Please read this post for my reasons behind this article series.

Using flash messages between web requests

Ok I admit, this is not entirely a C# vs. Ruby issue. It is more Ruby On Rails done with MVC3. But it uses some of the dynamic stuff from C#.

In Rails you can push messages between requests by using a special flash component:

controller UserController < ApplicationController

def Login
  user = (try-to-login-user)
  if not user.LoggedIn?
    flash[:error] = “Invalid credentials”
    redirect_to :controller => “LoginForm”
  end
end
…

This will use sessions to carry the error-message into the next request. You can now test for the flash-message in LoginForm's view:

Login

<% if flash[:error] %>
  
<%=flash[:error]%>
<% end %>

After this request, the flash message is erased automatically.

The same thing can be made in .NET by using a combination of the dictionary TempData" and C#'s extension methods. And we can formalize it a bit more by using an enum to control how many different message types, that we will accept.

Create a file called "FlashHelper.cs" with the following content:

using System;
using System.Collections.Generic;
using System.Text;

namespace System.Web.Mvc
{
    public enum FlashType
    {
            Info,
            Warning,
            Error
    }

    public class FlashData
    {
        public FlashType Key { get; set; }
        public string Message { get; set; }
    }

    public static class FlashHelpers
    {
        public static void Flash(this Controller controller, FlashType key, string message)
        {
            if (controller.TempData["flash"] == null)
                controller.TempData["flash"] = new List<FlashData>();

            var flashList = controller.TempData["flash"] as List<FlashData>;
            flashList.Add(new FlashData {Key = key, Message = message});
        }

        public static MvcHtmlString Flash(this HtmlHelper helper)
        {
            var sb = new StringBuilder();

            if (helper.ViewContext.TempData["flash"] != null)
            {
                foreach (FlashData flash in helper.ViewContext.TempData["flash"] as List<FlashData>)
                {
                    sb.AppendLine(string.Format("<div class=\"flash {0}\">", flash.Key.ToString().ToLower()));
                    sb.AppendLine(flash.Message);
                    sb.AppendLine("</div>");
                }
            }
            return MvcHtmlString.Create(sb.ToString());
        }
    }
}

What I am doing here is, that I extend the base MVC controller with a new method called "Flash()". You call this in your controller to set a new message, supplying the type of message and the message-string itself.

The same goes for extending the Html helper. I define a method, that will print out all added messages, each of them wrapped into their own DIV tag with classes set appropiate of the message type.

So now we can write a C# MVC 3 example like the Rails one above.

The controller:

public class UserController : Controller
{
  public RedirectToRouteResult DoLogin(string username, string password)
  {
    var user = (try-to-login-user);

    if (!user.LoggedIn()) {
      this.Flash(FlashType.Error, "Invalid credentials");
      return RedirectToAction("LoginForm");
    }
    ...

And in your _layout.cshtml view:

    @Html.Flash()

    @RenderBody()

If this example is run, and the user tries to login with invalid credentials, the @Html.Flash() call will render:

    
Invalid credentials
    ...

which you can then style with CSS.