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.

This is an issue(means you are not able to modify your dropdown selection item) or you are asking how this is happening? — Rajput, Oct 22 '16 at 19:03
No, i just want to know how working mechanism of state keeping, cause as we know "web is stateless". May be this "helpers" uses ModelState for it, i don`t know. — Vlad Melnik, Oct 22 '16 at 19:11
Please confirm whether the request is send to `server` on `submit` — L J, Oct 22 '16 at 19:14
When you press button _Submit_, selected value of your select simply sends to `Index` method. Note that name of select and `string` parameter in `Index` method must be equal. — Yurii N., Oct 22 '16 at 20:00

score 2 · Answer 1 · edited May 23 '17 at 12:19

2

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.

edited May 23 '17 at 12:19

Community

1
1

answered Oct 22 '16 at 20:06

GSerg

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@VladMelnik It also might be the browser restoring values in controls after page refresh. Set a breakpoint in the view and see if it has a value in its modelstate on the server side. – GSerg Oct 22 '16 at 22:33
[Yes, it is(]http://image.prntscr.com/image/6b73430b8bf14614b403c9eb069d4502.png) – Vlad Melnik Oct 22 '16 at 22:40
@VladMelnik Apparently the code you posted is not your actual code. On your screenshot the form is `method="get"`, so your `HttpPost` handler is never triggered, and the values persist because with `get` forms they are passed in the URL. In the code you posted, the form is `method="post"` (by default) so the `HttpPost` handler is triggered and the parameters are not passed in the URL. – GSerg Oct 23 '16 at 07:28

Vlad Melnik · Accepted Answer · 2016-10-22T22:10:05.110

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.

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

2 Answers2