using MessagePack;

using System;

public class Program

{

    public static void Main()

    {

        var obj = new Test()

        {

            Action = "test_action",

            Id = 150,

            Str = "str_123"

        };

        byte[] bytes = MessagePackSerializer.Serialize(obj);

        Console.WriteLine(bytes.Length);

        Console.WriteLine(string.Join(' ', bytes));

        Console.WriteLine(Convert.ToBase64String(bytes));

        Console.WriteLine(System.Text.Encoding.UTF8.GetString(bytes));

    }

}

​

[MessagePackObject]

public class Test

{

    [Key(0)]

    public string Action { get; init; }

    [Key(1)]

    public int Id { get; init; }

    [Key(2)]

    public string Str { get; init; }

}