "2", Text = "item 2" },
        new SelectListItem { Value = "3", Text = "item 3" },
        new SelectListItem { Value = "4", Text = "item 4" },
    };

    return View(model);
}

and in the view:

@model MyViewModel
...
@Html.ListBoxFor(x => x.SelectedValues, Model.Values)

It is the HTML helper that will automatically preselect the items whose values match those of the SelectedValues property.

For me it works also for @Html.DropDownListFor:

Model:

public class MyViewModel
{
    public int[] SelectedValues { get; set; }
    public IEnumerable<SelectListItem> Values { get; set; }
}

Controller:

public ActionResult Index()
{
    var model = new MyViewModel();

    // the list of available values
    model.Values = new[]
    {
        new SelectListItem { Value = "2", Text = "2", Selected = true },
        new SelectListItem { Value = "3", Text = "3", Selected = true },
        new SelectListItem { Value = "6", Text = "6", Selected = true }
    };

    return View(model);
}

Razor:

@Html.DropDownListFor(m => m.SelectedValues, Model.Values, new { multiple = "true" })

Submitted SelectedValues in controller looks like: