ASP.NET MVC Binding array to DropDownFor

Question

I have an array that i want to bind to a dropdown. Here is the code:

@for (int i = 1; i <= 5; i++)
{
    @for (int j = 1; j <= 7; j++)
    {
        @Html.DropDownListFor(m => @Model.Periods, new SelectList(@Model.Items, "Id", "Title", @Model.ScheduleList[i, j]))
    }
}

@Model.Periods is an ordinary empty integer array. @Model.ScheduleList is a two dimensional integer array and holds the values of the option that should be selected on binding. If I replace:

@Model.Periods

with:

@Model.Periods[i*10 + j]

The selected values are being shown but the generated select input has name like for example Periods[34], not just Periods and I get null in my view model on postback. So my question is: How can I bind an oridnary array to dropdown and pass it the selected value with another array?

It's hard to understand what result you are trying to achieve, you want to have 5 * 7 = 35 dropdowns? — Red
– Red, Commented Dec 28, 2015 at 20:06
Yes, I want 35 dropdowns that should be saved to the database and later the user can edit them. In @Model.ScheduleList I have the values from the database, but I can't change the generated select input to show tham insted of the default first element — snaksa
– snaksa, Commented Dec 28, 2015 at 20:16
@Model.Periods[i*10 + j] is 'almost' correct - to post back that data you have to post it back with all values from Periods[0] to Periods[34], if any of Periods[X] (where X is from 0 to 34 included) will be missing, you won't receive correct data. Also index of array should strictly start from 0. So probably @Model.Periods[(i-1)*10 + (j-1)] should be fine. — Red
– Red, Commented Dec 28, 2015 at 20:40
You are completely right! Maybe you can write this in a separate answer so I can mark it :) — snaksa
– snaksa, Commented Dec 28, 2015 at 21:00

Red · Accepted Answer · 2015-12-28 21:12:19Z

@Model.Periods[i*10 + j] is not 100% correct - to post that data to your COntroller you have to post it back with all values starting from Periods[0] straight to Periods[5 * 7 - 1].

If any of Periods[X] (where X is from 0 to 34 included) will be missing, you won't receive correct data. Also index of array should strictly start from 0.

So for example:

Periods[0] = 1
Periods[1] = 2
Periods[2] = 3
Periods[4] = 4

Will be bound to following array: [1, 2, 3], because there's no payload item with sequential index between 2 and 4.

For payload:

Periods[1] = 1
Periods[2] = 2
Periods[3] = 3
Periods[4] = 4

Result will be null - first index of the array is missing in the payload.

So for you probably @Model.Periods[(i-1)*10 + (j-1)] should be fine.

If you really need your indexes be non-sequential, you can use this work-around: for each element add hidden field:

<input type="hidden" name="Periods.Index" value="{your index}" />

So your code will look this way:

@for (int i = 1; i <= 5; i++)
{
    @for (int j = 1; j <= 7; j++)
    {
        <input type="hidden" name="Periods.Index" value="@(i * j)" /> 
        @Html.DropDownListFor(m => @Model.Periods, new SelectList(@Model.Items, "Id", "Title", @Model.ScheduleList[i, j]))
    }
}

But this is not to be used in your current scenario. More info on indexers can be found in the web - for ex. on http://haacked.com/archive/2008/10/23/model-binding-to-a-list.aspx/

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