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Migrating from MessagePack v1.x to MessagePack v2.x

MessagePack 2.0 contains many breaking changes since the 1.x versions. These include both binary and source breaking changes, meaning you may need to update your source code as well as recompile against the 2.x version.

The v1.x version will still be serviced for security fixes, but new features will tend to only be offered in the 2.x versions.

Update your package references from the 1.x version you use to the 2.x version. If your project compiles, you may be done. Otherwise work through each compiler error. Some common ones you may face are listed below with suggested fixes.

If you own an application that has a mix of MessagePack consumers and not all of them can be upgraded to v2.x at once, you can offer both MessagePack v1.x and v2.x assemblies with your application so that each user can find the one it needs. Here is a sample.

API changes


A new MessagePackSerializerOptions class becomes a first class citizen in this library. It encapsulates the IFormatterResolver that used to be passed around by itself. It also includes several other settings that may influence how MessagePackSerializerOptions or some of the formatters may operate.

Because this new class tends to get saved to public static properties, it is immutable to ensure it can be shared safely. Each property Foo on the class includes a WithFoo method which clones the instance and returns the new instance with just that one property changed.

To support this new options class and avoid unnecessary allocations from the copy-and-mutate methods, many of the popular resolvers now expose a public static Options property with the resolver preset to itself. So for example, you may use:

var msgpack = MessagePackSerializer.Serialize(objectGraph, StandardResolverAllowPrivate.Options);
var deserializedGraph = MessagePackSerializer.Deserialize<MyType>(msgpack, StandardResolverAllowPrivate.Options);

If you want to combine a particular resolver with other options changes (e.g. enabling LZ4 compression), you may do that too:

var options = StandardResolverAllowPrivate.Options.WithCompression(MessagePackCompression.Lz4BlockArray);
var msgpack = MessagePackSerializer.Serialize(objectGraph, options);
var deserializedGraph = MessagePackSerializer.Deserialize<MyType>(msgpack, options);

An equivalent options instance can be created manually:

var options = MessagePackSerializerOptions.Standard

MessagePackSerializer class


Serializing object graphs to msgpack is now based on IBufferWriter<byte> instead of ref byte[]. This allows for serializing very large object graphs without repeatedly allocating ever-larger arrays and copying the previously serialized msgpack bytes from the smaller buffer to the larger one. IBufferWriter<byte> can direct the written msgpack bytes directly to a pipe, a file, or anywhere else you choose, allowing you to avoid a buffer copy within your own code as well.

An IBufferWriter<byte> is always wrapped by the new MessagePackWriter struct.

Many overloads of the Serialize method exist which ultimately all call the overload that accepts a MessagePackWriter.


Deserializing msgpack sequences is now much more flexible. Instead of deserializing from byte[] or ArraySegment<byte> only, you can deserialize from any ReadOnlyMemory<byte> or ReadOnlySequence<byte> instance.

ReadOnlyMemory<byte> is like ArraySegment<byte> but more friendly and can refer to contiguous memory anywhere including native pointers. You can pass a byte[] or ArraySegment<byte> in anywhere that ReadOnlyMemory<byte> is expected and C# will implicitly cast for you (without any buffer copying).

ReadOnlySequence<byte> allows for deserialization from non-continguously allocated memory, enabling you to deserialize very large msgpack sequences without risking an OutOfMemoryException due simply to the inability to find large amounts of free contiguous memory.

Many overloads of the Deserialize method exists which ultimately all call the overload that accepts a MessagePackReader.

Deserializing from a Stream

Deserializing from a Stream has changed from v1.x to v2.0. The readStrict parameter has been removed and in v2.x the MessagePackSerializer.Deserialize{Async}(Stream) methods act as if readStrict: false in v1.x. This works great and is the preferred API to use when the entire Stream is expected to contain exactly one top-level messagepack structure that you want to deserialize.

For performance reasons, the entire Stream is read into memory before deserialization begins. If there is more data on the Stream than the messagepack structure to be deserialized, the deserialization will ignore the excess data, but the excess data wouldn't be on the Stream any more to be read later.

If the Stream is seekable (that is, its CanSeek property returns true) then after deserialization is complete the Stream will be repositioned to the first byte after the messagepack data structure that was deserialized. This means you'll get the Stream back as you might expect it, but only after you paid a perf cost of "reading" more data than was necessary to deserialize.

