Prerendering Razor Pages

Prerendering Razor Pages Background
6 min read

Prerendering static content is a popular technique used by JAMStack Apps to improve the performance, reliability and scalability of Web Apps that's able to save unnecessary computation at runtime by generating static content at deployment which can be optionally hosted from a CDN for even greater performance.

As such we thought it a valuable technique to include in the new vue-mjs template to show how it can be easily achieved within a Razor Pages Application. Since prerendered content is only updated at deployment, it's primarily only useful for static content like Blogs which is powered by the static markdown content in _blog/posts whose content is prerendered to:

Parsing Markdown Files

All the functionality to load and render Markdown files is maintained in Configure.Markdown.cs, most of which is spent populating the POCO below from the content and frontmatter of each Markdown file:

public class MarkdownFileInfo
    public string Path { get; set; } = default!;
    public string? Slug { get; set; }
    public string? FileName { get; set; }
    public string? HtmlFileName { get; set; }
    public string? Title { get; set; }
    public string? Summary { get; set; }
    public string? Splash { get; set; }
    public string? Author { get; set; }
    public List<string> Tags { get; set; } = new();
    public DateTime? Date { get; set; }
    public string? Content { get; set; }
    public string? Preview { get; set; }
    public string? HtmlPage { get; set; }
    public int? WordCount { get; set; }
    public int? LineCount { get; set; }

Which uses the popular Markdig library to parse the frontmatter into a Dictionary that it populates the POCO with using the built-in Automapping:

var content = VirtualFiles.GetFile(path).ReadAllText();
var document = Markdown.Parse(content, pipeline);
var block = document
var doc = block?
    .Lines // StringLineGroup[]
    .Lines // StringLine[]
    .Select(x => $"{x}\n")
    .Select(x => x.Replace("---", string.Empty))
    .Where(x => !string.IsNullOrWhiteSpace(x))
    .Select(x => KeyValuePairs.Create(x.LeftPart(':').Trim(), x.RightPart(':').Trim()))

Since this is a Jekyll inspired blog it derives the date and slug for each post from its file name which has the nice property of maintaining markdown blog posts in chronological order:

doc.Slug = file.Name.RightPart('_').LastLeftPart('.');
doc.HtmlFileName = $"{file.Name.RightPart('_').LastLeftPart('.')}.html";

var datePart = file.Name.LeftPart('_');
if (DateTime.TryParseExact(datePart, "yyyy-MM-dd", CultureInfo.InvariantCulture,
        DateTimeStyles.AdjustToUniversal, out var date))
    doc.Date = date;

The rendering itself is done using Markdig's HtmlRenderer which renders the Markdown content into a HTML fragment:

var pipeline = new MarkdownPipelineBuilder()
var writer = new StringWriter();
var renderer = new Markdig.Renderers.HtmlRenderer(writer);

doc.Preview = writer.ToString();

At this point we've populated Markdown Blog Posts into a POCO which is the data source used to implement all the blog's functionality.

We can now start prerendering entire HTML Pages by rendering the markdown inside the Post.cshtml Razor Page by populating its PageModel from the MarkdownFileInfo POCO. It also sets a Static flag that tells the Razor Page that this page is being statically rendered so it can render the appropriate links.

var page = razorPages.GetView("/Pages/Posts/Post.cshtml");
var model = new Pages.Posts.PostModel(this) { Static = true }.Populate(doc);
doc.HtmlPage = RenderToHtml(page.View, model);

public string RenderToHtml(IView? page, PageModel model)
    using var ms = MemoryStreamFactory.GetStream();
    razorPages.WriteHtmlAsync(ms, page, model).GetAwaiter().GetResult();
    ms.Position = 0;
    var html = Encoding.UTF8.GetString(ms.ReadFullyAsMemory().Span);
    return html;

The use of GetResult() on an async method isn't ideal, but something we have to live with until there's a better way to run async code on Startup.

The actual rendering of the Razor Page is done with ServiceStack's RazorPagesEngine feature which sets up the necessary Http, View and Page contexts to render Razor Pages, registered in ASP.NET Core's IOC at:

.ConfigureServices(services => {

The process of saving the prerendered content is then simply a matter of saving the rendered Razor Page at the preferred locations, done for each post and the /blog index page using the Posts/Index.cshtml Razor Page:

foreach (var file in files)
    // prerender /blog/{slug}.html
    if (renderTo != null)
        log.InfoFormat("Writing {0}/{1}...", renderTo, doc.HtmlFileName);
        fs.WriteFile($"{renderTo}/{doc.HtmlFileName}", doc.HtmlPage);

// prerender /blog/index.html
if (renderTo != null)
    log.InfoFormat("Writing {0}/index.html...", renderTo);
        new Pages.Posts.IndexModel { Static = true }, $"{renderTo}/index.html");

Prerendering Pages Task

Next we need to come up with a solution to run this from the command-line. App Tasks is ideal for this which lets you run one-off tasks within the full context of your App but without the overhead of maintaining a separate .exe with duplicated App configuration & logic, instead we can run the .NET App to run the specified Tasks then exit before launching its HTTP Server.

To do this we'll register this task with the prerender AppTask name:

AppTasks.Register("prerender", args => blogPosts.LoadPosts("_blog/posts", renderTo: "blog"));

Which we can run now from the command-line with:

$ dotnet run --AppTasks=prerender

To make it more discoverable, this is also registered as an npm script in package.json:

    "scripts": {
        "prerender": "dotnet run --AppTasks=prerender"

That can now be run to prerender this blog to /wwwroot/blog with:

$ npm run prerender

Prerendering at Deployment

To ensure this is always run at deployment it's also added as an MS Build task in MyApp.csproj:

<Target Name="AppTasks" AfterTargets="Build" Condition="$(APP_TASKS) != ''">
    <CallTarget Targets="Prerender" Condition="$(APP_TASKS.Contains('prerender'))" />
<Target Name="Prerender">
    <Message Text="Prerender..." />
    <Exec Command="dotnet run --AppTasks=prerender" />

Configured to run when the .NET App is published in the GitHub Actions deployment task in /.github/workflows/release.yml:

 # Publish .NET Project
 - name: Publish dotnet project
   working-directory: ./MyApp
   run: | 
     dotnet publish -c Release /p:APP_TASKS=prerender

Where it's able to control which App Tasks are run at deployment.

Pretty URLs for static .html pages

A nicety we can add to serving static .html pages is giving them Pretty URLs by registering the Plugin:

Plugins.Add(new CleanUrlsFeature());

Which allows prerendered pages to be accessed with and without its file extension: