问题
Am I right to think that there is no way to set the selected value in the C# class SelectList after it is created? Isn't that a bit silly?
回答1:
I think you are fighting the framework. The data going into your views should be created at the Last Possible Minute (LPM).
Thinking this way, a SelectList is a type to feed the DropDownList HTML helper. It is NOT a place to store data while you decide how to process it.
A better solution would be to retrieve your data into a List<T> and then initialize the SelectList(s) when you need to. An immediate benefit of this practice is that it allows you to reuse your List<T> for more than one DropDownList, such as:
Country of birth
Country of residence
These SelectLists all use the Countries list of type List<Country>.
You can use your List<T> at the 'last minute' like in this example:
public class TaxCheatsFormViewModel
{
private List<Country> countries { get; set; }
public TaxCheat Cheat { get; private set; }
public SelectList CountryOfBirth { get; private set; }
public SelectList CountryOfResidence { get; private set; }
public SelectList CountryOfDomicile { get; private set; }
public TaxCheatsFormViewModel(TaxCheat baddie)
{
TaxCheat = baddie;
countries = TaxCheatRepository.GetList<Country>();
CountryOfBirth = new SelectList(countries, baddie.COB);
CountryOfResidence = new SelectList(countries, baddie.COR);
CountryOfDomicile = new SelectList(countries, baddie.COD);
}
}
The point being that you should keep your data in a List<T> till you really need to output it; the last possible minute (LPM).
回答2:
Because this is such a high result in Google, I'm providing what I found to work here (despite the age):
If you are using a Strongly typed View (i.e. the Model has an assigned type), the SelectedValue provided to the Constructor of the SelectList is overwritten by the value of the object used for the Model of the page.
This works well on an edit page, but not for Create when you want to preselect a specific value, so one workaround is simply to assign the correct value on a default constructed object you pass to the view as the Model. e.g.
var model = new SubjectViewModel()
{
Subject = new Subject(),
Types = data.SubjectTypes.ToList()
}
model.Subject.SubjectType = model.Types.FirstOrDefault(t => t.Id == typeId);
ViewData.Model = model;
return View();
so the code in my Create View that looks like this:
Html.DropDownListFor(model => model.Subject.SubjectTypeId, new SelectList(model.Types, "Id", "Name"))
returns the Drop Down List with the value I assigned as the SelectedValue in the list.
回答3:
I know it' an old question, but it might help someone out there. I did what looks to be closely like what Erik did. I just created a new SelectList from my existing one...
SelectList myList = GetMySelectList();
SelectListItem selected = myList.FirstOrDefault(x => x.Text.ToUpper().Contains("UNITED STATES"));
if (selected != null)
{
myList = new SelectList(myList, "value", "text", selected.Value);
}
回答4:
The "SelectedValue" property is read-only. Setting it in the constructor is the only way.
Tor, the SelectList is an ASP.NET MVC construct used to create a drop-down list. Doing it your way should work too, but the SelectList should do it for you (and not in JS) if done properly.
回答5:
I think you can do this in the controller. If you are going to render a drop down list with name/ID StateCode, then you can set the selected state using this code after the SelectList is created:
ViewData["StateCode"] = "VT";
It seems that the drop down list HTML helper looks for a ViewData item with the same name as the drop down list that's being created.
I don't think I like using this technique, but it does seem to work.
I do agree that the SelectList class is a little weak at the moment. I'd like to be able to create a SelectList and then select a value later by either value or text.
回答6:
You mean client-side, in the browser?
var select = document.getElementById('mySelect');
select.options[newIndex].selected = true;
回答7:
The SelectList object is readonly after it was created. if you want to select something you better do it like:
<%= Html.DropDownList("clientId", ViewData["clients"] as List<SelectListItem>,)%>
And in the code behind:
ViewData["clientId"] = "ASD"; //This should be the value of item you want to select
ViewData["clients"] = clientItemList; //List<SelectListItem>
回答8:
Could do it pretty easy with jQuery;
$(function() {
$("#myselectlist option[@value='ItemToSelectValue'].attr('selected', 'true');
});
回答9:
I needed a dropdown in a editable grid myself with preselected dropdown values. Afaik, the selectlist data is provided by the controller to the view, so it is created before the view consumes it. Once the view consumes the SelectList, I hand it over to a custom helper that uses the standard DropDownList helper. So, a fairly light solution imo. Guess it fits in the ASP.Net MVC spirit at the time of writing; when not happy roll your own...
public static string DropDownListEx(this HtmlHelper helper, string name, SelectList selectList, object selectedValue)
{
return helper.DropDownList(name, new SelectList(selectList.Items, selectList.DataValueField, selectList.DataTextField, selectedValue));
}
回答10:
I agree with @awrigley and others that it is better to load the selected value using the selectlist constructor in the usual case when you have a single dropdownlist. However, sometimes, one needs to set the selected value on the view, such as when you have a page with n identical dropdowns. Since it is not practical to create such collections in the controller and may not even know in advance how many dropdowns you need, a better approach is to create one instance in the viewmodel as a template with no selected value set and then dynamically create the rest dynamically on the view.
I run into this often when iterating through a list of child items on edit views when the children need dropdowns. In such cases, you can do it like this with one line of code:
@Html.DropDownListFor(x => myViewModelFieldName, new SelectList(Model.MyRawSelectListFromMyViewModel.Select(x=> new {x.Value, x.Text}), "Value", "Text", TheValueYouWantToSelect))
来源：https://stackoverflow.com/questions/301854/set-selected-value-in-selectlist-after-instantiation