当值在数组中时，MVC5 Razor html.dropdownlistfor set selected

Question

I'm developing an ASP.NET MVC 5 application, with C# and .NET Framework 4.6.1.

I have this View :

@model MyProject.Web.API.Models.AggregationLevelConfViewModel

[...]

@Html.DropDownListFor(m => m.Configurations[0].HelperCodeType, (SelectList)Model.HelperCodeTypeItems, new { id = "Configurations[0].HelperCodeType" })

The ViewModel is:

public class AggregationLevelConfViewModel
{
    private readonly List<GenericIdNameType> codeTypes;
    private readonly List<GenericIdNameType> helperCodeTypes;

    public IEnumerable<SelectListItem> CodeTypeItems
    {
        get { return new SelectList(codeTypes, "Id", "Name"); }
    }

    public IEnumerable<SelectListItem> HelperCodeTypeItems
    {
        get { return new SelectList(helperCodeTypes, "Id", "Name"); }
    }

    public int ProductionOrderId { get; set; }

    public string ProductionOrderName { get; set; }

    public IList<Models.AggregationLevelConfiguration> Configurations { get; set; }

    public AggregationLevelConfViewModel()
    {
        // Load CodeTypes to show it as a DropDownList
        byte[] values = (byte[])Enum.GetValues(typeof(CodeTypes));

        codeTypes = new List<GenericIdNameType>();
        helperCodeTypes = new List<GenericIdNameType>();

        for (int i = 0; i < values.Length; i++)
        {
            GenericIdNameType cType = new GenericIdNameType()
            {
                Id = values[i].ToString(),
                Name = EnumHelper.GetDescription((CodeTypes)values[i])
            };

            if (((CodeTypes)values[i]) != CodeTypes.NotUsed)
                codeTypes.Add(cType);

            helperCodeTypes.Add(cType);
        }
    }
}

And Models.AggregationLevelConfiguration is:

public class AggregationLevelConfiguration
{
    public byte AggregationLevelConfigurationId { get; set; }
    public int ProductionOrderId { get; set; }
    public string Name { get; set; }
    public byte CodeType { get; set; }
    public byte HelperCodeType { get; set; }
    public int PkgRatio { get; set; }
    public int RemainingCodes { get; set; }
}

I need to set selected value in these properties:

public IEnumerable<SelectListItem> CodeTypeItems
{
    get { return new SelectList(codeTypes, "Id", "Name"); }
}

public IEnumerable<SelectListItem> HelperCodeTypeItems
{
    get { return new SelectList(helperCodeTypes, "Id", "Name"); }
}

But I can't set it in new SelectList(codeTypes, "Id", "Name"); or new SelectList(helperCodeTypes, "Id", "Name"); because the selected value are in Configurations array: fields AggregationLevelConfiguration.CodeType and AggregationLevelConfiguration.HelperCodeType .

I think I have to set selected value in the View, but I don't know how to do it.

How can I set the selected values?

Answer 1

Unfortunately @Html.DropDownListFor() behaves a little differently than other helpers when rendering controls in a loop. This has been previously reported as an issue on CodePlex (not sure if its a bug or just a limitation)

The are 2 option to solve this to ensure the correct option is selected based on the model property

Option 1 (using an EditorTemplate )

Create a custom EditorTemplate for the type in the collection. Create a partial in /Views/Shared/EditorTemplates/AggregationLevelConfiguration.cshtml (note the name must match the name of the type

@model yourAssembly.AggregationLevelConfiguration
@Html.DropDownListFor(m => m.HelperCodeType, (SelectList)ViewData["CodeTypeItems"])
.... // other properties of AggregationLevelConfiguration

and then in the main view, pass the SelectList to the EditorTemplate as additionalViewData

@using (Html.BeginForm())
{
  ...
  @Html.EditorFor(m => m.Configurations , new { CodeTypeItems = Model.CodeTypeItems })
  ...

