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Dynamically Loading Assemblies at Runtime in RazorPages


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I've been working on some standalone tools that are generically serving static content as well as ASP.NET Core content from an arbitrary folder. The tool I've built recently is a .NET based, standalone local dev server with built-in Live Reload functionality and its main purpose is to serve static content locally in a self-contained fashion.

With ASP.NET Core it's now possible using several different approaches to create standalone server applications that provide all sorts of utility with relative ease. I think there are lots of use cases for standalone servers for special purpose tasks, or as I do here, for more generic purposes. This Live Reload Web Server is a good example of what can be done with very little effort.

Grab the Tools

If you're interested you can grab the Dotnet Tool or a Standalone Exe (zipped).

To install the Dotnet Tool (requires .NET Core 3.0 SDK and runs on Windows, Mac and Linux):

dotnet tool install --global LiveReloadServer

Install from Chocolatey as a standalone EXE (no dependencies, Windows only):

choco install LiveReloadWebServer

Note: The Choco Package has not been approved yet - grab the latest which currently is 0.1.10.

Once installed you can run:

# dotnet tool
LiveReloadServer --WebRoot c:\temp\mysite\web 

# chocolatey install or EXE
LiveReloadWebServer --WebRoot c:\temp\mysite\web 

Note the different names for the dotnet tool and the standalone EXE, so it's possible to run both side by side in case both are installed.

There a few options for configuring the server, live reload, what to look for etc by using the --help command line switch.

Code for the LiveReload Middleware and generic Web Server can be found on GitHub:

Westwind.AspNetCore.LiveReload

and there's also a LiveReload middleware component that you can plug into your own ASP.NET Core applications for Live Reloading at development time for instant live reloading of server side UI content.

Install-Package Westwind.AspnetCore.LiveReload

Ok, before digging into the Razor Pages assembly loading, first a little background.

Static Content First

My original goal for this generic server implementation was borne out of the frequent need to generically serve local HTTP content. I maintain several old JavaScript libraries as well as working on a number of locally maintained (static) documentation sites, plus several legacy tools that work with static content. For these a local Web Server with built-in Live Reload functionality is a incredibly useful and productive.

The goal originally was to simply support Static Content because that's the most common use case. The idea is that you simply start LiveReloadServer out of a folder with Web content and go or use the --WebRoot <path> command line to point at a different folder and you're up and running with a Live Reload Web Server.

There are other tools like BrowserSync, but they are Node based. For me personally these node based tools have been pretty flakey. They work for a bit but eventually have to be restarted to keep reloading content or they slow down to the point of unusability. By building my own I can easily tweak the way it works and fix any issues as they come up. To top it off ASP.NET Core makes this functionality relatively trivial to implement and I can customize it for my exact needs.

For static content this has all been a no-brainer and it works beautifully.

Limited Razor Pages Content

But I also got to thinking: Wouldn't it be nice to support semi-dynamic content via Razor Pages in the referenced site. Razor Pages allow for self-contained .cshtml Razor pages on disk to be served including dynamic content via it's built-in support for C# Razor syntax.

Essentially you can create something like hello.cshtml and then serve that as https://localhost:5200/hello. The Razor page can then contain dynamic C# content.

Turns out it's very easy to route Razor pages to look for content in a non-install location:

if (UseRazor)
{
    services.AddRazorPages(opt => { opt.RootDirectory = "/"; })
        .AddRazorRuntimeCompilation(
            opt =>
            {
                opt.FileProviders.Add(new PhysicalFileProvider(WebRoot));
            });
}

In order for this dynamic Web Server concept to work, the first thing needed is to add .AddRazorRuntimeCompilation() and adding the following Nuget Package:

<ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation" Version="3.0.0" />
</ItemGroup>

Runtime compilation in ASP.NET Core 3.0 is disabled by default, which means you're expected to pre-compile all your RazorPages (and MVC Views), so there is no runtime recompilation when changes are made. The goal is for much faster startup time and that works, at the cost of development time convenience (or runtime changes).

Razor Compilation Adds Size and Overhead

By adding Runtime Compilation for Razor, you are adding about 8 megs of size to a Package or build output as this Razor Compilation adds the compiler features that are otherwise omitted. There's also startup overhead as the compilation engine spins up for the first time which can take several seconds of hesitation. After the initial hit however recompilation of content is fast.

