Rebinding MVC Models with multiple array selections

The correct way to handle this is to use a view model instead of passing your domain models to the view.

But if you don't want to follow good practices you could generate your dropdowns like this as a workaround:

for (int i = 0; i < Model.Schedules.Count; i++)
{
    @Html.DropDownList(
        "MySelectedCustomer[" + i + "]", 
        new SelectList(
            Model.Schedules[i].Customers, 
            "Id", 
            "FirstName", 
            Request["MySelectedCustomer[" + i + "]"]
        ), 
        "Select A Customer", 
        new { @class = "SelectedCustomer" }
    )
}

The correct way is to have a property of type int[] SelectedCustomers on your view model and use the strongly typed version of the DropDownListFor helper:

for (int i = 0; i < Model.Schedules.Count; i++)
{
    @Html.DropDownListFor(
        x => x.SelectedCustomers, 
        Model.Schedules[i].AvailableCustomers, 
        "Select A Customer", 
        new { @class = "SelectedCustomer" }
    )
}

and your POST controller action will obviously take the view model you defined as parameter:

[HttpPost]
public ViewResult ResponseForm(GuestResponseViewModel model)
{
    // The model.SelectedCustomers collection will contain the ids of the selected
    // customers in the dropdowns

    return View(model);
}

And since you mentioned the Redirect-After-Post design pattern, this is indeed the correct pattern to be used. In case of success you should redirect to a GET action:

[HttpPost]
public ViewResult ResponseForm(GuestResponseViewModel model)
{
    if (!ModelState.IsValid)
    {
        // the model is invalid => redisplay the view so that the user can fix
        // the errors
        return View(model);
    }

    // at this stage the model is valid => you could update your database with the selected
    // values and redirect to some other controller action which in turn will fetch the values
    // from the database and correctly rebind the model
    GuestResponse domainModel = Mapper.Map<GuestResponseViewModel, GuestResponse>(model);
    repository.Update(domainModel);

    return RedirectToAction("Index");
}

Note: I'm first addressing why it's not binding anything, but that's not addressing the array issue, which I will get to afterwards. Where most people go wrong with MVC is that they do not take advantage of the built-in features of MVC to deal with these situations. They insist on doing foreach's and manually rendering things, but do not take into account the collection status.

The reason why the values are reset is because you are using Html.DropDownList() rather than Html.DropDownListFor(), and you are renaming the posted property name to a different name than your model property name.

You could simply change it to this:

@Html.DropDownList("SelectedCustomer", // note the removal of "My"
    new SelectList(sched.Customers, "Id", "FirstName"), 
    "Select A Customer", new { @class = "SelectedCustomer" })

However, you would not have had this issue, and saved yourself a huge headache if you had just used the strongly typed version.

@Html.DropDownListFor(x => x.SelectedCustomer, 
    new SelectList(sched.Customers, "Id", "FirstName"), 
    "Select A Customer", new { @class = "SelectedCustomer" })

As for the Array, you should use an EditorTemplate for Schedules, and in that EditorTemplate you simply create your html as if it were a single item. That's the great thing about Editor/DisplayTemplates is that they automatically deal with collections.

Create a folder in your Views/Controller folder called EditorTemplates. In that folder, create an empty file called Schedule.cshtml (assuming Schedules is a List or array of Schedule). In that, you have code to render a single schedule.

EDIT:

Darin brings up a good point. I would make a small change to the model and add a Selected property to both Schedule and GuestResponse, then you can use Linq to return the selected schedule and it would simplify things.

EDIT2:

You some conflicts between the problem you've described and the code you've shown. I suggest you figure out exactly what you're trying to do, since your code does not really reflect a viable model for this.