NBMsgPack120: IMessagePackSerializationCallbacks is only supported on reference types

This diagnostic is reported when a struct implements IMessagePackSerializationCallbacks. The interface is only supported on reference types. Object map and array converters invoke OnBeforeSerialize, OnAfterSerialize, OnBeforeDeserialize, and OnAfterDeserialize by casting the value to the interface. When the value is a struct, that cast boxes a copy, so mutations made in the callback methods are lost.

The diagnostic includes every struct form: an ordinary struct, a readonly struct, a ref struct, and a record struct. Implementing the interface through another interface is reported as well.

Example violation

struct Counter : IMessagePackSerializationCallbacks
{
    public int Mutations;

    public void OnBeforeSerialize() => this.Mutations++;

    public void OnAfterSerialize() => this.Mutations++;

    public void OnBeforeDeserialize() => this.Mutations++;

    public void OnAfterDeserialize() => this.Mutations++;
}

record struct RecordCounter : IMessagePackSerializationCallbacks
{
    public int Mutations { get; set; }

    public void OnBeforeSerialize() => this.Mutations++;

    public void OnAfterSerialize() => this.Mutations++;

    public void OnBeforeDeserialize() => this.Mutations++;

    public void OnAfterDeserialize() => this.Mutations++;
}

Resolution

Implement the interface on a class or record class:

class Counter : IMessagePackSerializationCallbacks
{
    public int Mutations { get; set; }

    public void OnBeforeSerialize() => this.Mutations++;

    public void OnAfterSerialize() => this.Mutations++;

    public void OnBeforeDeserialize() => this.Mutations++;

    public void OnAfterDeserialize() => this.Mutations++;
}