Option 2 (generate a new SelectList in each iteration and set the selectedValue )

In this option your property CodeTypeItems should to be IEnumerable<GenericIdNameType> , not a SelectList (or just make codeTypes a public property). Then in the main view

@Html.DropDownListFor(m => m.Configurations[0].HelperCodeType, new SelectList(Model.CodeTypeItems, "Id", "Name", Model.Configurations[0].HelperCodeType)

Side note: there is no need to use new { id = "Configurations[0].HelperCodeType" - the DropDownListFor() method already generated that id attribute

Answer 2

I wrote this class to overcome an issue I was having with selecting an option in an html select list. I hope it helps someone.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Web;

namespace Login_page.Models
{
    public class HTMLSelect
    {
        public string id { get; set; }
        public IEnumerable<string> @class { get; set; }
        public string name { get; set; }
        public Boolean required { get; set; }
        public string size { get; set; }
        public IEnumerable<SelectOption> SelectOptions { get; set; }

        public HTMLSelect(IEnumerable<SelectOption> options)
        {

        }

        public HTMLSelect(string id, string name)
        {
            this.id = id;
            this.name = name;
        }

        public HTMLSelect(string id, string name, bool required, IEnumerable<SelectOption> options)
        {
            this.id = id;
            this.name = name;
            this.required = required;
        }

        private string BuildOpeningTag()
        {
            StringBuilder text = new StringBuilder();
            text.Append("<select");
            text.Append(this.id != null ? " id=" + '"' + this.id + '"' : "");
            text.Append(this.name != null ? " name=" + '"' + this.name + '"' : "");
            text.Append(">");
            return text.ToString();

        }

        public string GenerateSelect(IEnumerable<SelectOption> options)
        {
            StringBuilder selectElement = new StringBuilder();
            selectElement.Append(this.BuildOpeningTag());
            foreach (SelectOption option in options)
            {
                StringBuilder text = new StringBuilder();
                text.Append("\t");
                text.Append("<option value=" + '"' + option.Value + '"');
                text.Append(option.Selected != false ? " selected=" + '"' + "selected" + '"' + ">" : ">");
                text.Append(option.Text);
                text.Append("</option>");
                selectElement.Append(text.ToString());
            }
            selectElement.Append("</select");
            return selectElement.ToString();
        }
    }

    public class SelectOption
    {
        public string Text { get; set; }
        public Boolean Selected { get; set; }
        public string Value { get; set; }
    }
}

And

public IEnumerable<SelectOption> getOrderTypes()
{
    List<SelectOption> orderTypes = new List<SelectOption>();
                        if (this.orderType == "OptionText")
                        {
                            orderTypes.Add(new SelectOption() { Value = "1", Text = "OptionText", Selected = true });
                        } else
                        {
                            orderTypes.Add(new SelectOption() { Value = "2", Text = "OptionText2" });
                        }
}

And to use it:

@{
    Login_page.Models.HTMLSelect selectElement = new Login_page.Models.HTMLSelect("order-types", "order-types");

}
@Html.Raw(selectElement.GenerateSelect(Model.getOrderTypes()));

Answer 3

I leave this in case it helps someone else. I had a very similar problem and none of the answers helped.

We had in a view this line at the top:

IEnumerable<SelectListItem> exitFromTrustDeed = (ViewData["ExitFromTrustDeed"] as IEnumerable<string>).Select(e => new SelectListItem() { 
    Value = e, 
    Text = e,
    Selected = Model.ExitFromTrustDeed == e
});

and then below in the view:

@Html.DropDownListFor(m => m.ExitFromTrustDeed, exitFromTrustDeed, new { @class = "form-control" })

We had a property in my ViewData with the same name as the selector for the lambda expression and for some reason that makes the dropdown to be rendered without any option selected.

We changed the name in ViewData to ViewData["ExitFromTrustDeed2"] and that made it work as expected.

Weird though.