New View Engine or New ActionResult?
One question we are often asked is when someone should create a custom view engine as opposed to a new ActionResult type. For example, suppose that you want to return objects in a custom XML format. Should you write a custom view engine or a new MyCustomXmlFormatActionResult?
The general rule of thumb for choosing between one and the other is whether or not it makes sense to have some sort of template file that guides how the markup is rendered. If there's only one way to convert an object to the output format, then writing a custom ActionResult type makes more sense.
For example, the ASP.NET MVC Framework includes a JsonResult, by default, which serializes an object to JSON syntax. In general, there's only one way to serialize an object to JSON. You wouldn't change the serialization of the same object to JSON according to which action method or view is being returned. Serialization is generally not controlled via a template.
But suppose that you wanted to use XSLT to transform XML into HTML. In this case, you may have multiple ways to transform the same XML into HTML depending on which action you're invoking. In this case, you would create an XsltViewEngine, which uses XSLT files as the view templates.