Use MessagePack Hub Protocol in SignalR for ASP.NET Core

By Brennan Conroy

This article assumes the reader is familiar with the topics covered in Get Started.

What is MessagePack?

MessagePack is a binary serialization format that is fast and compact. It's useful when performance and bandwidth are a concern because it creates smaller messages compared to JSON. Because it's a binary format, messages are unreadable when looking at network traces and logs unless the bytes are passed through a MessagePack parser. SignalR has built-in support for the MessagePack format, and provides APIs for the client and server to use.

Configure MessagePack on the server

To enable the MessagePack Hub Protocol on the server, install the Microsoft.AspNetCore.SignalR.Protocols.MessagePack package in your app. In the Startup.cs file add AddMessagePackProtocol to the AddSignalR call to enable MessagePack support on the server.


JSON is enabled by default. Adding MessagePack enables support for both JSON and MessagePack clients.


To customize how MessagePack will format your data, AddMessagePackProtocol takes a delegate for configuring options. In that delegate, the FormatterResolvers property can be used to configure MessagePack serialization options. For more information on how the resolvers work, visit the MessagePack library at MessagePack-CSharp. Attributes can be used on the objects you want to serialize to define how they should be handled.

    .AddMessagePackProtocol(options =>
        options.FormatterResolvers = new List<MessagePack.IFormatterResolver>()

Configure MessagePack on the client


JSON is enabled by default for the supported clients. Clients can only support a single protocol. Adding MessagePack support will replace any previously configured protocols.

.NET client

To enable MessagePack in the .NET Client, install the Microsoft.AspNetCore.SignalR.Protocols.MessagePack package and call AddMessagePackProtocol on HubConnectionBuilder.

var hubConnection = new HubConnectionBuilder()


This AddMessagePackProtocol call takes a delegate for configuring options just like the server.

JavaScript client

MessagePack support for the JavaScript client is provided by the @microsoft/signalr-protocol-msgpack npm package. Install the package by executing the following command in a command shell:

npm install @microsoft/signalr-protocol-msgpack

MessagePack support for the JavaScript client is provided by the @aspnet/signalr-protocol-msgpack npm package. Install the package by executing the following command in a command shell:

npm install @aspnet/signalr-protocol-msgpack

After installing the npm package, the module can be used directly via a JavaScript module loader or imported into the browser by referencing the following file:



In a browser, the msgpack5 library must also be referenced. Use a <script> tag to create a reference. The library can be found at node_modules\msgpack5\dist\msgpack5.js.


When using the <script> element, the order is important. If signalr-protocol-msgpack.js is referenced before msgpack5.js, an error occurs when trying to connect with MessagePack. signalr.js is also required before signalr-protocol-msgpack.js.

<script src="~/lib/signalr/signalr.js"></script>
<script src="~/lib/msgpack5/msgpack5.js"></script>
<script src="~/lib/signalr/signalr-protocol-msgpack.js"></script>

Adding .withHubProtocol(new signalR.protocols.msgpack.MessagePackHubProtocol()) to the HubConnectionBuilder will configure the client to use the MessagePack protocol when connecting to a server.

const connection = new signalR.HubConnectionBuilder()
    .withHubProtocol(new signalR.protocols.msgpack.MessagePackHubProtocol())


At this time, there are no configuration options for the MessagePack protocol on the JavaScript client.

MessagePack quirks

There are a few issues to be aware of when using the MessagePack Hub Protocol.

MessagePack is case-sensitive

The MessagePack protocol is case-sensitive. For example, consider the following C# class:

public class ChatMessage
    public string Sender { get; }
    public string Message { get; }

When sending from the JavaScript client, you must use PascalCased property names, since the casing must match the C# class exactly. For example:

connection.invoke("SomeMethod", { Sender: "Sally", Message: "Hello!" });

Using camelCased names won't properly bind to the C# class. You can work around this by using the Key attribute to specify a different name for the MessagePack property. For more information, see the MessagePack-CSharp documentation.

DateTime.Kind is not preserved when serializing/deserializing

The MessagePack protocol doesn't provide a way to encode the Kind value of a DateTime. As a result, when deserializing a date, the MessagePack Hub Protocol assumes the incoming date is in UTC format. If you're working with DateTime values in local time, we recommend converting to UTC before sending them. Convert them from UTC to local time when you receive them.

For more information on this limitation, see GitHub issue aspnet/SignalR#2632.

DateTime.MinValue is not supported by MessagePack in JavaScript

The msgpack5 library used by the SignalR JavaScript client doesn't support the timestamp96 type in MessagePack. This type is used to encode very large date values (either very early in the past or very far in the future). The value of DateTime.MinValue is January 1, 0001 which must be encoded in a timestamp96 value. Because of this, sending DateTime.MinValue to a JavaScript client isn't supported. When DateTime.MinValue is received by the JavaScript client, the following error is thrown:

Uncaught Error: unable to find ext type 255 at decoder.js:427

Usually, DateTime.MinValue is used to encode a "missing" or null value. If you need to encode that value in MessagePack, use a nullable DateTime value (DateTime?) or encode a separate bool value indicating if the date is present.

For more information on this limitation, see GitHub issue aspnet/SignalR#2228.

MessagePack support in "ahead-of-time" compilation environment

The MessagePack-CSharp library used by the .NET client and server uses code generation to optimize serialization. As a result, it isn't supported by default on environments that use "ahead-of-time" compilation (such as Xamarin iOS or Unity). It's possible to use MessagePack in these environments by "pre-generating" the serializer/deserializer code. For more information, see the MessagePack-CSharp documentation. Once you have pre-generated the serializers, you can register them using the configuration delegate passed to AddMessagePackProtocol:

    .AddMessagePackProtocol(options =>
        options.FormatterResolvers = new List<MessagePack.IFormatterResolver>()

Type checks are more strict in MessagePack

The JSON Hub Protocol will perform type conversions during deserialization. For example, if the incoming object has a property value that is a number ({ foo: 42 }) but the property on the .NET class is of type string, the value will be converted. However, MessagePack doesn't perform this conversion and will throw an exception that can be seen in server-side logs (and in the console):

InvalidDataException: Error binding arguments. Make sure that the types of the provided values match the types of the hub method being invoked.

For more information on this limitation, see GitHub issue aspnet/SignalR#2937.