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Introducing xcore

Now published in pre-release (alpha). Not suitable for production code yet.

Current version: 1.0.1-alpha.5

OS Support: Windows 10 x64 (more will come in the future)

Node version: Tested with Node 6.5.0


npm install xcorenode

Once installed you can find sample javascript files in the node_modules/xcorenode

demo video

What is xcore

xcore is a Node.js® plugin enabling the use of .NET netstandard libraries with javascript. netstandard is a new specification by Microsoft representing a set of .NET API which can be used across many .NET implementations such as .NET Framework and .NET Core. -

Basically xcore allows any Node.js script to use .NET code. xcore is entirely authored by Raffaele Rialdi - @raffaeler

Please refer to the repo github issues to report bugs, ask questions or suggestions.

Show me the code ...

Load and initialize xcore

var xcore = require(__dirname + '\\node_modules\\xcorenode\\xcorenode.node');
var netPath = __dirname + "\\node_modules\\xcorenode\\binw10x64";
// load the dll (initializer is optional), then load the class OrderManager (full qualified name)
xcore.initialize(netPath, "SampleLibrary.dll", "SampleLibrary.Initializer");

Load the .net metadata. This operation is done for just the entry-point class of a graph.

xcore.loadClass("SampleLibrary.OrderSample.�    OrderManager, SampleLibrary");

Create an instance of the .NET class

var om = new xcore.OrderManager("raf");

Call methods

console.log(om.Add("Hello, ", "world"));

Walk the graph. Metadata for the Order class is loaded automatically


Asynchronous calls

om.AddAsync(2, 3, function(res){


Subscribing events

var cookie = om.addEventHandler("OrderReady", function(sender, args){
    console.log("sender: " + sender.SelectedOrderName, " args:" + args.Name);

Unsubcscribe events

om.removeEventHandler("OrderReady", cookie);

Much more is supported!

Features already implemented

  • Creating .NET objects – calling constructors
    • Constructors and methods supports overloads
  • Invoking methods, including asynchronous Task methods
    • They will be executed in the context of the calling thread (the main JS thread)
    • If asynchronous, the callback will be enqueued back to the main thread
  • Using properties and indexers
    • Normally used to access the state of the created instance
  • Subscribing events (support for events invoked on secondary threads)
    • The code can refer to the "cookie" (see example) or directly to the subscribing function
  • Automatic upcasting and downcasting
    • In the event sample, the "sender" is an object, but is downcasted to the runtime object

How does it work?


xcore on the C++ side

  • C++ layer is invisible to both JS and .NET developers
  • Host the CoreCLR and load the required infrastructure
  • Creates proxies "on demand" for each .NET object you want to access
  • Dynamically map all the required constructors/methods/properties/events
  • .NET metadata is used to build a complete map
  • Each call from Javascript is mapped by xcore to the .NET instance/member
  • If a method returns Task or Task, a different (asynchronous) mapping occurs
  • Interop occurs to talk "Reverse PInvoke" to .NET code

xcore on the .NET side

  • .NET layer is invisible to both JS and .NET developers
  • Receives the requests from C++ code for each constructor/method/property/event
  • Each time a method is accessed, xcore generates code doing the interop
  • After the first call, each access to a member is an optimized fast delegate
  • Execute the .NET user code
  • Maintains a cache of the marshaled instances (tied to the JS garbage collector)
  • Calls back the C++ code with the responses (PInvoke)
  • A special treatment is done for asynchronous calls

Asynchronous code - promises - async / await

  • V8 engine uses libuv to manage the threading story
  • Javascript / Node.js / V8 must execute code in the one and only specific thread
  • The xcore plugin leverages the V8 engine to "fix" the calls from different threads
  • Constructors, properties and methods normally are all executed on the JS thread
  • Methods returning tasks and events are executed asynchrously and calls are pushed back to custom queues fixing the thread mismatch
  • Two arguments (functions) are added at the end of the signature of the JS proxy
    • onSuccess and onError will be called, if provided, as soon as the results are available


  • Javascript has no direct support for events

  • The xcore plugin add two members to all the proxied objects:

    addEventHandler("OrderReady", function(sender, args) { … }); removeEventHandler("OrderReady", function(sender, args) { … });

  • The name of the function can be used instead of the function implementation

  • To ease unsubscription, it can also be used a different syntax:

    var cookie = addEventHandler("OrderReady", function(sender, args) { … }); removeEventHandler("OrderReady", cookie);

  • The program execution will not end until all the subscriptions are correctly unsubscribed.

Performance profile

  • There are still many improvements that can be done
  • The add use-case is not realistic and is used to measure the infrastructure
    • Four marshaling operations will be always needed (JS -> C++ -> .NET -> C++ -> JS)

Initialization code

var om = new xcore.OrderManager();

om.Add(2, 2);		// jitting
LocalAdd(2,2);		// jitting
xcore.add(2, 2);	// jitting
var loop = 1000000;	// 1 Million

function LocalAdd(x, y)
    { return x+y; }

Javascript only

for(var i=0; i<loop; i++)
    LocalAdd(i, i);

C++ static function, optimized ad-hoc Add

for(var i=0; i<loop; i++)
    xcore.add(i, i);

.NET Add performed via xcore

for(var i=0; i<loop; i++)
    om.NetAdd(i, i);

Results for 1 million calls

	  js:	 6.156 ms
	 c++:	56.352 ms
	.net: 1294.254 ms

Performance considerations

  • The "Add" case is the worst possible case
    • JS code is compiled to machine code
    • C++ code is optimized but we pay the cost of V8 engine (parameters marshaling)
    • .NET code pays the V8 overhead + the .NET marshaling
      • On the .NET side the dynamically created code is fast (no reflection involved)
      • We still have to find the right instance to call the NetAdd method on
  • More can be done by branching and modifying the CLR itself
    • Certainly not coming soon (if ever)


  1. Node.js applications
  2. UI created with the Electron framework
  3. Using JS to script Windows Powershell
  4. Nativescript Mobile applications
  5. Anything else based on V8

Possible future steps

  • Port the whole thing on Linux and mobile platforms
  • Create Typescript mappings automatically
  • Support straight delegates to support reactive extensions like paradigm (easy to do, the event infrastructure already provides the needed infrastructure)
  • Support "projections" (renaming members to adapt the javascript naming conventions)
  • The necessary infrastructure is already there, but still not exposed
  • Support for fields (easy to do, not sure if it is really needed)
  • Support for javascript iterators (for – of)
  • Specialized plugin to use Javascript to script Windows Powershell
  • Other dark secret plans :)

Source code

I have no current plans to release the source code for xcode.

Release date

Follow this repo or myself on twitter @raffaeler for any news. ###Suggestions and requests### Please open an issue on this GitHub repo to ask, discuss and propose anything about xcore. Even better, post the javascript + .NET essential snippets you'd like to use but not sure it would work.


Makes easy using netstandard libraries from Node.JS






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