How to pass ViewModel data between POST requests in ASP.NET MVC?

Question

I have a ViewModel, for example:

public class ProductEditModel
{
    public string Name { get; set; }
    public int CategoryId { get; set; }
    public SelectList Categories { get; set; }

    public ProductEditModel()
    {
        var categories = Database.GetCategories(); // made-up method
        Categories = new SelectList(categories, "Key", "Value");
    }
}

Then I have two controller methods that use this model:

public ActionResult Create()
{
    var model = new ProductEditModel();
    return View(model);
}

[HttpPost]
public ActionResult Create(ProductEditModel model)
{
    if (ModelState.IsValid)
    {
        // convert the model to the actual entity
        var product = Mapper.Map(model, new Product());
        Database.Save(product);
        return View("Success");
    }
    else
    {
        return View(model); // this is where it fails
    }
}

When you first access the view, the Createuser receives a list of categories. However, if they do not confirm the validation, the view returns to them, except that the property Categoriesis null. This is understandable because it is ModelBindernot saved Categoriesif it was not in the POST request. My question is: what is the best way to save Categoriespreserved? I can do something like this:

[HttpPost]
public ActionResult Create(ProductEditModel model)
{
    if (ModelState.IsValid)
    {
        // convert the model to the actual entity
        var product = Mapper.Map(model, new Product());
        Database.Save(product);
        return View("Success");
    }
    else
    {
        // manually populate Categories again if validation failed
        model.Categories = new SelectList(categories, "Key", "Value");
        return View(model); // this is where it fails
    }
}

But this is an ugly decision. How else can I go on with this? I cannot use a hidden field because it is a collection.

Answer 1

( ) readonly. , , , .

public SelectList Categories
{
    get
    {
        var categories = Database.GetCategories(); // made-up method
        return new SelectList(categories, "Key", "Value");
    }
}

(.. ) , , .

Answer 2

, :

[HttpPost]
public ActionResult Create(ProductEditModel model)
{
    if (!ModelState.IsValid)
    {
        // manually populate Categories again if validation failed
        model.Categories = Repository.GetCategories();
        return View(model);
    }

    // convert the model to the actual entity
    var product = Mapper.Map(model, new Product());
    Database.Save(product);

    // I would recommend you to redirect here
    return RedirectToAction("Success");
}

, " " .

Answer 3

BaseModel, .

:

public IEnumerable<SelectListItem> CountryList
{
    get
    {
        return GetCountryList().Select(
            t => new SelectListItem { Text = t.Name, Value = Convert.ToString(t.CountryID) });
    }
}

GetCountryList() - , Singleton .

, , , SelectListItem.

, , Singleton.