MVC Binding to Child Objects in a View Model

Hi There

Quick post to talk about MVC binding which can seem like magic at times.

If you use MVC out of the box and get Visual Studio to create your views for you it all just seems to work, its only when you start using View Models that you have to write your own views for that it all seems to fall apart.

I’ve been doing a lot of this lately and there’s a few simple rules to bear in mind that help a lot.

If the bit of data does NOT end up on the page in a HTML field then its not coming back in the posted model.

1. There is no magic (or ‘state’) if its not on that page then it cannot be passed back to you.
2. If you need that bit of data make sure you put it on the page.
3. It has to held in a HTML field, text won’t cut it, no matter how you mark it up.
4. Use EditorFor if its editable.
5. If its not editable but it must be passed back use HiddenFor, this is really only for primary keys though.
6. If you are not going to use the changed value then don’t pass it back just use DisplayFor.

That’s it, except for collections, here the critical thing to bear in mind is that the MVC binders use numbered fields to rebuild the collection so you must use the following construct:

       

Lists

                    @*Must use for rather than foreach for default binding to work *@
@for (var i = 0; i
{

}

@Html.LabelFor(model => model.Lists.First().Title)	@Html.LabelFor(model => model.Lists.First().WebRelativeUrl)	@Html.LabelFor(model => model.Lists.First().IsIncluded)
@Html.HiddenFor(model => model.Lists[i].Title) @Html.DisplayFor(model => model.Lists[i].Title)	@Html.HiddenFor(model => model.Lists[i].WebRelativeUrl) @Html.DisplayFor(model => model.Lists[i].WebRelativeUrl)	@Html.EditorFor(model => model.Lists[i].IsIncluded)

</div>

The key is @for (var i = 0; i < Model.Lists.Count; i++) which allows us to use model.Lists[i].Title and thus the default binder will realise its a connection.

You must NOT use @foreach (var list in Model.Lists) e ven though this feels more ‘with it’ as it prevents the binder from realising that it is a list.

Cheers

Sebastian

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