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NATS - .NET C# Client

A C# .NET client for the NATS messaging system multi targetting .NET4.6+ and .NETStandard1.6.

This NATS Maintainer supported client parallels the NATS GO Client.

License Apache 2.0 API Documentation Build Status NuGet

Getting started

The easiest and recommended way to start using NATS in your .NET projects, is to use the NuGet package. For examples on how to use the client, see below or in any of the included sample projects.

Get up and running with the source code

First, download the source code:

git clone [email protected]:nats-io/

Project files

The repository contains several projects, all located under src\

  • NATS - The NATS.Client assembly
  • Tests
    • IntegrationTests - XUnit tests, verifying the client integration with nats-server.exe (ensure you have nats-server.exe in your path to run these).
    • UnitTests - XUnit tests that requires no dependencies
  • Samples
    • Publish Subscribe
      • Publish - A sample publisher.
      • Subscribe - A sample subscriber.
    • QueueGroup - An example queue group subscriber.
    • Request Reply
      • Requestor - A requestor sample.
      • Replier - A sample replier for the Requestor application.

All examples provide statistics for benchmarking.


.NET Core SDK style projects are used, so ensure your environment (command line, VSCode, Visual Studio, etc) supports the targetted .NET Core SDK in src\global.json as well as .NET Framework 4.6 or greater.

Visual Studio

The recommendation is to load src\NATS.sln into Visual Studio 2019 (Visual Studio 2017 works as well). .NET Core SDK style projects are used to multitarget different frameworks, so when working with the source code (debugging, running tets etc) you might need to mind the "context" of the current framework.

XML documentation is generated (in Release), so code completion, context help, etc, will be available in the editor.

Command line

Since .NET Core SDK style projects are used, you can use the .NET SDK to build, run tests, pack etc.

E.g. to build:

dotnet build src\NATS.sln -c Release

This will build the respective NATS.Client.dll, examples etc in Release mode, with only requiring the .NET Core SDK and the .NET Platform.

Building the API Documentation

Doxygen is used for building the API documentation. To build the API documentation, change directories to documentation and run the following command:


Doxygen will build the NATS .NET Client API documentation, placing it in the documentation\NATS.Client\html directory. Doxygen is required to be installed and in the PATH. Version 1.8 is known to work.

Current API Documentation

Basic Usage

NATS .NET C# Client uses interfaces to reference most NATS client objects, and delegates for all types of events.

Creating a NATS .NET Application

First, reference the NATS.Client assembly so you can use it in your code. Be sure to add a reference in your project or if compiling via command line, compile with the /r:NATS.Client.DLL parameter. While the NATS client is written in C#, any .NET langage can use it.

Below is some code demonstrating basic API usage. Note that this is example code, not functional as a whole (e.g. requests will fail without a subscriber to reply).

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

// Reference the NATS client.
using NATS.Client;

Here are example snippets of using the API to create a connection, subscribe, publish, and request data.

            // Create a new connection factory to create
            // a connection.
            ConnectionFactory cf = new ConnectionFactory();

            // Creates a live connection to the default
            // NATS Server running locally
            IConnection c = cf.CreateConnection();

            // Setup an event handler to process incoming messages.
            // An anonymous delegate function is used for brevity.
            EventHandler<MsgHandlerEventArgs> h = (sender, args) =>
                // print the message

                // Here are some of the accessible properties from
                // the message:
                // args.Message.Data;
                // args.Message.Reply;
                // args.Message.Subject;
                // args.Message.ArrivalSubcription.Subject;
                // args.Message.ArrivalSubcription.QueuedMessageCount;
                // args.Message.ArrivalSubcription.Queue;

                // Unsubscribing from within the delegate function is supported.

