Why is my default value for DropDownListFor not working properly?

Question

For whatever reason, my default value for my Html.DropDownListFor isn't working:

@Html.DropDownListFor(model => model.DomainList,
    new SelectList(Model.DomainList, "DomainId", "Name", Model.SingleDomain.DomainId),
    new { @class = "form-control" })

Any idea why?

UPDATE

With the answer below, I updated my code to the following:

@Html.DropDownListFor(model => model.SelectedDomain, 
    new SelectList(Model.DomainList, "DomainId", "Name", Model.SelectedDomain),
    "Select a Domain", 
    new { @class = "form-control" })

Answer 1

Try this:

In your ViewModel, add an integer to store the selected domain id:

public int SelectedDomainId { get; set; }

Change your DropDownListFor to:

@Html.DropDownListFor(model => model.SelectedDomainId,
    new SelectList(Model.DomainList, "DomainId", "Name", Model.SingleDomain.DomainId),
    new { @class = "form-control" })

Now on your post-back the selected id will be posted in SelectedDomainId.

Or, you could add the single domain object to your VM (to indicate which one was selected):

public Domain SelectedDomain { get; set; }

And change your DropDownListFor to:

@Html.DropDownListFor(model => model.SelectedDomain,
        new SelectList(Model.DomainList, "DomainId", "Name", Model.SingleDomain.DomainId),
        new { @class = "form-control" })

I usually use the selected ID, but I think either should work

Here is another good example:

MVC3 DropDownListFor - a simple example?