If the Stream is not seekable (e.g. a network stream) or contains multiple top-level messagepack data structures consecutively, MessagePack 2.0 adds a new, more performant way to read each messagepack structure. It's analogous to v1.x's readStrict: true mode, but is much more performant. It comes in the form of the new MessagePackStreamReader class, and can be easily used as follows:

static async Task<List<T>> DeserializeListFromStreamAsync<T>(Stream stream, CancellationToken cancellationToken)
    var dataStructures = new List<T>();
    using (var streamReader = new MessagePackStreamReader(stream))
        while (await streamReader.ReadAsync(cancellationToken) is ReadOnlySequence<byte> msgpack)
            dataStructures.Add(MessagePackSerializer.Deserialize<T>(msgpack, cancellationToken: cancellationToken));

    return dataStructures;

Default behavior

The DefaultResolver static property has been replaced with the DefaultOptions static property. Just as with v1.x, in v2.x this static property influences how serialization occurs when the value is not explicitly specified when invoking one of the MessagePackSerializer methods.

WARNING: When developing a simple application where you control all MessagePack-related code it may be safe to rely on this mutable static to control behavior. For all other libraries or multi-purpose applications that use MessagePackSerializer you should explicitly specify the MessagePackSerializerOptions to use with each method invocation to guarantee your code behaves as you expect even when sharing an AppDomain or process with other MessagePack users that may change this static property.

Non-generic methods

In v1.x non-generic methods for serialization/deserialization were exposed on the nested MessagePackSerializer.NonGeneric class. In v2.x these overloads are moved to the MessagePackSerializer class itself.

The MessagePackSerializer.Typeless nested class in v1.x remains in v2.x, but with a modified set of overloads.

JSON converting methods

In v1.x the MessagePackSerializer class exposed methods both to serialize an object graph to JSON, as well as converting between msgpack and JSON. These two translations were very different but were mere overloads of each other. In v2.x these methods have been renamed for clarity. The methods ConvertFromJson and ConvertToJson translates between JSON and msgpack binary. The method SerializeToJson translates an object graph to JSON.


The LZ4MessagePackSerializer class has been removed. Instead, use MessagePackSerializer and pass in a MessagePackSerializerOptions with WithCompression set to MessagePackCompression.Lz4Block or MessagePackCompression.Lz4BlockArray.

For example, make this change:

-byte[] buffer = LZ4MessagePackSerializer.Serialize("hi");
+static readonly lz4Options = MessagePackSerializerOptions.Standard.WithCompression(MessagePackCompression.Lz4BlockArray);
+byte[] buffer = MessagePackSerializer.Serialize("hi", lz4Options);

Lz4Block is same as v1 LZ4MessagePackSerializer. Lz4BlockArray is new compression mode of v2. Regardless of which Lz4 option is set at the deserialization, both data can be deserialized. For example, when the option is Lz4BlockArray, binary data of both Lz4Block and Lz4BlockArray can be deserialized.

Thrown exceptions

In v1.x any exception thrown during serialization or deserialization was uncaught and propagated to the application. In v2.x all exceptions are caught by the MessagePackSerializer and rethrown as an inner exception of MessagePackSerializationException. This makes it easier to write code to catch exceptions during serialization since you can now catch just one specific type of exception.

Built-in resolvers

The following resolvers have been removed:

Removed v1.x formatter v2.x alternative
UnsafeBinaryResolver NativeDecimalResolver, NativeGuidResolver


In v1.x the CompositeResolver type could only be used once and mutated a static property. In v2.x the CompositeResolver type no longer mutates any statics and thus can be used safely by many callers that simply want to aggregate many formatters and/or resolvers into one resolver. This often removes the need for you to define your own IFormatterResolver.

