在MVC Html.DropDownListFor中包括默认的“选择选项”

Question

My Model contains an IEnumerable<SelectListItem> and the View contains a DropDownListFor that works fine with the exception that I need to include a default "Select an Option" with a value of -1 in some cases.

In the Controller:

model.Items = _context.SomeTable.Select(m => new SelectListItem()
{
    Text = m.SomeName,
    Value = m.SomeId
});

In the View:

@Html.DropDownListFor(
    w => Model.ItemId,
    new SelectList(Model.Items, "Value", "Text", Model.ItemId),
    new { @class = "form-control text-left" })

Between the Controller or View, how can I add the following as the first option and conditionally selected (if ItemId == -1)?

<option value="Select an Item">Select an Item</option>

Since the logic that determines whether there is an option currently selected is in the Controller I would prefer to add this in the Controller - if possible. Or, if i can get the <option ... added in the View and be selected when the Controller passes -1 as ItemId that would work too.

Answer 1

To add a default item you can try

model.Items.Insert(0, new SelectListItem { Text = "Please Select", Value = "-1" });

That will push the item to the top of the select list for you. Do this after you have called

model.Items = _context.SomeTable.Select(m => new SelectListItem()
{
    Text = m.SomeName,
    Value = m.SomeId
}).ToList();

It's important that you add a call to ToList() though, so that you are working with a projected list of items, not just the enumerator.

Answer 2

There is an override for 'DropDownListFor' that takes an 'optionLabel' as an argument

@Html.DropdownListFor(m => m.id, "select one", Model.selectList, new {})

You might want to check what value that it generates for that option. I think its either 0 or -1