Question

I’m using Ajax to refresh a partial view and find that the ViewBag contents are not available when the controller is invoked to perform the refresh.

Specifically, I place an item in ViewBag.ItemToMonitor. It is available the first time the partial view is rendered (here I use it in the controller, but I also tried using it in the view). When the partial view is refreshed via Ajax, the ViewBag does not contain anything for ItemToMonitor.

How can I make data available upon refresh?

Here’s a simple example, based on the MVC 3 project template:

Index.cshtml

<div>
    Select an item to monitor:

    <ul>
    <li>@Html.ActionLink("One", "Monitor", new { itemToMonitor = "Item One" })</li>
    <li>@Html.ActionLink("Two", "Monitor", new { itemToMonitor = "Item Two" })</li>
    <li>@Html.ActionLink("Three", "Monitor", new { itemToMonitoritem = "Item Three" })</li>
    </ul>
</div>

Monitor.cshtml

@model string
<script type="text/javascript">
    setInterval(function () {
        $.post('@Url.Action("_MonitorDetails")', function (data) {
            $('#refreshMe').html(data);
        }
    );
    }, 5000);
</script>

<h2>Monitor</h2>

<div>Monitoring @ViewBag.ItemToMonitor</div>
<div id="refreshMe">
@Html.Partial("_MonitorDetails")
</div>

_MonitorDetails.cshtml

@model string

<div style="background-color:Blue; color:White">
This is my partial view to monitor '@Model', updated at @DateTime.Now.ToString("u").
</div>

HomeController.cs

public ActionResult Index()
{
    ViewBag.Message = "Welcome to ASP.NET MVC!";

    return View();
}

public ActionResult Monitor(string itemToMonitor)
{
    ViewBag.ItemToMonitor = itemToMonitor;

    return View((object)itemToMonitor);
}

public ActionResult _MonitorDetails()
{
    string itemToMonitor = ViewBag.ItemToMonitor;
    // itemToMonitor is null.  Why?
    return PartialView((object)itemToMonitor);
}

Answer 1

Don’t think a ViewBag in MVC is like a ViewState in WebForms – it is NOT refilled at every request like the ViewState. You have to set everything in the ViewBag again on the next request.

So you need another solution, either

• pass the itemToMonitor string to Monitor in your ajax request
• save the itemToMonitor in the session and access that in _MonitorDetails