C# MVC4 Binding Array As Nested Type

Question

I have a model that looks like this

 public class ViewModel
 {
    public Dictionary<int, int[]> SelectedRoomIds 
 }

On my page I have something like this

@foreach(var floor in floors)
{
        @Html.Hidden("SelectedRoomIds.Index", floor)
        @Html.Hidden("SelectedRoomIds[" + floor + "].Key", floor)
        @Html.ListBox("SelectedRoomIds[" + floor + "].Value", Model.FloorSelectList)
}

This works (binds) great if at least one item is selected in any of the list boxes, but it throws an exception that I can't seem to trap somewhere if nothing is selected. If nothing is selected I would expect either the key exists with a null int array or the entire dictionary be null. Neither is the case. Any suggestions are greatly appreciated.

sorry i was using mvc 5, i guess the issue has been fixed in 5, i have reproduced an issue in mvc 4 — BhavO
– BhavO, Commented Aug 6, 2015 at 23:55

BhavO · Accepted Answer · 2015-08-09 20:49:29Z

1

The MVC 4 default model binder seems to throw a invalid cast exception when a value is not posted for a specific key, this problem exist for any array type in the generic dictionary value, where no value is posted on the form, whether it be a string[] or object[].

I haven't had time to dig into the MVC 4 source code yet, the issue is fixed in MVC 5, if you can upgrade do so, if not you can create your own model binder to deal with the issue, if you don't have time to do that you can try the below and change your int array to be a array of nullable ints:

public Dictionary<int, int?[]> SelectedRoomIds { get; set; }

and add a empty value placeholder in your view before your select list you it will work, only side effect is you will always have a null item in your array:

@Html.Hidden("SelectedRoomIds.Index", floor)
@Html.Hidden("SelectedRoomIds[" + floor + "].Key", floor)
@Html.Hidden("SelectedRoomIds[" + floor + "].Value", "")
@Html.ListBox("SelectedRoomIds[" + floor + "].Value", Model.FloorSelectList)

to make the dictionary easier to consume you can create a backing field and add a getter to remove all null values from the dictionary values.

edited Aug 9, 2015 at 20:49

answered Aug 7, 2015 at 1:04

BhavO

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1 Comment

user3170736 Over a year ago

This is a possible solution and does work so I will mark it as the answer. I opted to go with a custom model binder I found through other questions and a few modifications made it suit my needs. github.com/loune/MVCStuff/blob/master/Extensions/…

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