asp.net mvc 5 DropDownList nullable int默认选项不为null

Question

I have a problem with mvc DropDownList, lots of topics about that, but not one with the same problem.

I want a default selected option for my DropDownList, but i also need additional option for "all" items selection.

So my controller is binding default value of 2 to dropdown

 public ActionResult Index(int? All = 2){ ...

In cshtml i have

 @Html.DropDownList("All","All items")

All list is filled like this

 ViewData["All"] = new SelectList(CommonLists.property_types.Select(x => new { v = x.Value, t = x.Key.ToLower() }), "v", "t", All);

property_types

  public static Dictionary<string, int> property_types = new Dictionary<string, int>() { 
    { "House", 1 }, { "Flat", 2 }, { "Garden", 3 }

So it should work like this

• when first entered the default selected value is 2, equals to "flat", only "flat" items are displayed (works)
• user can change it to other option from the list (works)
• user can change to view all items at once no matter the category selecting "All items" (this is not working)

I have assumed it should work, but to my surprise, when i select "All items" mvc does not return null it just returns the default int value 2 , so basically no way to query for all items.

Is this suppose to work like that? The auto generated "All items" is empty value so i assumed mvc would translate it to null, but it is not.

How to fix this?

Answer 1

The problem isn't that null isn't returned by the POST, but that null IS returned by the POST.

Since your action has a default value of 2 . When the action is receiving null the it's initializing the variable with the default value, in that case 2

public ActionResult Index(int? All = 2)

What you want to do it to be able to differentiate between "All items" and the default behavior of the app. It that case, the default behavior is to show only flats

You can achieve that goal by using building the "All items" inside of your controller.

public ActionResult Index(int? All = 2)
{
    var propertyTypesSelectList = new SelectList(property_types.Select(x => new {v = x.Value, t = x.Key.ToLower()}), "v", "t", All).ToList();
    propertyTypesSelectList.Insert(0, new SelectListItem() { Value = "0", Text = "All items"});

    ViewData["All"] = propertyTypesSelectList;
    return View();
}

And changing your View to this

@Html.DropDownList("All")