Google Protocol Buffers (protobuf): C# parsing library

Google Protocol Buffers (AKA protobuf) is a popular data serialization scheme used for communication protocols, data storage, etc. There are implementations are available for almost every popular language. The focus points of this scheme are brevity (data is encoded in a very size-efficient manner) and extensibility (one can add keys to the structure, while keeping it readable in previous version of software).

Protobuf uses semi-self-describing encoding scheme for its messages. It means that it is possible to parse overall structure of the message (skipping over fields one can't understand), but to fully understand the message, one needs a protocol definition file (.proto). To be specific:

  • "Keys" in key-value pairs provided in the message are identified only with an integer "field tag". .proto file provides info on which symbolic field names these field tags map to.
  • "Keys" also provide something called "wire type". It's not a data type in its common sense (i.e. you can't, for example, distinguish sint32 vs uint32 vs some enum, or string from bytes), but it's enough information to determine how many bytes to parse. Interpretation of the value should be done according to the type specified in .proto file.
  • There's no direct information on which fields are optional / required, which fields may be repeated or constitute a map, what restrictions are placed on fields usage in a single message, what are the fields' default values, etc, etc.

KS implementation details

License: MIT
Minimal Kaitai Struct required: 0.7

References

This page hosts a formal specification of Google Protocol Buffers (protobuf) using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

Usage

Runtime library

All parsing code for C# generated by Kaitai Struct depends on the C# runtime library. You have to install it before you can parse data.

The C# runtime library is available in the NuGet Gallery. Installation instructions can also be found there.

Code

Parse a local file and get structure in memory:

var data = GoogleProtobuf.FromFile("path/to/local/file.google_protobuf");

Or parse structure from a byte array:

byte[] someArray = new byte[] { ... };
var data = new GoogleProtobuf(new KaitaiStream(someArray));

After that, one can get various attributes from the structure by accessing properties like:

data.Pairs // => Key-value pairs which constitute a message

C# source code to parse Google Protocol Buffers (protobuf)

GoogleProtobuf.cs

// This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild

using System.Collections.Generic;

namespace Kaitai
{

    /// <summary>
    /// Google Protocol Buffers (AKA protobuf) is a popular data
    /// serialization scheme used for communication protocols, data storage,
    /// etc. There are implementations are available for almost every
    /// popular language. The focus points of this scheme are brevity (data
    /// is encoded in a very size-efficient manner) and extensibility (one
    /// can add keys to the structure, while keeping it readable in previous
    /// version of software).
    /// 
    /// Protobuf uses semi-self-describing encoding scheme for its
    /// messages. It means that it is possible to parse overall structure of
    /// the message (skipping over fields one can't understand), but to
    /// fully understand the message, one needs a protocol definition file
    /// (`.proto`). To be specific:
    /// 
    /// * &quot;Keys&quot; in key-value pairs provided in the message are identified
    ///   only with an integer &quot;field tag&quot;. `.proto` file provides info on
    ///   which symbolic field names these field tags map to.
    /// * &quot;Keys&quot; also provide something called &quot;wire type&quot;. It's not a data
    ///   type in its common sense (i.e. you can't, for example, distinguish
    ///   `sint32` vs `uint32` vs some enum, or `string` from `bytes`), but
    ///   it's enough information to determine how many bytes to
    ///   parse. Interpretation of the value should be done according to the
    ///   type specified in `.proto` file.
    /// * There's no direct information on which fields are optional /
    ///   required, which fields may be repeated or constitute a map, what
    ///   restrictions are placed on fields usage in a single message, what
    ///   are the fields' default values, etc, etc.
    /// </summary>
    /// <remarks>
    /// Reference: <a href="https://developers.google.com/protocol-buffers/docs/encoding">Source</a>
    /// </remarks>
    public partial class GoogleProtobuf : KaitaiStruct
    {
        public static GoogleProtobuf FromFile(string fileName)
        {
            return new GoogleProtobuf(new KaitaiStream(fileName));
        }

        public GoogleProtobuf(KaitaiStream p__io, KaitaiStruct p__parent = null, GoogleProtobuf p__root = null) : base(p__io)
        {
            m_parent = p__parent;
            m_root = p__root ?? this;
            _read();
        }
        private void _read()
        {
            _pairs = new List<Pair>();
            {
                var i = 0;
                while (!m_io.IsEof) {
                    _pairs.Add(new Pair(m_io, this, m_root));
                    i++;
                }
            }
        }