            // The simple way to create an asynchronous subscriber
            // is to simply pass the event in.  Messages will start
            // arriving immediately.
            IAsyncSubscription s = c.SubscribeAsync("foo", h);

            // Alternatively, create an asynchronous subscriber on subject foo,
            // assign a message handler, then start the subscriber.   When
            // multicasting delegates, this allows all message handlers
            // to be setup before messages start arriving.
            IAsyncSubscription sAsync = c.SubscribeAsync("foo");
            sAsync.MessageHandler += h;

            // Simple synchronous subscriber
            ISyncSubscription sSync = c.SubscribeSync("foo");

            // Using a synchronous subscriber, gets the first message available,
            // waiting up to 1000 milliseconds (1 second)
            Msg m = sSync.NextMessage(1000);

            c.Publish("foo", Encoding.UTF8.GetBytes("hello world"));

            // Unsubscribing

            // Publish requests to the given reply subject:
            c.Publish("foo", "bar", Encoding.UTF8.GetBytes("help!"));

            // Sends a request (internally creates an inbox) and Auto-Unsubscribe the
            // internal subscriber, which means that the subscriber is unsubscribed
            // when receiving the first response from potentially many repliers.
            // This call will wait for the reply for up to 1000 milliseconds (1 second).
            m = c.Request("foo", Encoding.UTF8.GetBytes("help"), 1000);

            // Draining and closing a connection

            // Closing a connection

RX Usage

Importing the namespace NATS.Client.Rx you will be able to use an extension method connection.Observe(subject) to turn the connection to an observable. If you have a more advanced IAsyncSubscription, you can use asyncSubscription.ToObservable().

You can now import the namespace NATS.Client.Rx.Ops. After this you get builtin support for:

  • Subscribe
  • SubscribeSafe (will not fail an observer if it misbehaves)
  • Where
  • Select

If you want, you could instead take an external dependency on System.Reactive and use that instead of NATS.RX.Ops.

using System;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using NATS.Client;
using NATS.Client.Rx;
using NATS.Client.Rx.Ops; //Can be replaced with using System.Reactive.Linq;

namespace RxSample
    class Program
        static void Main(string[] args)
            using (var cn = new ConnectionFactory().CreateConnection())
                var temperatures =
                        .Where(m => m.Data?.Any() == true)
                        .Select(m => BitConverter.ToInt32(m.Data, 0));

                temperatures.Subscribe(t => Console.WriteLine($"{t}C"));

                temperatures.Subscribe(t => Console.WriteLine($"{(t * 9 / 5) + 32}F"));

                var cts = new CancellationTokenSource();

                Task.Run(async () =>
                    var rnd = new Random();

                    while (!cts.IsCancellationRequested)
                        cn.Publish("temperatures", BitConverter.GetBytes(rnd.Next(-10, 40)));

                        await Task.Delay(1000, cts.Token);
                }, cts.Token);


Basic Encoded Usage

The .NET NATS client mirrors go encoding through serialization and deserialization. Simply create an encoded connection and publish objects, and receive objects through an asynchronous subscription using the encoded message event handler. The .NET 4.6 client has a default formatter serializing objects using the BinaryFormatter, but methods used to serialize and deserialize objects can be overridden. The NATS core version does not have serialization defaults and they must be specified.

        using (IEncodedConnection c = new ConnectionFactory().CreateEncodedConnection())
            EventHandler<EncodedMessageEventArgs> eh = (sender, args) =>
                // Here, obj is an instance of the object published to
                // this subscriber.  Retrieve it through the
                // ReceivedObject property of the arguments.
                MyObject obj = (MyObject)args.ReceivedObject;

                System.Console.WriteLine("Company: " + obj.Company);

            // Subscribe using the encoded message event handler
            IAsyncSubscription s = c.SubscribeAsync("foo", eh);

            MyObject obj = new MyObject();
            obj.Company = "MyCompany";

            // To publish an instance of your object, simply
            // call the IEncodedConnection publish API and pass
            // your object.
            c.Publish("foo", obj);

Other Types of Serialization

Optionally, one can override serialization. Depending on the level of support or third party packages used, objects can be serialized to JSON, SOAP, or a custom scheme. XML was chosen as the example here as it is natively supported by .NET 4.6.

