Question

I would like in .ToDictionary() to concatenate both category_code and category_name in to one Value. Right now I’m only sending category_name, but I want category_name + category_code. Is it possible?

public ActionResult Create()
{
    Party party = new Party();
    var list = repository.GetCategories()
        .Select(p => new { p.category_id, p.category_name, p.category_code })
        .AsEnumerable()
        .ToDictionary(m => m.category_id, m => m.category_name);

    party.categories = new SelectList(list, "Key", "Value", "SelectedValue");
    return View(party);
}

Answer 1

Why are you using .ToDictionary(), AsEnumerable, Select with an anonymous object instead of something shorter:

var list = repository
    .GetCategories()
    .Select(x => new SelectListItem 
    { 
        Value = x.category_id.ToString(), 
        Text = x.category_name + x.category_code
    });
party.categories = new SelectList(list, "Value", "Text", "SelectedValue");

Not to mention that this kind of stuff are better adapted to your mapping layer. For example if you are using AutoMapper your controller action could look like this:

public ActionResult Create()
{
    var categories = repository.GetCategories();
    var categoriesViewModel = Mapper.Map<IEnumerable<Category>, IEnumerable<CategoryViewModel>>(categories);
    var party = new PartyViewModel
    {
        Categories = categoriesViewModel
    }
    return View(party);
}