I recently published a new package for .NET Core developers that want to implement a plugin system. Dynamic assembly loading in .NET Core is difficult to get right. The API in this package wrangles the complexity through a feature called ‘load contexts’. In this post, I’ll walk through problems that motivated the creation of this project, and explain what the API can do. My hope is that this plugin API will let you focus more on writing your app, and put an end to the inevitable mess of creating your own assembly loading code.

TL;DR?

Introducing McMaster.NETCore.Plugins

The foundation of McMaster.NETCore.Plugins is AssemblyLoadContext (more on this below). The API in McMaster.NETCore.Plugins ties together the understanding of how ALC works, and how dotnet.exe (aka corehost) reads deps.json files and runtimeconfig.json files to find dependencies. In the end, you should be able to use this new API with just a little bit of code.

using McMaster.NETCore.Plugins;

PluginLoader loader = PluginLoader.CreateFromAssemblyFile("./plugins/MyPlugin1.dll",
                        sharedTypes: new[] { typeof(ILogger) });
Assembly pluginDll = loader.LoadDefaultAssembly();

Once you have an Assembly object, you can use reflection to initialize and run code from the plugin.

using System.Reflection;

// For example, you could find and invoke a static method named Start on a type named Plugin.
Type pluginType = pluginDll.GetTypes().First(t => t.Name == "Plugin");
MethodInfo startMethod = pluginType.GetMethod("Start");
startMethod.Invoke(null, new object[] { myLogger, "arg1", "arg2" });

The plugins API provides a solution for managing common problems with assembly loading code, such as

  • finding dependencies of assemblies to load
  • finding unmanaged binaries to load
  • dealing with conflicts between different dependency versions
  • type unification - establishing consistent type identity between plugin and host app
  • isolation - keeping assemblies and their dependencies separated from each other and the host app

Motivations: the trouble with Assembly.LoadFrom

If you’ve ever tried to use Assembly.LoadFrom, you may be familiar with these issues. If you’re not, let me give you a quick demo.

Let’s say you want to load a new .dll file into an app. Assembly.LoadFrom is a tempting choice because it will get you part of what you want.

var pluginDll = Assembly.LoadFrom("./plugin/MyPlugin1.dll");

For simple plugins, it works great until…

Problem 1 - locating dependency assemblies

Let’s say MyPlugin1 uses JSON.NET, but the app calling Assembly.LoadFrom does not. If you try to do anything with the Assembly object you get from LoadFrom, you’ll get

System.IO.FileNotFoundException: Could not load file or assembly ‘Newtonsoft.Json, Version=11.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed’. The system cannot find the file specified.

Even if you copy Newtonsoft.Json.dll into the same folder, .NET Core will not load it. Workarounds exist for this problem, such as hooking into assembly resolving events, but these don’t resolve the next set of issues.

Problem 2 - dependency mismatch

Let’s say MyPlugin1 uses JSON.NET 11, but the app calling LoadFrom used JSON.NET 10. You will still run into System.IO.FileNotFoundException.

You won’t always get this output when dependency versions don’t match. If the situation is reversed – MyPlugin1 depends on a lower version of something than the app has – you can get different errors if there were breaking changes. If you’re lucky, these surface as MissingMethodException or TypeLoadException. If you’re unlucky, your plugin will just function in a way you don’t expect because it’s running on a different version of its dependency.

Problem 3 - side by side and race conditions

If you resolved Problem 1 with some clever workarounds, you will still have issues when you need to load multiple plugins with mixed dependencies. The first plugin to load “wins”. So, if MyPlugin1 depends on JSON.NET 1 and MyPlugin2 depends on 11, you might get 10 or 11, but it will vary based on the order in which you called Assembly.LoadFrom.

var pluginDll1 = Assembly.LoadFrom("./plugin/MyPlugin1/MyPlugin1.dll");
var pluginDll2 = Assembly.LoadFrom("./plugin/MyPlugin2/MyPlugin2.dll");

These problems may be resolvable if you know ahead of time all the plugins and you can merge them to use a same version of common dependencies, but assuming this is not possible or reasonable, you can easily get into a situation where plugins will break each other. And, you have to pick a version. Highest wins is not always right, and Assembly.LoadFrom doesn’t give you a way to use multiple versions of an assembly by the same name.

Problem 4 - native libraries

If you need to use [DllImport] and extern to P/Invoke unmanaged code, Assembly.LoadFrom doesn’t work. In fact, I’m not really sure how you would do this without AssemblyLoadContext.