        // Example XML serialization.
        byte[] serializeToXML(Object obj)
            MemoryStream  ms = new MemoryStream();
            XmlSerializer x = new XmlSerializer(((SerializationTestObj)obj).GetType());

            x.Serialize(ms, obj);

            byte[] content = new byte[ms.Position];
            Array.Copy(ms.GetBuffer(), content, ms.Position);

            return content;

        Object deserializeFromXML(byte[] data)
            XmlSerializer x = new XmlSerializer(new SerializationTestObj().GetType());
            MemoryStream ms = new MemoryStream(data);
            return x.Deserialize(ms);


        // Create an encoded connection and override the OnSerialize and
        // OnDeserialize delegates.
        IEncodedConnection c = new ConnectionFactory().CreateEncodedConnection();
        c.OnDeserialize = deserializeFromXML;
        c.OnSerialize = serializeToXML;

        // From here on, the connection will use the custom delegates
        // for serialization.

One can also use Data Contract to serialize objects. Below are simple example overrides that work with .NET core:

public class JsonObject
    public string Value = "";

internal object jsonDeserializer(byte[] buffer)
    using (MemoryStream stream = new MemoryStream())
        var serializer = new DataContractJsonSerializer(typeof(JsonObject));
        stream.Write(buffer, 0, buffer.Length);
        stream.Position = 0;
        return serializer.ReadObject(stream);

internal byte[] jsonSerializer(object obj)
    if (obj == null)
        return null;

    var serializer = new DataContractJsonSerializer(typeof(JsonObject));

    using (MemoryStream stream = new MemoryStream())
        serializer.WriteObject(stream, obj);
        return stream.ToArray();


// Create an encoded connection and override the OnSerialize and
// OnDeserialize delegates.
IEncodedConnection c = new ConnectionFactory().CreateEncodedConnection();
c.OnDeserialize = jsonDeserializer;
c.OnSerialize = jsonSerializer;

// From here on, the connection will use the custom delegates
// for serialization.

Wildcard Subscriptions

The * wildcard matches any token, at any level of the subject:

IAsyncSubscription s = c.SubscribeAsync("foo.*.baz");

This subscriber would receive messages sent to:

  • foo.a.baz
  • etc...

It would not, however, receive messages on:

  • foo.baz
  • etc...

The > wildcard matches any length of the tail of a subject, and can only be the last token.

IAsyncSubscription s = c.SubscribeAsync("foo.>");

This subscriber would receive any message sent to:

  • etc...

However, it would not receive messages sent on:

  • foo
  • etc...

Publishing on this subject would cause the two above subscriber to receive the message:

c.Publish("", null);

Queue Groups

All subscriptions with the same queue name will form a queue group. Each message will be delivered to only one subscriber per queue group, using queue sematics. You can have as many queue groups as you wish. Normal subscribers will continue to work as expected.

ISyncSubscription s1 = c.SubscribeSync("foo", "job_workers");


IAsyncSubscription s = c.SubscribeAsync("foo", "job_workers", myHandler);

To unsubscribe, call the ISubscriber Unsubscribe method:


When finished with NATS, close the connection.


Advanced Usage

Connection and Subscriber objects implement IDisposable and can be created in a using statement. Here is all the code required to connect to a default server, receive ten messages, and clean up, unsubcribing and closing the connection when finished.

            using (IConnection c = new ConnectionFactory().CreateConnection())
                using (ISyncSubscription s = c.SubscribeSync("foo"))
                    for (int i = 0; i < 10; i++)
                        Msg m = s.NextMessage();
                        System.Console.WriteLine("Received: " + m);

Or to publish ten messages:

            using (IConnection c = new ConnectionFactory().CreateConnection())
                for (int i = 0; i < 10; i++)
                    c.Publish("foo", Encoding.UTF8.GetBytes("hello"));

Flush a connection to the server - this call returns when all messages have been processed. Optionally, a timeout in milliseconds can be passed.



Setup a subscriber to auto-unsubscribe after ten messsages.

        IAsyncSubscription s = c.SubscribeAsync("foo");
        s.MessageHandler += (sender, args) =>
           Console.WriteLine("Received: " + args.Message);


Note that an anonymous function was used. This is for brevity here - in practice, delegate functions can be used as well.

Other events can be assigned delegate methods through the options object.

