Last year I posted a detailed article on how to render Razor Views to string both inside of ASP.NET MVC and outside of it. In that article I showed several different approaches to capture the rendering output. The first and easiest is to use an existing MVC Controller Context to render a view by simply passing the controller context which is fairly trivial and I demonstrated a simple ViewRenderer class that simplified the process down to a couple lines of code.

However, if no Controller Context is available the process is not quite as straight forward and I referenced an old, much more complex example that uses my RazorHosting library, which is a custom self-contained implementation of the Razor templating engine that can be hosted completely outside of ASP.NET. While it works inside of ASP.NET, it’s an awkward solution when running inside of ASP.NET, because it requires a bit of setup to run efficiently.

Well, it turns out that I missed something in the original article, namely that it is possible to create a ControllerContext, if you have a controller instance, even if MVC didn’t create that instance.

Creating a Controller Instance outside of MVC

The trick to make this work is to create an MVC Controller instance – any Controller instance – and then configure a ControllerContext through that instance. As long as an HttpContext.Current is available it’s possible to create a fully functional controller context as Razor can get all the necessary context information from the HttpContextWrapper().

The key to make this work is the following method:

/// <summary>
/// Creates an instance of an MVC controller from scratch 
/// when no existing ControllerContext is present       
/// </summary>
/// <typeparam name="T">Type of the controller to create</typeparam>
/// <returns>Controller Context for T</returns>
/// <exception cref="InvalidOperationException">thrown if HttpContext not available</exception>
public static T CreateController<T>(RouteData routeData = null)
            where T : Controller, new()
{
    // create a disconnected controller instance
    T controller = new T();

    // get context wrapper from HttpContext if available
    HttpContextBase wrapper = null;
    if (HttpContext.Current != null)
        wrapper = new HttpContextWrapper(System.Web.HttpContext.Current);
    else
        throw new InvalidOperationException(
            "Can't create Controller Context if no active HttpContext instance is available.");

    if (routeData == null)
        routeData = new RouteData();

    // add the controller routing if not existing
    if (!routeData.Values.ContainsKey("controller") && !routeData.Values.ContainsKey("Controller"))
        routeData.Values.Add("controller", controller.GetType().Name
                                                    .ToLower()
                                                    .Replace("controller", ""));

    controller.ControllerContext = new ControllerContext(wrapper, routeData, controller);
    return controller;
}

This method creates an instance of a Controller class from an existing HttpContext which means this code should work from anywhere within ASP.NET to create a controller instance that’s ready to be rendered. This means you can use this from within an Application_Error handler as I needed to or even from within a WebAPI controller as long as it’s running inside of ASP.NET (ie. not self-hosted). Nice.

So using the ViewRenderer class from the previous article I can now very easily render an MVC view outside of the context of MVC. Here’s what I ended up in my Application’s custom error HttpModule:

protected override void OnDisplayError(WebErrorHandler errorHandler, ErrorViewModel model)
{
    var Response = HttpContext.Current.Response;
    Response.ContentType = "text/html";
    Response.StatusCode = errorHandler.OriginalHttpStatusCode;

    var context = ViewRenderer.CreateController<ErrorController>().ControllerContext;
    var renderer = new ViewRenderer(context);
    string html = renderer.RenderView("~/Views/Shared/GenericError.cshtml", model);

    Response.Write(html);           
}

That’s pretty sweet, because it’s now possible to use ViewRenderer just about anywhere in any ASP.NET application, not only inside of controller code.

This also allows the constructor for the ViewRenderer from the last article to work without a controller context parameter, using a generic view as a base for the controller context when not passed:

public ViewRenderer(ControllerContext controllerContext = null)
{
    // Create a known controller from HttpContext if no context is passed
    if (controllerContext == null)
    {
        if (HttpContext.Current != null)
            controllerContext = CreateController<ErrorController>().ControllerContext;
        else
            throw new InvalidOperationException(
                "ViewRenderer must run in the context of an ASP.NET " +
                "Application and requires HttpContext.Current to be present.");
    }
    Context = controllerContext;
}

In this case I use the ErrorController class which is a generic controller instance that exists in the same assembly as my ViewRenderer class and that works just fine since ‘generically’ rendered views tend to not rely on anything from the controller other than the model which is explicitly passed.

While these days most of my apps use MVC I do still have a number of generic pieces in most of these applications where Razor comes in handy. This includes modules like the above, which when they error often need to display error output. In other cases I need to generate string template output for emailing or logging data to disk. Being able to render simply render an arbitrary View to and pass in a model makes this super nice and easy at least within the context of an ASP.NET application!

You can check out the updated ViewRenderer class below to render your ‘generic views’ from anywhere within your ASP.NET applications. Hope some of you find this useful.

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