how to set the selected value in the drop down list populated with ViewData in mvc - asp.net-mvc-4

Question

I am new to MVC, please help me to set the selected value in drop down list populated using View Data. I have gone through so many solutions but every solutions deals with selecting value for single drop down. I have same drop down listed using the foreach loop. Setting selected value for each dropdown in that foreach loop.
My code is shown below.
[In view]
int i = 0;
foreach (var item in Model.Select((x, j) => new { Data = x, Index = j + 1 })) {
#Html.DropDownListFor(model => item.Data.CategoryID,(IEnumerable<SelectListItem>)ViewData["categories"], new { #id = "category" + i })
i++;
}
[in controller]
SelectList selectList = new SelectList((IEnumerable<Category>)ConvertCategoryToList(dt1), "CategoryID", "CategoryName");
ViewData["categories"] = selectList;

Answer 1

There is a lot missing from your sample (e.g.what a category looks like, what the Edit actions look like, what is dt1, what the model is you pass to the view etc), but I will try based on what is shown.
Problem 1: Can't use foreach in binding
You can't use a foreach with a collection of form items. The items have no way of knowing which individual element to bind with (when they are sent back to the server they need more information to make each entry unique).
Solution:
It only knows how to bind if you use indexing (e.g. using for (int i=0; < count; i++){ #Html.DropDownListFor(model=>item[i]...)
Problem 2: Selected values only can come from SelectList!
This is the most irritating feature of DropDownListFor and appears to be a bug. It will not take the current selected value from the bound data. It will only take a current value from a SelectList, which means you need a unique SelectList for every dropdown.
Solution:
Create a unique SelectList in the view, per drop down required, with the item value as the current selection.
With some faking of your data, I got the following working:
Example
Controller:
Only store the list of items for the SelectList in the ViewBag:
ViewData["categories"] = ConvertCategoryToList(dt1);
View:
1. You need to pass a List and not an IEnumerable as IEnumerable cannot be indexed.
#model List<MyApplication.Item>
2. Create a SelectList for each dropdown
SelectList newSelectList = new SelectList((IEnumerable<MyApplication.Category>)ViewData["categories"], "CategoryID", "CategoryName", Model[i].Id);
3. Use the indexing method for addressing your items in the collection.
#Html.DropDownListFor(m => Model[i].Id, newSelectList, "Select an item")
Problem 3: What data to pass?
The other issue with your code, as-it-stands, is what to pass back and forth to/from the edit view? At the moment I would guess (from the variable name dt1) you are passing a DataTable(?). You will need to be very explicit about the data you are using in order to get a clean solution to this one. I would suggest posting a second question with all your code and Razor view HTML.
If you need to see more of my sample code (below), please post your own code so I can make them consistent.
Full dummy controller code below
public class TestController : Controller
{
public List<Category> dt1 { get; set; }
public TestController()
{
this.dt1 = new List<Category>();
for (int i = 1; i <= 10; i++)
{
this.dt1.Add(new Category() { CategoryId = i, CategoryName = string.Format("Category {0}", i) });
}
}
[HttpGet]
public ActionResult Edit()
{
//SelectList selectList = new SelectList((IEnumerable<Category>)ConvertCategoryToList(dt1), "CategoryID", "CategoryName");
ViewData["categories"] = ConvertCategoryToList(dt1);
List<Item> items = new List<Item>();
items.Add(new Item(){Id = 1});
items.Add(new Item(){Id = 2});
items.Add(new Item(){Id = 3});
items.Add(new Item(){Id = 4});
items.Add(new Item(){Id = 5});
return View(items);
}
[HttpPost]
public ActionResult Edit(FormCollection form)
{
// Turn form submission back into a compatible view model
List<Item> items = new List<Item>();
foreach (string key in form.Keys)
{
string val = form[key];
items.Add(new Item() { Id = int.Parse(val) });
}
// Recreate the select list
ViewData["categories"] = ConvertCategoryToList(dt1);
return View(items);
}
List<Category> ConvertCategoryToList(IEnumerable<Category> dt)
{
return dt.ToList();
}
}
Note: The post version of Edit simply recreates the list of data (using the selected values posted back) and returns to the Edit view. This is just for testing.
Screen shot
Dummy category class
public class Category
{
public int CategoryId { get; set; }
public string CategoryName { get; set; }
}
Full test View
#model List<MyApplication.Item>
#using (Html.BeginForm())
{
for (int i = 0; i < Model.Count; i++)
{
SelectList newSelectList = new SelectList((IEnumerable<MyApplication.Category>)ViewData["categories"], "CategoryID", "CategoryName", Model[i].Id);
#Html.DropDownListFor(m => Model[i].Id, newSelectList, "Select an item")
}
<input type="submit" value="Submit"/>
}