Once the above configuration has been added I can now easily create a Razor Page (hello.cshtml) somewhere in the target folder hierarchy and then add something like this:

@page
<html>
<body>
<h1>Hello World</h1>

<p>Time is: @DateTime.Now.ToString("hh:mm:ss tt")</p>

<hr>

@{
    var client = new System.Net.WebClient();
    var xml = await client.DownloadStringTaskAsync("https://west-wind.com/files/MarkdownMonster_version.xml");
    var start = xml.IndexOf("<Version>") + 9;        
    var end = xml.LastIndexOf("</Version>");
    var version = xml.Substring(start, end - start);
}

<h3>Latest Markdown Monster Version: @version</h3>
<hr>

</body>
</html>

Not surprisingly this works just fine because all the dependencies in this code are directly contained in the .NET Core and ASP.NET Core runtimes and the script is self-contained. When ASP.NET recompiles the Razor Page at runtime, it has access to all the components it needs. Cool!

Not quite Full Razor Pages

So far everything I've described works just fine with runtime compilation. And it works because I've used only built-in features that are part of the .NET Core and ASP.NET runtimes plus whatever dependencies I compile in - Live Reload mainly.

Out of the Box this sort of generic Razor Rendering has a couple of drawbacks that don't work:

  • Loading external Assemblies
    Because runtime compiled Razor Pages are not pre-compiled you can't easily add assemblies to access at runtime. All that's available by default is what is compiled into the application when the static server was built originally - it doesn't look for other assemblies in the startup folder or elsewhere at least not automatically.

  • Compiling ‘code-behind’ code for Page Models
    Razor Pages supports both script-only and Page Model pages. With Page Models you provide a C# class that inherits from PageModel that has page lifecycle event hooks that can be overridden and provide the ability to create support functions to minimize code inside of scripted Razor Page. These CodeBehind code files also don't work at runtime - even with RazorCompilation enabled.

    While quite useful, Runtime Compilation is not something I have a solution for, nor is it something that really fits the generic Web Server scenario that is supposed to provide ‘static pages with benefits’.

Dynamically Load Assemblies for Razor Pages at Runtime

It turns out that runtime loading of assemblies however is possible, although it requires some special handling using a built in, and not very obvious support feature built into ASP.NET Core and MVC/Pages for just this purpose.

To make this work, my idea is to allow the --WebRoot folder that is the base Web folder to have a \PrivateBin subfolder into which assemblies can be placed.

When the server starts, the server looks for all the assemblies in that folder and then loads the assemblies at runtime.

Sounds simple enough right?

I can hook up the assembly loading when Razor Pages is configured:

#if USE_RAZORPAGES
    if (UseRazor)
    {
        var mvcBuilder = services.AddRazorPages(opt => opt.RootDirectory = "/")
            .AddRazorRuntimeCompilation(
                opt => { opt.FileProviders.Add(new PhysicalFileProvider(WebRoot)); });

        LoadPrivateBinAssemblies(mvcBuilder);
    }
#endif

Then:

private void LoadPrivateBinAssemblies(IMvcBuilder mvcBuilder)
{
    var binPath = Path.Combine(WebRoot, "privatebin");
    if (Directory.Exists(binPath))
    {
        var files = Directory.GetFiles(binPath);
        foreach (var file in files)
        {
            if (!file.EndsWith(".dll", StringComparison.CurrentCultureIgnoreCase))
                continue;

            try
            {
                var asm = AssemblyLoadContext.Default.LoadFromAssemblyPath(file);
            }
            catch (Exception ex)
            {
                FailedPrivateAssemblies.Add(file + "\n    - " + ex.Message);
            }
        }
    }
}

Well it turns out it's not quite that simple and the above code doesn't work to make the assemblies available in Razor.

While this loads the assemblies into the server process, but they are not actually visible to the RazorPages engine. Say what?

This was frustrating as heck: I could see the assemblies being loaded into the server process - it shows in loaded assembly list, and I can even see it in Process Explorer's view of loaded assemblies. But even so Razor Pages refuses to reference any of the embedded types resulting in type loading errors.

It turns out, not only does the assembly have to be loaded, but you have to also let ASP.NET know that it's available.

There are a couple of ways to do this but the recommended way is to using mvcBuilder.AddApplicationPart():

try
{
    // Load the assembly manually
    var asm = AssemblyLoadContext.Default.LoadFromAssemblyPath(file);
    
    // Let Razor know about the assembly
    mvc@DamianEdwards Builder.AddApplicationPart(asm);
}

You need to explicitly load the assembly and then notify the MVC/Razor engine that this assembly is available so that it can be referenced in our runtime compiled Razor Pages.

And voila this actually works to allow me to access my assemblies in Razor Pages now.

Assemblies? What's that?

We've been trained to use NuGet packages, so much so that it took me a bit to figure out a good way to actually retrieve the needed assemblies for a given package I wanted to use.

For example, I wanted to a Markdown library in one of my applications using the Westwind.AspNetCore.Markdown package.

