This project provides API for loading .NET Core assemblies dynamically, executing them as extensions to the main application, and finding and isolating the dependencies of the plugin from the main application.

Unlike other approaches to dynamic assembly loading, like Assembly.LoadFrom, this API attempts to imitate the behavior of .deps.json and runtimeconfig.json files to probe for dependencies, load native (unmanaged) libraries, and to find binaries from runtime stores or package caches. In addition, it allows for fine-grained control over which types should be unified between the loader and the plugin, and which can remain isolated from the main application.

Blog post introducing this project: .NET Core Plugins: Introducing an API for loading .dll files (and their dependencies) as 'plugins'

Pre-release builds and symbols: https://www.myget.org/gallery/natemcmaster/

You can install the plugin loading API using the McMaster.NETCore.Plugins NuGet package.

dotnet add package McMaster.NETCore.Plugins

The main API to use is PluginLoader.CreateFromAssemblyFile.

PluginLoader.CreateFromAssemblyFile(
    assemblyFile: "./plugins/MyPlugin/MyPlugin1.dll",
    sharedTypes: new [] { typeof(IPlugin), typeof(IServiceCollection), typeof(ILogger) },
    isUnloadable: true)

• assemblyFile = the file path to the main .dll of the plugin
• sharedTypes = a list of types which the loader should ensure are unified
• isUnloadable = (.NET Core 3+ only). Allow this plugin to be unloaded from memory at some point in the future. (Requires ensuring that you have cleaned up all usages of types from the plugin before unloading actually happens.)

See example projects in samples/ for more detailed, example usage.

Using plugins requires at least two projects: (1) the 'host' app which loads plugins and (2) the plugin, but typically also uses a third, (3) an abstractions project which defines the interaction between the plugin and the host.

For a fully functional sample of this, see samples/hello-world/ .

You can define your own plugin abstractions. A minimal plugin might look like this.

public interface IPlugin
{
    string GetName();
}

Typically, it is best to implement plugins by targeting netcoreapp2.0 or higher. They can target netstandard2.0 as well, but using netcoreapp2.0 is better because it reduces the number of redundant System.* assemblies in the plugin output.

A minimal implementation of the plugin could be as simple as this.

internal class MyPlugin1 : IPlugin
{
    public string GetName() => "My plugin v1";
}

The host application can load plugins using the PluginLoader API. The host app needs to define a way to find the assemblies for the plugin on disk. One way to do this is to follow a convention, such as:

plugins/
    $PluginName1/
        $PluginName1.dll
        (additional plugin files)
    $PluginName2/
        $PluginName2.dll

For example, you could prepare the sample plugin above by running

dotnet publish MyPlugin1.csproj --output plugins/MyPlugin1/

An implementation of a host which finds and loads this plugin might look like this:

using McMaster.NETCore.Plugins;

public class Program
{
    public static void Main(string[] args)
    {
        var loaders = new List<PluginLoader>();

        // create plugin loaders
        var pluginsDir = Path.Combine(AppContext.BaseDirectory, "plugins");
        foreach (var dir in Directory.GetDirectories(pluginsDir))
        {
            var dirName = Path.GetFileName(dir);
            var pluginDll = Path.Combine(dir, dirName + ".dll");
            if (File.Exists(pluginDll))
            {
                var loader = PluginLoader.CreateFromAssemblyFile(
                    pluginDll,
                    sharedTypes: new [] { typeof(IPlugin) });
                loaders.Add(loader);
            }
        }

        // Create an instance of plugin types
        foreach (var loader in loaders)
        {
            foreach (var pluginType in loader
                .LoadDefaultAssembly()
                .GetTypes()
                .Where(t => typeof(IPlugin).IsAssignableFrom(t) && !t.IsAbstract))
            {
                // This assumes the implementation of IPlugin has a parameterless constructor
                IPlugin plugin = (IPlugin)Activator.CreateInstance(pluginType);

                Console.WriteLine($"Created plugin instance '{plugin.GetName()}'.");
            }
        }
    }
}

A common usage for plugins is to load class libraries that contain MVC controllers or Razor Pages. You can set up an ASP.NET Core to load controllers and views from a plugin using the McMaster.NETCore.Plugins.Mvc package.

dotnet add package McMaster.NETCore.Plugins.Mvc

The main API to use is .AddPluginFromAssemblyFile(), which can be chained onto the call to .AddMvc() or .AddRazorPages() in the Startup.ConfigureServices method.

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        var pluginFile = Path.Combine(AppContext.BaseDirectory, "plugins/MyRazorPlugin/MyRazorPlugin.dll");
        services
            .AddMvc()
            // The AddPluginFromAssemblyFile method comes from McMaster.NETCore.Plugins.Mvc
            .AddPluginFromAssemblyFile(pluginFile);
    }
}

See example projects in samples/aspnetcore-mvc/ for more detailed, example usage.