For example if you have written a custom formatter and want to use that in addition to what the StandardResolver offers, you can easily compose an aggregate resolver like this:

var resolver = CompositeResolver.Create(
    new IMessagePackFormatter[] { MyCustomFormatter.Instance },
    new IFormatterResolver[] { StandardResolver.Instance }
var options = MessagePackSerializerOptions.Standard.WithResolver(resolver);
var msgpack = MessagePackSerializer.Serialize(objectGraph, options);
var deserializedGraph = MessagePackSerializer.Deserialize<MyType>(msgpack, options);

Built-in formatters

The following formatters have been removed:

Removed v1.x formatter v2.x alternative
BinaryDecimalFormatter NativeDecimalFormatter
BinaryGuidFormatter NativeGuidFormatter
FourDimentionalArrayFormatter FourDimensionalArrayFormatter
OldSpecBinaryFormatter Use MessagePackSerializerOptions.OldSpec or MessagePackWriter.OldSpec instead.
OldSpecStringFormatter Use MessagePackSerializerOptions.OldSpec or MessagePackWriter.OldSpec instead.
QeueueFormatter<T> QueueFormatter<T>
TaskUnitFormatter Store values instead of promises
TaskValueFormatter<T> Store values instead of promises
ThreeDimentionalArrayFormatter ThreeDimensionalArrayFormatter
TwoDimentionalArrayFormatter TwoDimensionalArrayFormatter
ValueTaskFormatter<T> Store values instead of promises

A few formatters that remain have changed to remove mutable properties where those formatters may be exposed as public static instances. This helps to avoid malfunctions when one MessagePack user changes a static setting to suit their need but in a way that conflicts with another MessagePack user within the same process.

TypelessFormatter changes

The TypelessFormatter.BindToType static property has been removed. If you were using this property, you can find equivalent functionality in the virtual Type MessagePackSerializerOptions.LoadType(string typeName) method. The TypelessFormatter will call this method on the MessagePackSerializerOptions instance passed to it during deserialization.

For example, you can override this virtual method in your own derived type:

class LoadTypeCustomizedOptions : MessagePackSerializerOptions
    internal LoadTypeCustomizedOptions(MessagePackSerializerOptions copyFrom)
        : base(copyFrom)

    internal LoadTypeCustomizedOptions(IFormatterResolver resolver)
        : base(resolver)

    public override Type LoadType(string typeName)
        Type type = base.LoadType(typeName);
        if (type == null)
            // custom logic here

        return type;

You can then instantiate this options type and pass it to your deserializer:

var options = new LoadTypeCustomizedOptions(MessagePackSerializerOptions.Standard);
T value = MessagePackSerializer.Deserialize<T>(sequence, options);

Custom formatters

If you have written a custom IMessagePackFormatter<T> implementation you will have to adapt to the interface changes and APIs used to implement such a class.

The interface has been changed as described here:

 public interface IMessagePackFormatter<T> : IMessagePackFormatter
-    int Serialize(ref byte[] bytes, int offset, T value, IFormatterResolver  formatterResolver);
+    void Serialize(ref MessagePackWriter writer, T value, MessagePackSerializerOptions options);
-    T Deserialize(byte[] bytes, int offset, IFormatterResolver  formatterResolver, out int readSize);
+    T Deserialize(ref MessagePackReader reader, MessagePackSerializerOptions options);

Notice the simpler method signature for each method. You no longer have to deal with raw arrays and offsets. The MessagePackBinary static class from v1.x that a formatter used to write msgpack codes is replaced with MessagePackWriter and MessagePackReader. These two structs include the APIs to write and read msgpack, and they manage the underlying buffers so you no longer need to.

Consider the following v1.x formatter for the Int16 type:

class NullableInt16Formatter : IMessagePackFormatter<Int16?>
    public int Serialize(ref byte[] bytes, int offset, Int16? value, IFormatterResolver formatterResolver)
        if (value == null)
            return MessagePackBinary.WriteNil(ref bytes, offset);
            return MessagePackBinary.WriteInt16(ref bytes, offset, value.Value);

    public Int16? Deserialize(byte[] bytes, int offset, IFormatterResolver formatterResolver, out int readSize)
        if (MessagePackBinary.IsNil(bytes, offset))
            readSize = 1;
            return null;
            return MessagePackBinary.ReadInt16(bytes, offset, out readSize);

After migration for v2.x, it looks like this:

class NullableInt16Formatter : IMessagePackFormatter<Int16?>
    public void Serialize(ref MessagePackWriter writer, Int16? value, MessagePackSerializerOptions options)
        if (value == null)

    public Int16? Deserialize(ref MessagePackReader reader, MessagePackSerializerOptions options)
        if (reader.TryReadNil())
            return default;
            return reader.ReadInt16();

Notice the structure is very similar, but arrays and offsets are no longer necessary. The underlying msgpack format is unchanged, allowing code to be upgraded to v2.x while maintaining compatibility with a file or network party that uses MessagePack v1.x.

