How does @Html.ListBox() and @Html.DropDownList() keep the state in asp.net mvc?

Question

Controller:

public class HomeController : Controller
            {
                //
                // GET: /Home/
                public ActionResult Index()
                {   
                    return View();
                }
                [HttpPost]
                public ActionResult Index(string list1)
                {
                    return View();
                }


            }

View(uses default layout):

@{
    ViewBag.Title = "Index";
}

<h2>Index</h2>

@using (Html.BeginForm())
{

    @Html.DropDownList("list1",
                        new SelectList(new[] { "Item 1", "Item 2", "Item 3" }))

    <input type="submit"/>
}
<br />
<br />

After selecting value in DropDownList and submitting the form my application have same state that was before. For example if i had selected "item 3" after pressing "submit" i have selected "item 3". How does it work?

Upd:

I dont want to avoid it, just want to understand how its work.

Answer 1

In your [Post] handler you are not redirecting anywhere and returning a view directly. Hence the values persist because they are still found in the model state.

A common pattern is to redirect to a [Get] handler after a successful Post , in which case values would not persist.

Answer 2

From MVC sources:

SelectExtensions.cs:

.....
    object defaultValue = (allowMultiple) ? htmlHelper.GetModelStateValue(fullName, typeof(string[])) : htmlHelper.GetModelStateValue(fullName, typeof(string));
.....

HtmlHelper.cs:

internal object GetModelStateValue(string key, Type destinationType)
        {
            ModelState modelState;
            if (ViewData.ModelState.TryGetValue(key, out modelState))
            {
                if (modelState.Value != null)
                {
                    return modelState.Value.ConvertTo(destinationType, null /* culture */);
                }
            }
            return null;
        }

Well, it means that Action sends "ModelState" to view and controls(i know that it's not correct definition) use "ModelState" for fill themselves.