            Options opts = ConnectionFactory.GetDefaultOptions();

            opts.AsyncErrorEventHandler += (sender, args) =>
                Console.WriteLine("Error: ");
                Console.WriteLine("   Server: " + args.Conn.ConnectedUrl);
                Console.WriteLine("   Message: " + args.Error);
                Console.WriteLine("   Subject: " + args.Subscription.Subject);

            opts.ServerDiscoveredEventHandler += (sender, args) =>
                Console.WriteLine("A new server has joined the cluster:");
                Console.WriteLine("    " + String.Join(", ", args.Conn.DiscoveredServers));

            opts.ClosedEventHandler += (sender, args) =>
                Console.WriteLine("Connection Closed: ");
                Console.WriteLine("   Server: " + args.Conn.ConnectedUrl);

            opts.DisconnectedEventHandler += (sender, args) =>
                Console.WriteLine("Connection Disconnected: ");
                Console.WriteLine("   Server: " + args.Conn.ConnectedUrl);

            IConnection c = new ConnectionFactory().CreateConnection(opts);

After version 0.5.0, the C# .NET client supports async Requests.

public async void MyRequestDataMethod(IConnection c)
    var m = await c.RequestAsync("foo", null);

    m = c.RequestAsync("foo", null);
    // do some work
    await m;

    // cancellation tokens are supported.
    var cts = new CancellationTokenSource();

    var msg = c.RequestAsync("foo", null, cts.Token);
    // do stuff
    if (requestIsNowIrrevelant())

    await msg;
    // be sure to handle OperationCancelled Exception.

The NATS .NET client supports the cluster discovery protocol. The list of servers known to a connection is automatically updated when a connection is established, and afterword in realtime as cluster changes occur. A current list of known servers in a cluster can be obtained using the IConnection.Servers property; this list will be used if the client needs to reconnect to the cluster.

Clustered Usage

            string[] servers = new string[] {

            Options opts = ConnectionFactory.GetDefaultOptions();
            opts.MaxReconnect = 2;
            opts.ReconnectWait = 1000;
            opts.Servers = servers;

            IConnection c = new ConnectionFactory().CreateConnection(opts);


The NATS .NET client supports TLS 1.2. Set the secure option, add the certificate, and connect. Note that .NET requires both the private key and certificate to be present in the same certificate file.

        Options opts = ConnectionFactory.GetDefaultOptions();
        opts.Secure = true;

        // .NET requires the private key and cert in the
        //  same file. 'client.pfx' is generated from:
        // openssl pkcs12 -export -out client.pfx
        //    -inkey client-key.pem -in client-cert.pem
        X509Certificate2 cert = new X509Certificate2("client.pfx", "password");


        IConnection c = new ConnectionFactory().CreateConnection(opts);

Many times, it is useful when developing an application (or necessary when using self-signed certificates) to override server certificate validation. This is achieved by overriding the remove certificate validation callback through the NATS client options.

    private bool verifyServerCert(object sender,
        X509Certificate certificate, X509Chain chain,
                SslPolicyErrors sslPolicyErrors)
            if (sslPolicyErrors == SslPolicyErrors.None)
                return true;

            // Do what is necessary to achieve the level of
            // security you need given a policy error.


        Options opts = ConnectionFactory.GetDefaultOptions();
        opts.Secure = true;
        opts.TLSRemoteCertificationValidationCallback = verifyServerCert;

        IConnection c = new ConnectionFactory().CreateConnection(opts);

The NATS server default cipher suites may not be supported by the Microsoft .NET framework. Please refer to nats-server --help_tls usage and configure the NATS server to include the most secure cipher suites supported by the .NET framework.

NATS 2.0 Authentication (Nkeys and User Credentials)

This requires server with version >= 2.0.0

NATS servers have a new security and authentication mechanism to authenticate with user credentials and Nkeys. The simplest form is to use the helper method UserCredentials(credsFilepath).

IConnection c = new ConnectionFactory().CreateConnection("nats://", "user.creds")

The helper methods creates two callback handlers to present the user JWT and sign the nonce challenge from the server. The core client library never has direct access to your private key and simply performs the callback for signing the server challenge. The helper will load and wipe and erase memory it uses for each connect or reconnect.