        /// <summary>
        /// Key-value pair
        /// </summary>
        public partial class Pair : KaitaiStruct
        {
            public static Pair FromFile(string fileName)
            {
                return new Pair(new KaitaiStream(fileName));
            }


            public enum WireTypes
            {
                Varint = 0,
                Bit64 = 1,
                LenDelimited = 2,
                GroupStart = 3,
                GroupEnd = 4,
                Bit32 = 5,
            }
            public Pair(KaitaiStream p__io, GoogleProtobuf p__parent = null, GoogleProtobuf p__root = null) : base(p__io)
            {
                m_parent = p__parent;
                m_root = p__root;
                f_wireType = false;
                f_fieldTag = false;
                _read();
            }
            private void _read()
            {
                _key = new VlqBase128Le(m_io);
                switch (WireType) {
                case WireTypes.Varint: {
                    _value = new VlqBase128Le(m_io);
                    break;
                }
                case WireTypes.LenDelimited: {
                    _value = new DelimitedBytes(m_io, this, m_root);
                    break;
                }
                case WireTypes.Bit64: {
                    _value = m_io.ReadU8le();
                    break;
                }
                case WireTypes.Bit32: {
                    _value = m_io.ReadU4le();
                    break;
                }
                }
            }
            private bool f_wireType;
            private WireTypes _wireType;

            /// <summary>
            /// &quot;Wire type&quot; is a part of the &quot;key&quot; that carries enough
            /// information to parse value from the wire, i.e. read correct
            /// amount of bytes, but there's not enough informaton to
            /// interprete in unambiguously. For example, one can't clearly
            /// distinguish 64-bit fixed-sized integers from 64-bit floats,
            /// signed zigzag-encoded varints from regular unsigned varints,
            /// arbitrary bytes from UTF-8 encoded strings, etc.
            /// </summary>
            public WireTypes WireType
            {
                get
                {
                    if (f_wireType)
                        return _wireType;
                    _wireType = (WireTypes) (((WireTypes) (Key.Value & 7)));
                    f_wireType = true;
                    return _wireType;
                }
            }
            private bool f_fieldTag;
            private int _fieldTag;

            /// <summary>
            /// Identifies a field of protocol. One can look up symbolic
            /// field name in a `.proto` file by this field tag.
            /// </summary>
            public int FieldTag
            {
                get
                {
                    if (f_fieldTag)
                        return _fieldTag;
                    _fieldTag = (int) ((Key.Value >> 3));
                    f_fieldTag = true;
                    return _fieldTag;
                }
            }
            private VlqBase128Le _key;
            private object _value;
            private GoogleProtobuf m_root;
            private GoogleProtobuf m_parent;

            /// <summary>
            /// Key is a bit-mapped variable-length integer: lower 3 bits
            /// are used for &quot;wire type&quot;, and everything higher designates
            /// an integer &quot;field tag&quot;.
            /// </summary>
            public VlqBase128Le Key { get { return _key; } }

            /// <summary>
            /// Value that corresponds to field identified by
            /// `field_tag`. Type is determined approximately: there is
            /// enough information to parse it unambiguously from a stream,
            /// but further infromation from `.proto` file is required to
            /// interprete it properly.
            /// </summary>
            public object Value { get { return _value; } }
            public GoogleProtobuf M_Root { get { return m_root; } }
            public GoogleProtobuf M_Parent { get { return m_parent; } }
        }
        public partial class DelimitedBytes : KaitaiStruct
        {
            public static DelimitedBytes FromFile(string fileName)
            {
                return new DelimitedBytes(new KaitaiStream(fileName));
            }

            public DelimitedBytes(KaitaiStream p__io, GoogleProtobuf.Pair p__parent = null, GoogleProtobuf p__root = null) : base(p__io)
            {
                m_parent = p__parent;
                m_root = p__root;
                _read();
            }
            private void _read()
            {
                _len = new VlqBase128Le(m_io);
                _body = m_io.ReadBytes(Len.Value);
            }
            private VlqBase128Le _len;
            private byte[] _body;
            private GoogleProtobuf m_root;
            private GoogleProtobuf.Pair m_parent;
            public VlqBase128Le Len { get { return _len; } }
            public byte[] Body { get { return _body; } }
            public GoogleProtobuf M_Root { get { return m_root; } }
            public GoogleProtobuf.Pair M_Parent { get { return m_parent; } }
        }
        private List<Pair> _pairs;
        private GoogleProtobuf m_root;
        private KaitaiStruct m_parent;

        /// <summary>
        /// Key-value pairs which constitute a message
        /// </summary>
        public List<Pair> Pairs { get { return _pairs; } }
        public GoogleProtobuf M_Root { get { return m_root; } }
        public KaitaiStruct M_Parent { get { return m_parent; } }
    }
}