This package has a dependency on another package - MarkDig - and so in order to actually use this functionality I have to make sure I get both dependencies into the PrivateBin folder where my custom application assembly loader looks for assemblies.

Easy enough, but it now becomes your responsibility to make sure all dependencies are present, can be found and can be loaded. With some complex packages that have lots of nested dependencies this may not be so easy.

Finding actual raw assemblies to pick out of NuGet packages is actually not so easy anymore in .NET Core, because unless you built a full, self-contained runtime publish pass, the output generated doesn't actually include all of the dependencies.

Short of extracting files directly from a .nupkg zip file, the only good way I could think of getting my raw assemblies was to create quick dotnet new console, adding the packages I want to use to the project and then doing a full self-contained publish into a specific folder to get at the assemblies dumped there.

If you can think of an easier way to pick assemblies out of packages short of unzipping .nupkg files, please leave a comment.

At the end of the day this is a pain, but if you need to use external functionality it's possible by compiling code into an assembly that's referenced this way. Cool.

Is this Useful?

It depends - you certainly wouldn't want to create a complex site with this, but it's quite useful for a number of simple use cases:

  • Create a local site that has some simple dynamic content
    • Adding current dates to pages
    • Looking up and displaying version numbers
    • Displaying Build Status on Page
    • Displaying incremental download counts
    • Logging SEO referral codes
    • Looking up and display status information retrieved from monitoring sites
  • Cross site search support for a documentation site

Lots of things even the most static of static sites can use dynamic content for at times.

As Damien Edwards mentioned a while back:

“Are you trying to re-invent ASP.NET Web Pages”

No, but sort of, yes 😄.

I do want the ability to just dump files into a folder with a configuration file, that can re-use a single installation of a generic server, rather than having to deploy a separate full ASP.NET Core application where by default you can't even update Razor Pages and see the changes. And I want to be able to just use simple scripting inside of pages that I can easily update on the server whether it's with a simple Git Push operation or some sort of File deploy mechanism that's more than appropriate for (mostly) static content sites.

I can also use this same server approach for hosting many Web sites generically off the same binary installation! In other words I can install a tool like this as a generic server that can be referenced from many Web sites and re-use the single installed instance to serve content. Very useful that!

Unfortunately, ASP.NET Core 3.0 does not support serving EXE servers in IIS hosting scenarios - you have to reference a .DLL for IIS Hosting to work. This is kind of a downer, and and it required me to publish a separate deploy package that isn't an Exe that can be hosted.

How this works is a topic for another blog post. But again, it's very useful as it solves a very common use case for me.

Summary

Loading assemblies at runtime is not something that a typical application does, but it's something a generic tool like my static Web server requires. While it's not obvious how to load assemblies and it requires some explicit notification APIs to let Razor know about dynamically loaded assemblies, the good news is that it's quite possible to do this which opens up a lot of functionality that otherwise wouldn't be available.

Resources

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Posted in ASP.NET Core  

The Voices of Reason


 

Rick Tuttle
November 14, 2019

# re: Dynamically Loading Assemblies at Runtime in RazorPages

Thanks for this post. I've always wanted to use ASP.NET/C# to do what some PHP frameworks do quite easily by adding plugins that are simply folders of code that are dynamically loaded. What do you think about using this method for being able to say, following a pre-defined folder convention, add .cshtml, .cs, .dlls, etc. to a folder that could then be loaded and compiled dynamically by the server to then extend functionality of a website? I've looked at things like MEF but way too complicated.


Rick Strahl
November 14, 2019

# re: Dynamically Loading Assemblies at Runtime in RazorPages

@RickTuttle - Yes it's possible to do this. In a way that's exactly what this is doing Except for the raw Code Loading and compilation. I suppose it's possible to walk the folder hierarchy and pick up any .cs files, and compile them all into an assembly that can then be loaded. For simple cases that'll likely just work, but there are a lot of details to attend to around all that. Roslyn compilation would make the compilation bit relatively easy.

I'll probably noodle with this when I have more time...


MV10
November 16, 2019

# re: Dynamically Loading Assemblies at Runtime in RazorPages

It would be interesting to see whether Razor Components (.razor files) work this way. I doubt they would, the RC/Blazor folks don't seem to be interested in runtime load/unload. For Blazor server-side apps I was able to get them working (including unloading) via AssemblyLoadContext and some probably-sketchy reflection against a private field, but it crashes the VS2019 debugger (stand-alone seems to run fine indefinitely). If anyone is interested I opened an issue with a repository link here, although I'm not expecting MS to care much about this edge-case, unless it happens to expose a bigger problem.


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