layout.cshtml

  var toastTypes = new[] { "success", "error", "info", "warning" };
  foreach (var type in toastTypes)
  {
    if (TempData.ContainsKey(type))
    {
      string message = (string)TempData[type];
      string title = null;
      if (TempData.ContainsKey("title"))
      {
        title = (string)TempData["title"];
        <script language="javascript">
          toastr['@type']('@message', '@title');
        </script>
      }
      else
      {
        <script language="javascript">
          toastr['@type']('@message');
        </script>
      }
    }
  }

toastrExtensions.cs

  public static class ToastrExtension
  {
    public static void Success(this Controller controller, string message, string title = null)
    {
      controller.TempData["success"] = message;
      if (title != null)
      {
        controller.TempData["title"] = title;
      }
    }

    public static void Info(this Controller controller, string message, string title = null)
    {
      controller.TempData["info"] = message;
      if (title != null)
      {
        controller.TempData["title"] = title;
      }
    }

    public static void Warning(this Controller controller, string message, string title = null)
    {
      controller.TempData["warning"] = message;
      if (title != null)
      {
        controller.TempData["title"] = title;
      }
    }

    public static void Error(this Controller controller, string message, string title = null)
    {
      controller.TempData["error"] = message;
      if (title != null)
      {
        controller.TempData["title"] = title;
      }
    }
  }