The helper also can take two entries, one for the JWT and one for the NKey seed file.

IConnection c = new ConnectionFactory().CreateConnection("nats://", "user.jwt", "user.nk");

You can also set the event handlers directly and manage challenge signing directly.

EventHandler<UserJWTEventArgs> jwtEh = (sender, args) =>
    // Obtain a user JWT...
    string jwt = getMyUserJWT();

    // You must set the JWT in the args to hand off
    // to the client library.
    args.JWT = jwt;

EventHandler<UserSignatureEventArgs> sigEh = (sender, args) =>
    // get a private key seed from your environment.
    string seed = getMyUserSeed();

    // Generate a NkeyPair
    NkeyPair kp = NKeys.FromSeed(seed);

    // Sign the nonce and return the result in the SignedNonce
    // args property.  This must be set.
    args.SignedNonce = kp.Sign(args.ServerNonce)
Options opts = ConnectionFactory.GetDefaultOptions();
opts.SetUserCredentialHandlers(jwtEh, sigEh);

IConnection c = new ConnectionFactory().CreateConnection(opts));

Bare Nkeys are also supported. The nkey seed should be in a read only file, e.g. seed.txt

> cat seed.txt

This is a helper function which will load and decode and do the proper signing for the server nonce. It will clear memory in between invocations. You can choose to use the low level option and provide the public key and a signature callback on your own.

Options opts = ConnectionFactory.GetDefaultOptions();

// Direct
IConnection c = new ConnectionFactory().CreateConnection(opts));

Message Headers

The NATS.Client version 0.11.0 and NATS server version 2.2 support message headers. Message headers are represented as a string name value pair just as HTTP headers are.

Setting Message Headers

IConnection c = new new ConnectionFactory().CreateConnection();

Msg m = new Msg();
m.Header["Content-Type"] = "json";
m.Subject = "foo";

Getting Message Headers

IConnection c = new new ConnectionFactory().CreateConnection();
var s = c.SubscribeSync("foo")

Msg m = s.NextMessage();
string contentType = m.Header["Content-Type"];


The NATS .NET client can throw the following exceptions:

  • NATSException - The generic NATS exception, and base class for all other NATS exception.
  • NATSConnectionException - The exception that is thrown when there is a connection error.
  • NATSProtocolException - This exception that is thrown when there is an internal error with the NATS protocol.
  • NATSNoServersException - The exception that is thrown when a connection cannot be made to any server.
  • NATSSecureConnRequiredException - The exception that is thrown when a secure connection is required.
  • NATSConnectionClosedException - The exception that is thrown when a an operation is performed on a connection that is closed.
  • NATSSlowConsumerException - The exception that is thrown when a consumer (subscription) is slow.
  • NATSStaleConnectionException - The exception that is thrown when an operation occurs on a connection that has been determined to be stale.
  • NATSMaxPayloadException - The exception that is thrown when a message payload exceeds what the maximum configured.
  • NATSBadSubscriptionException - The exception that is thrown when a subscriber operation is performed on an invalid subscriber.
  • NATSTimeoutException - The exception that is thrown when a NATS operation times out.

Benchmarking the NATS .NET Client

Benchmarking the NATS .NET Client is simple - run the benchmark application with a default NATS server running. Tests can be customized, run benchmark -h for more details. In order to get the best out of your test, update the priority of the benchmark application and the NATS server:

start /B /REALTIME nats-server.exe

And the benchmark:

start /B /REALTIME benchmark.exe

The benchmarks include:

  • PubOnly - publish only
  • PubSub - publish and subscribe
  • ReqReply - request/reply
  • Lat - latency.

Note, kb/s is solely payload, excluding the NATS message protocol. Latency is measure in microseconds.

Here is some sample output, from a VM running on a Macbook Pro, simulating a cloud environment. Running the benchmarks in your environment is highly recommended; these numbers below should reflect the low end of performance numbers.

