Question

I want to add a post with category, my Add action in BlogController is like this :

private readonly IBlogPostRepository _blogPostRepository;
private readonly ICategoryRepository _categoryRepository;

public BlogController()
        {
            _blogPostRepository = new BlogPostRepository(new SiteContext());
            _categoryRepository = new CategoryRepository(new SiteContext());
        }


        public BlogController(IBlogPostRepository blogPostRepository, ICategoryRepository      categoryRepository)
        {
            _blogPostRepository = blogPostRepository;
            _categoryRepository = categoryRepository;
        }

        public ActionResult Add()
        {
            ViewData["categoryList"] = _categoryRepository.GetAllCategory();
            return View("Add");
        }

        [HttpPost]
        public ActionResult Add(BlogPost blogPost)
        {
            if (ModelState.IsValid)
            {
                blogPost.PublishDate = DateTime.Now;

                _blogPostRepository.AddPost(blogPost);
                _blogPostRepository.Save();
                return RedirectToAction("Add");
            }
            return new HttpNotFoundResult("An Error Accoured while requesting your order!");
        }

My first question is that why I can’t cast category list to SelectList in razor to select a category via DropDownList? My code in view is like this:

    <div>
        @Html.LabelFor(b => b.Category)
        @Html.DropDownList("Category", ViewData["categoryList"] as SelectList)
        @Html.ValidationMessageFor(b => b.Category)
    </div>

My second question is: how can I add category in Add action with POST request?

Answer 1

To use a collection for a dropdown, it has to be of type IEnumerable<SelectListItem>, which SelectList is. You can’t take an arbitrary collection of objects and just cast it to a SelectList.

I like to use an extension method for creating an IEnumerable<SelectListItem>, check out this:

public static class SelectListExtensions
{
    public static IEnumerable<SelectListItem> ToSelectList<T>(this IEnumerable<T> source)
    {
        return ToSelectList(source, null);
    }

    public static IEnumerable<SelectListItem> ToSelectList<T>(this IEnumerable<T> source, string defaultOption)
    {
        return ToSelectList(source, value => value, value => value.ToString(), null);
    }

    public static IEnumerable<SelectListItem> ToSelectList<T, TDataValue>(
        this IEnumerable<T> source,
        Func<T, TDataValue> dataValueSelector,
        Func<T, string> dataTextSelector)
    {
        return ToSelectList(source, (value, index) => dataValueSelector(value), dataTextSelector, null);
    }

    public static IEnumerable<SelectListItem> ToSelectList<T, TDataValue>(
        this IEnumerable<T> source,
        Func<T, int, TDataValue> dataValueSelector,
        Func<T, string> dataTextSelector)
    {
        return ToSelectList(source, dataValueSelector, dataTextSelector, null);
    }

    public static IEnumerable<SelectListItem> ToSelectList<T, TDataValue>(
        this IEnumerable<T> source,
        Func<T, TDataValue> dataValueSelector,
        Func<T, string> dataTextSelector,
        string defaultOption)
    {
        return ToSelectList(source, (value, index) => dataValueSelector(value), dataTextSelector, defaultOption);
    }

    public static IEnumerable<SelectListItem> ToSelectList<T, TDataValue>(
        this IEnumerable<T> source,
        Func<T, int, TDataValue> dataValueSelector,
        Func<T, string> dataTextSelector,
        string defaultOption)
    {
        var list = source.Select((item, index) => new SelectListItem
        {
            Value = dataValueSelector(item, index).ToString(),
            Text = dataTextSelector(item),
        }).ToList();

        if (defaultOption == null) return list;

        list.Insert(0, new SelectListItem { Text = defaultOption, Value = "-1" });

        return list;
    }
}

You can then use this extension method in your controller:

public ActionResult Add()
{
    ViewData["categoryList"] = _categoryRepository.GetAllCategory()
        .ToSelectList(c => c.Id, c => c.Name);

    return View("Add");
}

Assuming that your Category has the properties Id and Name…

And then use it in your view:

@Html.DropDownList("Category", ViewData["categoryList"] as IEnumerable<SelectListItem>)

On a side note, I would recommend that you use models to pass to your (strongly typed) views, as opposed to using ViewData/ViewBag. It saves you a lot of trouble the day you’re going to refactor something and it also gives you good tooling help (intellisense).