Success message from Controller to View

Expanding from Brad Christie's answer, I created a NuGet package, BootstrapNotifications, that will do this for you with built-in Bootstrap3 support. This package also supports multiple notification types (error, warning, success, and info) with pre-styled alerts and is easily extendable.

The extension supports multiple notifications per request of the same type and different types elegantly.

The Code

NotificationExtensions.cs:

public static class NotificationExtensions
{
    private static IDictionary<String, String> NotificationKey = new Dictionary<String, String>
    {
        { "Error",      "App.Notifications.Error" }, 
        { "Warning",    "App.Notifications.Warning" },
        { "Success",    "App.Notifications.Success" },
        { "Info",       "App.Notifications.Info" }
    };


    public static void AddNotification(this ControllerBase controller, String message, String notificationType)
    {
        string NotificationKey = getNotificationKeyByType(notificationType);
        ICollection<String> messages = controller.TempData[NotificationKey] as ICollection<String>;

        if (messages == null)
        {
            controller.TempData[NotificationKey] = (messages = new HashSet<String>());
        }

        messages.Add(message);
    }

    public static IEnumerable<String> GetNotifications(this HtmlHelper htmlHelper, String notificationType)
    {
        string NotificationKey = getNotificationKeyByType(notificationType);
        return htmlHelper.ViewContext.Controller.TempData[NotificationKey] as ICollection<String> ?? null;
    }

    private static string getNotificationKeyByType(string notificationType)
    {
        try
        {
            return NotificationKey[notificationType];
        }
        catch (IndexOutOfRangeException e)
        {
            ArgumentException exception = new ArgumentException("Key is invalid", "notificationType", e);
            throw exception;
        }
    }
}

public static class NotificationType
{
    public const string ERROR = "Error";
    public const string WARNING = "Warning";
    public const string SUCCESS = "Success";
    public const string INFO = "Info";

}

_Notifications.cshtml:

@using YourApp.Extensions
@{
    var errorList = Html.GetNotifications(NotificationType.ERROR);
    var warningList = Html.GetNotifications(NotificationType.WARNING);
    var successList = Html.GetNotifications(NotificationType.SUCCESS);
    var infoList = Html.GetNotifications(NotificationType.INFO);
}
<!-- display errors -->
@if (errorList != null)
{
    <div class="alert alert-danger alert-dismissable">
        <button type="button" class="close" data-dismiss="alert" aria-hidden="true">&times;</button>
        @if(errorList.Count() > 1){
            <strong><span class="glyphicon glyphicon-remove"></span> There are @errorList.Count() errors: </strong>
            <ul>
                @foreach (String message in errorList)
                {
                    <li>@Html.Raw(message)</li>
                }
            </ul>
        }
        else{
            <strong><span class="glyphicon glyphicon-remove"></span> Error: </strong>
            @Html.Raw(errorList.First())
        }
    </div>
}

<!-- display warnings -->
@if (warningList != null)
{
    <div class="alert alert-warning alert-dismissable">
        <button type="button" class="close" data-dismiss="alert" aria-hidden="true">&times;</button>
        @if(warningList.Count() > 1){
            <strong><span class="glyphicon glyphicon-warning-sign"></span> There are @warningList.Count() warnings: </strong>
            <ul>
                @foreach (String message in warningList)
                {
                    <li>@Html.Raw(message)</li>
                }
            </ul>
        }
        else{
            <strong><span class="glyphicon glyphicon-warning-sign"></span> Warning: </strong>
            @Html.Raw(warningList.First())
        }
    </div>
}

<!-- display success -->
@if (successList != null)
{
    <div class="alert alert-success alert-dismissable">
        <button type="button" class="close" data-dismiss="alert" aria-hidden="true">&times;</button>
        @if(successList.Count() > 1){
            <strong><span class="glyphicon glyphicon-ok"></span> There are @successList.Count() successful notifications: </strong>
            <ul>
                @foreach (String message in successList)
                {
                    <li>@Html.Raw(message)</li>
                }
            </ul>
        }
        else{
            <strong><span class="glyphicon glyphicon-ok"></span> Success! </strong>
            @Html.Raw(successList.First())
        }
    </div>
}

<!-- display success -->
@if (infoList != null)
{
    <div class="alert alert-info alert-dismissable">
        <button type="button" class="close" data-dismiss="alert" aria-hidden="true">&times;</button>
        @if(infoList.Count() > 1){
            <strong><span class="glyphicon glyphicon-info-sign"></span> There are @infoList.Count() notifications: </strong>
            <ul>
                @foreach (String message in infoList)
                {
                    <li>@Html.Raw(message)</li>
                }
            </ul>
        }
        else{
            <strong><span class="glyphicon glyphicon-info-sign"></span> </strong>
            @Html.Raw(infoList.First())
        }
    </div>
}

To see all of this code and how its used, you can download a full working demo from github.

There are a few ways to skin this cat. You could use the ViewBag:

ViewBag.SuccessMessage = "<p>Success!</p>";

Then in your view you could render it to the page:

@ViewBag.SuccessMessage

I'm not a fan of the ViewBag, so I typically have a ViewModel object created that holds all the data I would need for my particular view. And a success message would be just that kind of data:

public MyViewModel{
    public bool IsSuccess {get;set;}
}

Then in your controller, you would pass this ViewModel to your stongly-typed view

[HttpPost]
public ActionResult Update(MyViewModel vm){
    //Glorious code!

   return View(vm)
}

Finally, just check it in your view and print a message if it succeeds:

@if(vm.IsSuccess){
     <p>Here is an amazing success message!</p>
}

Also, instead of that, you can use TempData, which works like the ViewBag but only lasts until the end of your next request and is then discarded:

TempData["SuccessMessage"] = "Success!";