Miłosz Orzeł

Let's suppose that we have some model that has a list property and we want to render some radio buttons for items of that list. Take the following basic setup as an example.

Main model class with list:

using System.Collections.Generic;

public class Team
{
    public string Name { get; set; }
    public List<Player> Players { get; set; }
}

List item class:

public class Player
{
    public string Name { get; set; }
    public string Level { get; set; }
}

There are three accepted values for player’s skill Level property: BEG (Beginner), INT (Intermediate) and ADV (Advanced) so we want three radio buttons (with labels) for each player in a team. Yup, normally we would rather use enum instead of a string for Level property, but let’s skip it here for the sake of simplicity…

Controller action method that returns sample data:

public ActionResult Index()
{
    var team = new Team() {
        Name = "Some Team",
        Players = new List<Player> {
               new Player() {Name = "Player A", Level="BEG"},
               new Player() {Name = "Player B", Level="INT"},
               new Player() {Name = "Player C", Level="ADV"}
        }
    };

    return View(team);
}

Here is our Index.cshtml view:

@model Team

<section>
    <h1>@Model.Name</h1>        

    @Html.EditorFor(model => model.Players)            
</section>

Notice that markup for Players is not created inside a loop. Instead, EditorTemplate is used. It’s a good practice since it makes code more clear and maintainable. Framework is smart enough to use code from template for each player on a list, because Team.Players property implements IEnumerable interface...

And here is Players.cshtml EditorTemplate:

@model Player
<div>
    <strong>@Model.Name:</strong>

    @Html.RadioButtonFor(model => model.Level, "BEG")
    @Html.LabelFor(model => model.Level, "Beginner")

    @Html.RadioButtonFor(model => model.Level, "INT")
    @Html.LabelFor(model => model.Level, "Intermediate")

    @Html.RadioButtonFor(model => model.Level, "ADV")
    @Html.LabelFor(model => model.Level, "Advanced")        
</div>

The code looks fine, nice strongly typed helpers that relay on lambda expressions (for compile-time checking and easier refactoring)... but there’s a catch: HTML markup that is generated by such code is actually seriously flawed. Check this snipped of web page source generated for the first player:

<div>
    <strong>Player A:</strong>  
 
    <input name="Players[0].Level" id="Players_0__Level" type="radio" checked="checked" value="BEG">
    <label for="Players_0__Level">Beginner</label>
 
    <input name="Players[0].Level" id="Players_0__Level" type="radio" value="INT">
    <label for="Players_0__Level">Intermediate</label>
 
    <input name="Players[0].Level" id="Players_0__Level" type="radio" value="ADV">
    <label for="Players_0__Level">Advanced</label>        
</div>

Players_0__Level id is used for three different radio buttons! Lack of uniqueness not only violates HTML specification and makes scripting hard but also causes label tags to not work properly (clicking on them doesn’t check their corresponding input element). 

Fortunately MVC framework contains TemplateInfo class that has GetFullHtmlFieldId method. This method returns id for DOM element. That id is constructed by appending name provided as method argument to an automatically determined prefix. This prefix takes into account nesting level and list item's index. Internally, GetFullHtmlFieldId uses TemplateInfo.HtmlFieldPrefix property and TagBuilder.CreateSanitizedId method so even if you pass some illegal characters to id suffix they will be replaced.

@model Player
<div>
    <strong>@Model.Name:</strong>

    @{string rbBeginnerId = ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldId("rbBeginner"); }
    @Html.RadioButtonFor(model => model.Level, "BEG", new { id = rbBeginnerId })
    @Html.LabelFor(model => model.Level, "Beginner",  new { @for = rbBeginnerId} )

    @{string rbIntermediateId = ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldId("rbIntermediate"); }
    @Html.RadioButtonFor(model => model.Level, "INT", new { id = rbIntermediateId })
    @Html.LabelFor(model => model.Level, "Intermediate",  new { @for = rbIntermediateId })

    @{string rbAdvancedId = ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldId("rbAdvanced"); }
    @Html.RadioButtonFor(model => model.Level, "ADV", new { id = rbAdvancedId })
    @Html.LabelFor(model => model.Level, "Advanced",  new { @for = rbAdvancedId })
</div>

Calls to ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldId method let us obtain ids for radio buttons which are also used to set for attributes of labels. In MVC 3 there was no overload of LabelFor extension method that accepted htmlAttributes object. Luckily version 4 has it build-in.

Above code produces such markup:

<div>
    <strong>Player A:</strong>
 
    <input name="Players[0].Level" id="Players_0__rbBeginner" type="radio" checked="checked" value="BEG">
    <label for="Players_0__rbBeginner">Beginner</label>
 
    <input name="Players[0].Level" id="Players_0__rbIntermediate" type="radio" value="INT">
    <label for="Players_0__rbIntermediate">Intermediate</label>
 
    <input name="Players[0].Level" id="Players_0__rbAdvanced" type="radio" value="ADV">
    <label for="Players_0__rbAdvanced">Advanced</label>
</div>

Now inputs ids are unique and labels properly reference radio buttons via for attribute. Alright :)