Dropdown in MVC

问题

In my MVC application, I need to add a dropdown that would show a list of domain names.

I already have a ViewModel that contains multiple properties. I am not sure what the sequence of the steps should be:

1. Add a new property to my ViewModel? What should be the type? List?
2. Define a method that populates the above property with values.
3. Use that property in the View? Use HTML.DropdownFor?

I know I should put some code in my Question, but right now I am having difficulty getting started with this...

EDIT: Added the following property to the ViewModel:

 public IEnumerable<SelectListItem> DomainList { get; set; }

and implemented a method to return a list of Domains:

internal static List<Domain> FetchAllDomains()

Next in my controller action, I have:

var domains = FetchAllDomains().Select(d => d.DomainName);
return new EmailModel() {DomainList = domains };

But I get the following error:

Cannot implicitly convert type 'System.Collections.Generic.IEnumerable' to 'System.Collections.Generic.IEnumerable'. An explicit conversion exists (are you missing a cast?)

回答1:

1) Add a new property to my ViewModel? What should be the type? List?

You need 2 properties to be more precise: an IEnumerable<SelectListItem> to hold all the available options and a scalar property to hold the selected value

2) Define a method that populates the above property with values.

Yes.

3) Use that property in the View? Use HTML.DropdownFor?

No, not in the view. The view doesn't call any methods. A view works with the view model. It is the responsibility of the controller to pass a properly filled view model to the view.

So for example:

public class MyViewModel
{
    public string SelectedValue { get; set; }
    public IEnumerable<SelectListItem> Values { get; set; }

    ... some other properties that your view might need
}

and then a controller action that will populate this view model:

public ActionResult Index()
{
    var model = new MyViewModel();
    model.Values = new[]
    {
        new SelectListItem { Value = "1", Text = "item 1" },
        new SelectListItem { Value = "2", Text = "item 2" },
        new SelectListItem { Value = "3", Text = "item 3" },
    };
    return View(model);
}

and finally the strongly typed view in which you will display the dropdown list:

@model MyViewModel
@Html.DropDownListFor(x => x.SelectedValue, Model.Values)

UPDATE:

According to your updated question you are have an IEnumerable<SelectListItem> property on your view model to which you are trying to assign a value of type IEnumerable<string> which obviously is impossible. You could convert this to an IEnumerable<SelectListItem> like this:

var domains = FetchAllDomains().Select(d => new SelectListItem
{
    Value = d.DomainName,
    Text = d.DomainName
});
return new EmailModel { DomainList = domains };

回答2:

If you do not mind, I'll show you an example list of exams

public class SomeClass
{
    //Properties

    public int ExamId { get; set; }
    public SelectList ExamList { get; set; }
}

Helper:

public static void PopulateExamList(MarkEditViewModel model,
    List<Exam> examList)
{
    model.ExamList = new SelectList(examList, "Id", "ExamName");
}

You must to add to marks, in my example, list of all exams. After this in View you will get a dropdown list of exams
And View (foreach every element in Model):

@Html.DropDownListFor(model => model.ExamId,
                                        Model.ExamList,
                                        "Choose exam")

回答3:

Once you have the list on your controller pass it to the view (as model or using ViewBag) and then use the helper:

Html.DropDownList(
    string name,
    IEnumerable<SelectListItem> selectList,
    string optionLabel,
    object htmlAttributes) 

Or (MVC 3)

@Html.DropDownListFor(model => model.Property.ID,
    new SelectList(model.PropertyList, "ID", "Type"))

回答4:

In your ViewModel

public class ViewModelClass
{
    public string DomainName { get; set; }

    public IEnumerable<SelectListItem> DomainList
    {
        get
        {
            return GetDomainNames()  //your method to get domain names
             .Select(d => new SelectListItem() { Text = d, Value = d });
        }
    }
}

your strongly typed view

@model ViewModelClass

@Html.DropDownListFor(model => model.DomainName, Model.DomainList)

回答5:

1. Yes you need a property in your model to handle the value of the DropDownList
2. Depends, normally could be an Integer that refers to the value of the DropDownlist The List that you're going to used to populate elements could be define as a element in a ViewBag, or an Another Attribute in your model
3. The razor Syntax depends entirely of the what do you try achieve you should use @Html.DropDownListFor() refers to an attribute of your model for guarantee that the view was stronglyTyped

回答6:

You are in the right track, Have 2 properties in your ViewModel. One to hold the list of items and other for the selected Item Id from the UI

public class ProductViewModel
{
    public IEnumerable<SelectListItem> Items { get; set; }
    public string SelectedItemId { get; set; }
    //Other Properties
}

And in your Get Action method, Return the ViewModel with the data

 public ActionResult Index()
 {
    var objProduct = new ProductViewModel();
    objProduct.Items = new[]
    {
          new SelectListItem { Value = "1", Text = "Domain 1" },
          new SelectListItem { Value = "3", Text = "Domain 2" },
          new SelectListItem { Value = "3", Text = "Domain 3" }
    };
    // can replace the above line with loading data from Data access layer.
    return View(objProduct);
}

and in your view which is strongly typed to ProductViewModel

@model ProductViewModel
@using (Html.BeginForm())
{ 
  @Html.DropDownListFor(x => x.SelectedItemId, new SelectList(Model.Items,"Value","Text"), "Select..")
  <input type="submit" value="save" />
}

and in your HTTPpost Action, You will have the selected value available

[HttpPost]
public ActionResult Index(ProductViewModel model)
{
 //  check model.SElectedItemId here

}

来源：https://stackoverflow.com/questions/10484774/dropdown-in-mvc