AssemblyLoadContext: the dark horse

System.Runtime.Loader.AssemblyLoadContext, aka ALC, provides some essential API for defining dynamic assembly loading behavior. This is one of my favorite, little-known APIs in .NET Core. This API provides:

  • Assembly loading in partial isolation. You can create multiple load contexts. Each context can load independent versions of an assembly with the same name.
  • Bring-your-own-resolution. AssemblyLoadContext is an abstract class with some virtual base members you can override. This allows you to implement your own resolution for dependency look up.
  • AssemblyLoadContext.LoadUnmanagedDll. This is basically the only good way to load unmanaged binaries dynamically.

While AssemblyLoadContext is a great API, it’s currently lacking docs. (It’s on the TODO list). It’s also fairly low-level, so you have to have a certain level of understanding to implement a load context. By default, ALC does not provide any resolution logic. You might expect there to be some sort of API in .NET Core for reading .deps.json and runtime.json files, but there isn’t. This is why I called ALC a ‘dark horse’. It’s a really good API, but few know much about it.

AssemblyLoadContextBuilder: build your own ALC

To make ALC easier to work with, I’ve written an API called McMaster.NETCore.Plugins.Loader.AssemblyLoadContextBuilder. This API creates a new AssemblyLoadContext with resolving behavior based on information from various sources. Some of the methods available on this builder include:

  • SetBaseDirectory - This directory is used as the starting point for loading assemblies.
  • PreferLoadContextAssembly / PreferDefaultLoadContextAssembly - specify, by assembly name, which assemblies should be resolved to a common version shared by every plugin and the app (the default load context), and which assemblies can use versions which are unique.
  • AddProbingPath - additional locations for finding dependencies
  • AddAdditionalProbingPathFromRuntimeConfig - add additional probing paths from a runtimeconfig.json file
  • AddManagedLibrary - add specific details about an assembly dependency to be loaded
  • AddNativeLibrary - add specific details about an unmanaged binary to be loaded
  • AddDependencyContext - add managed and native libraries as described in a .deps.json file
  • Finally, .Build() produce a new ALC. Multiple contexts can be created with the same info.

PluginLoader: bring it all together

McMaster.NETCore.Plugins.PluginLoader simplifies assembly loading even more by hiding most of the details about AssemblyLoadContextBuilder behind a smaller API. This is the default entrypoint which should be sufficient for many plugin scenarios. It uses the ALC builder and a set of some well-known conventions to construct a rich load context.

As mentioned above, you need to first create a loader.

PluginLoader loader = PluginLoader.CreateFromAssemblyFile(
    assemblyFile: "./plugins/MyPlugin1.dll",
    sharedTypes: new[] { typeof(ILogger) });

The sharedTypes parameter is important: this is used to define types which must exchange between the plugin and the host. These types are used to ensure consistent type identity.

Furthermore, I’ve begun work to define a way to express plugin behavior through config files. While this is in its early stages, the vision for config files is that you can define plugin behavior externally from the app (if you want) so you can make decisions about how plugins interact with the host app or other plugins.

PluginLoader loader = PluginLoader.CreateFromConfigFile(
    assemblyFile: "./plugins/config.xml",
    sharedTypes: new[] { typeof(ILogger) });

Once you have the loader, you can then use PluginLoader.LoadDefaultAssembly() or LoadAssembly(AssemblyName) to get System.Reflection.Assembly objects. You can get from this object to executing code using a little bit of reflection.

Demo

A full example of the API in action can be seen here: https://github.com/natemcmaster/DotNetCorePlugins/tree/v0.1.0/samples/aspnetcore.

This demo includes a fully-working ASP.NET Core app which has two plugins loaded side-by-side. The plugins use type unification to ensure the plugin can interact with the IServiceCollection and IApplicationBuilder of the host application.

Closing

More reading

For more information, I recommend the following articles:

Why it’s still experimental

In PluginLoader, I’ve done my best to imitate most of the behaviors of corehost, however, there are some gaps which I can’t cover.

  • Unloading - once a plugin is loaded, the files it uses are locked by the process. The only way to unload a plugin is by killing the host app. Hopefully one day, .NET Core will implement collectible ALC’s, which will enable this feature.
  • Localization and resource assemblies - if you have locale-specific resource assemblies, they’re not automagically loaded yet.
  • Conflict resolution - I haven’t yet defined behavior yet for what to do when there are multiple sources for the same assembly. For example, what if both the shared runtime and an local copy of the same binary exist which only differ by file version? TBD.
  • Perf - I haven’t taken time to investigate performance, yet. Before I would recommend this for production, I want to take a closer look at memory impact, CPU throughput, etc.

Plus, there is more work to be done on the “plugin config file” idea, API refinements, bugs to squash, etc.

I would not recommend this yet for production critical apps, but I hope to get it there. The project is open source, and I’m happy to take contributions. Give it a shot let me know what you think.