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Embed a JS runtime in a .NET application

::: warning ⚠️ WARNING This functionality is still experimental. It works, but the process is not as streamlined as it should be. :::

Acquiring the the required libnode binary

This project depends on a PR to Node.js that adds simpler ABI-stable embedding APIs to libnode. Until that PR is merged and the Node.js project starts building shared libnode, we offer the [Microsoft.JavaScript.LibNode](https://www.nuget.org/packages/Microsoft.JavaScript.LibNode) NuGet package that installs pre-built libnode` for Windows, MacOSX, and Linux (Ubuntu). This package depends on the runtime ID specific NuGet packages which can be used directly if needed.

Since the PR for the ABI-stable embedding API is still work in progress, the built libnode will have breaking changes between versions. See the Directory.Packages.props file in the root of the node-api-dotnet project for the matching version of the Microsoft.JavaScript.LibNode package.

Importing JS modules into .NET

  1. Load libnode and initialize a Node.js "platform" and "runtime" instance:
// Find the path to the libnode binary for the current platform.
string baseDir = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location)!;
string libnodePath = Path.Combine(baseDir, "libnode.dll");
NodeEmbeddingPlatform nodejsPlatform = new(libnodePath, null);
NodeEmbeddingThreadRuntime nodejsRuntime = nodejsPlatform.CreateThreadRuntime(baseDir,
    new NodeEmbeddingRuntimeSettings
    {
        MainScript =
            "globalThis.require = require('module').createRequire(process.execPath);\n"
    });

There can only be one NodeEmbeddingPlatform instance per process, so typically it would be stored in a static variable. From the platform, multiple NodeEmbeddingRuntime or NodeEmbeddingThreadRuntime instances may be created and disposed.

The directory provided to the NodeEmbeddingThreadRuntime instance is the base for package resolution. Any packages or modules imported in the next step must be installed (in a node_modules directory) in that base directory or a parent directory.

  1. Invoke a simple script from C#:
await nodejsRuntime.RunAsync(() =>
{
    JSValue.RunScript("console.log('Hello from JS!');");
});

Be aware of JavaScript's single-threaded execution model. All JavaScript operations must be performed from the JS environment thread, unless otherwise noted. Use any of the Run(), RunAsync(), Post(), or PostAsync() methods on the JS NodeEmbeddingThreadRuntime instance to switch to the JS thread. Also keep in mind any JavaScript values of type JSValue (or any of the more specific JS value struct types) are not valid after switching off the JS thread. A JSReference can hold on to a JS value for future use. See also JS Threading and Async Continuations and JS References.

  1. Import modules or module properties:
// Import * from the `fluid-framework` module. Items exported from the module will be
// available as properties on the returned JS object.
JSValue fluidPackage = nodejsRuntime.Import("fluid-framework");

// Import just the `SharedString` class from the `fluid-framework` module.
JSValue sharedStringClass = nodejsRuntime.Import("fluid-framework", "SharedString");

As explained above, importing modules must be done on the JS thread.

Debugging the JavaScript code in a .NET process

int pid = Process.GetCurrentProcess().Id;
Uri inspectionUri = nodejsRuntime.StartInspector();
Debug.WriteLine($"Node.js ({pid}) inspector listening at {inspectionUri}");

Then attach a JavaScript debugger such as VS Code or Chrome to the inspection URI.