PubOnlyNo	  10000000	   4715384 msgs/s	       0 kb/s
PubOnly8b	  10000000	   4058182 msgs/s	   31704 kb/s
PubOnly32b	  10000000	   3044199 msgs/s	   95131 kb/s
PubOnly256b	  10000000	    408034 msgs/s	  102008 kb/s
PubOnly512b	  10000000	    203681 msgs/s	  101840 kb/s
PubOnly1k	   1000000	     94106 msgs/s	   94106 kb/s
PubOnly4k	    500000	     52653 msgs/s	  210612 kb/s
PubOnly8k	    100000	      8552 msgs/s	   68416 kb/s
PubSubNo	  10000000	   1101135 msgs/s	       0 kb/s
PubSub8b	  10000000	   1075814 msgs/s	    8404 kb/s
PubSub32b	  10000000	    990223 msgs/s	   30944 kb/s
PubSub256b	  10000000	    391208 msgs/s	   97802 kb/s
PubSub512b	    500000	    190811 msgs/s	   95405 kb/s
PubSub1k	    500000	     97392 msgs/s	   97392 kb/s
PubSub4k	    500000	     23714 msgs/s	   94856 kb/s
PubSub8k	    100000	     11870 msgs/s	   94960 kb/s
ReqReplNo	     20000	      3245 msgs/s	       0 kb/s
ReqRepl8b	     10000	      3237 msgs/s	      25 kb/s
ReqRepl32b	     10000	      3076 msgs/s	      96 kb/s
ReqRepl256b	      5000	      2446 msgs/s	     611 kb/s
ReqRepl512b	      5000	      2530 msgs/s	    1265 kb/s
ReqRepl1k	      5000	      2973 msgs/s	    2973 kb/s
ReqRepl4k	      5000	      1944 msgs/s	    7776 kb/s
ReqRepl8k	      5000	      1394 msgs/s	   11152 kb/s
LatNo (us)	500 msgs, 141.74 avg, 86.00 min, 600.40 max, 23.28 stddev
Lat8b (us)	500 msgs, 141.52 avg, 70.30 min, 307.00 max, 26.77 stddev
Lat32b (us)	500 msgs, 139.64 avg, 93.70 min, 304.20 max, 15.88 stddev
Lat256b (us)	500 msgs, 175.55 avg, 101.80 min, 323.90 max, 14.93 stddev
Lat512b (us)	500 msgs, 182.56 avg, 103.00 min, 1982.00 max, 81.50 stddev
Lat1k (us)	500 msgs, 193.15 avg, 86.20 min, 28705.20 max, 1277.13 stddev
Lat4k (us)	500 msgs, 291.09 avg, 99.70 min, 43679.10 max, 2047.67 stddev
Lat8k (us)	500 msgs, 363.56 avg, 131.50 min, 39428.50 max, 1990.81 stddev

About the code and contributing

A note: The NATS C# .NET client was originally developed with the idea in mind that it would support the .NET 4.0 code base for increased adoption, and closely parallel the GO client (internally) for maintenance purposes. So, some of the nice .NET APIs/features were intentionally left out. While this has certainly paid off, after consideration, and some maturation of the NATS C# library, the NATS C# code will move toward more idiomatic .NET coding style where it makes sense.

To that end, with any contributions, certainly feel free to code in a more .NET idiomatic style than what you see. PRs are always welcome!


  • [ ] Another performance pass - look at stream directly over socket, contention, fastpath optimizations, rw locks.
  • [ ] Rx API (unified over NATS Streaming?)
  • [ ] Expand Unit Tests to test internals (namely Parsing)
  • [X] Travis CI (Used AppVeyor instead)
  • [ ] Allow configuration for performance tuning (buffer sizes), defaults based on plaform.
  • [X] .NET Core compatibility, TLS required.
  • [ ] Azure Service Bus Connector
  • [ ] Visual Studio Starter Kit
  • [X] Convert unit tests to xunit
  • [X] Comprehensive benchmarking
  • [X] TLS
  • [X] Encoding (Serialization/Deserialization)
  • [X] Update delegates from traditional model to custom
  • [X] NuGet package
  • [X] Strong name the assembly

Any suggestions and/or contributions are welcome!

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