Subtle change in method naming

When writing integers, the method name pattern has changed such that although your v1.x->v2.0 code will compile it may produce slightly different (and less efficient) msgpack binary than before. Here is the translation table:

v1.x v2.x
MessagePackBinary.WriteMapHeaderForceMap32Block (removed)
MessagePackBinary.WriteArrayHeaderForceArray32Block (removed)
MessagePackBinary.WriteByteForceByteBlock MessagePackWriter.WriteUInt8(byte)
MessagePackBinary.WriteSByteForceSByteBlock MessagePackWriter.WriteInt8(sbyte)
MessagePackBinary.WriteInt16ForceInt16Block MessagePackWriter.WriteInt16(short)
MessagePackBinary.WriteInt64ForceInt64Block MessagePackWriter.WriteInt64(long)
MessagePackBinary.MessagePackBinary.WriteInt32ForceInt32Block MessagePackWriter.WriteInt32(int)
MessagePackBinary.WriteUInt16ForceUInt16Block MessagePackWriter.WriteUInt16(ushort)
MessagePackBinary.WriteUInt32ForceUInt32Block MessagePackWriter.WriteUInt32(uint)
MessagePackBinary.WriteUInt64ForceUInt64Block MessagePackWriter.WriteUInt64(ulong)
MessagePackBinary.WriteStringForceStr32Block (removed)
MessagePackBinary.WriteExtensionFormatHeaderForceExt32Block (removed)
MessagePackBinary.WriteMapHeader MessagePackWriter.WriteMapHeader
MessagePackBinary.WriteArrayHeader MessagePackWriter.WriteArrayHeader
MessagePackBinary.WriteByte MessagePackWriter.Write(byte)
MessagePackBinary.WriteBytes MessagePackWriter.Write(byte[])
MessagePackBinary.WriteSByte MessagePackWriter.Write(sbyte)
MessagePackBinary.WriteSingle MessagePackWriter.Write(float)
MessagePackBinary.WriteDouble MessagePackWriter.Write(double)
MessagePackBinary.WriteInt16 MessagePackWriter.Write(short)
MessagePackBinary.WriteInt32 MessagePackWriter.Write(int)
MessagePackBinary.WriteInt64 MessagePackWriter.Write(long)
MessagePackBinary.WriteUInt16 MessagePackWriter.Write(ushort)
MessagePackBinary.WriteUInt32 MessagePackWriter.Write(uint)
MessagePackBinary.WriteUInt64 MessagePackWriter.Write(ulong)
MessagePackBinary.WriteChar MessagePackWriter.Write(char)
MessagePackBinary.WriteStringBytes MessagePackWriter.WriteString(ReadOnlySpan<byte>)
MessagePackBinary.WriteString MessagePackWriter.Write(string)
MessagePackBinary.WriteExtensionFormatHeader MessagePackWriter.WriteExtensionFormatHeader
MessagePackBinary.WriteExtensionFormat MessagePackWriter.WriteExtensionFormat
MessagePackBinary.WriteDateTime MessagePackWriter.Write(DateTime) (notes)

The essence here is that you can typically just call MessagePackWriter.Write(*) for primitive types and the most efficient msgpack binary will be written out. You only should call the explicit WriteX(x) methods if you need to force a particular (fixed length) format of a value to be written out.

As for the integer reading methods, these are much more interchangeable than in v1.x. You can call any ReadInt* or ReadUInt* method and it will successfully read an integer value and fit it into the desired return type so long as the value doesn't overflow. So for example you can call Write(byte) and later read the value with ReadInt32(). You can even call Write(long) and later read it with ReadByte() and it will work so long as the actual value fits inside a byte. An OverflowException is thrown if the integer value exceeds the max or min value that can be stored by the required return type.

Behavioral changes


When writing out DateTime v1.x would always call DateTime.ToUniversalTime() before serializing the value. In v2.x we only call this method if DateTime.Kind == DateTimeKind.Local. The impact of this is that if you were writing DateTimeKind.Unspecified the serialized value will no longer be changed under some unjustified assumption that the underlying value was Local. Your should specify DateTimeKind explicitly for all your DateTime values. When upgrading to MessagePack v2.x this is a breaking change if your Unspecified values actually represented the Local time zone and needed the conversion.

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