Equivalent in C# of Python's “struct.pack/unpack”?

Question

I am a seasoned Python developer and have come to love a lot of its conveniences. I have actually known C# for some time but recently have gotten into some more advanced cod

Answer 1

I ended up writing my own class to handle this. It's pretty complex, but it does seem to work. It's also incomplete, but it works for what I need at this point. Feel free to use it, and if there's any good improvements, please let me know.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Diagnostics;

// This is a crude implementation of a format string based struct converter for C#.
// This is probably not the best implementation, the fastest implementation, the most bug-proof implementation, or even the most functional implementation.
// It's provided as-is for free. Enjoy.

public class StructConverter
{
    // We use this function to provide an easier way to type-agnostically call the GetBytes method of the BitConverter class.
    // This means we can have much cleaner code below.
    private static byte[] TypeAgnosticGetBytes(object o)
    {
        if (o is int) return BitConverter.GetBytes((int)o);
        if (o is uint) return BitConverter.GetBytes((uint)o);
        if (o is long) return BitConverter.GetBytes((long)o);
        if (o is ulong) return BitConverter.GetBytes((ulong)o);
        if (o is short) return BitConverter.GetBytes((short)o);
        if (o is ushort) return BitConverter.GetBytes((ushort)o);
        if (o is byte || o is sbyte) return new byte[] { (byte)o };
        throw new ArgumentException("Unsupported object type found");
    }

    private static string GetFormatSpecifierFor(object o)
    {
        if (o is int) return "i";
        if (o is uint) return "I";
        if (o is long) return "q";
        if (o is ulong) return "Q";
        if (o is short) return "h";
        if (o is ushort) return "H";
        if (o is byte) return "B";
        if (o is sbyte) return "b";
        throw new ArgumentException("Unsupported object type found");
    }

    /// <summary>
    /// Convert a byte array into an array of objects based on Python's "struct.unpack" protocol.
    /// </summary>
    /// <param name="fmt">A "struct.pack"-compatible format string</param>
    /// <param name="bytes">An array of bytes to convert to objects</param>
    /// <returns>Array of objects.</returns>
    /// <remarks>You are responsible for casting the objects in the array back to their proper types.</remarks>
    public static object[] Unpack(string fmt, byte[] bytes)
    {
        Debug.WriteLine("Format string is length {0}, {1} bytes provided.", fmt.Length, bytes.Length);

        // First we parse the format string to make sure it's proper.
        if (fmt.Length < 1) throw new ArgumentException("Format string cannot be empty.");

        bool endianFlip = false;
        if (fmt.Substring(0, 1) == "<")
        {
            Debug.WriteLine("  Endian marker found: little endian");
            // Little endian.
            // Do we need to flip endianness?
            if (BitConverter.IsLittleEndian == false) endianFlip = true;
            fmt = fmt.Substring(1);
        }
        else if (fmt.Substring(0, 1) == ">")
        {
            Debug.WriteLine("  Endian marker found: big endian");
            // Big endian.
            // Do we need to flip endianness?
            if (BitConverter.IsLittleEndian == true) endianFlip = true;
            fmt = fmt.Substring(1);
        }

        // Now, we find out how long the byte array needs to be
        int totalByteLength = 0;
        foreach (char c in fmt.ToCharArray())
        {
            Debug.WriteLine("  Format character found: {0}", c);
            switch (c)
            {
                case 'q':
                case 'Q':
                    totalByteLength += 8;
                    break;
                case 'i':
                case 'I':
                    totalByteLength += 4;
                    break;
                case 'h':
                case 'H':
                    totalByteLength += 2;
                    break;
                case 'b':
                case 'B':
                case 'x':
                    totalByteLength += 1;
                    break;
                default:
                    throw new ArgumentException("Invalid character found in format string.");
            }
        }

        Debug.WriteLine("Endianness will {0}be flipped.", (object) (endianFlip == true ? "" : "NOT "));
        Debug.WriteLine("The byte array is expected to be {0} bytes long.", totalByteLength);

        // Test the byte array length to see if it contains as many bytes as is needed for the string.
        if (bytes.Length != totalByteLength) throw new ArgumentException("The number of bytes provided does not match the total length of the format string.");

        // Ok, we can go ahead and start parsing bytes!
        int byteArrayPosition = 0;
        List<object> outputList = new List<object>();
        byte[] buf;

        Debug.WriteLine("Processing byte array...");
        foreach (char c in fmt.ToCharArray())
        {
            switch (c)
            {
                case 'q':
                    outputList.Add((object)(long)BitConverter.ToInt64(bytes,byteArrayPosition));
                    byteArrayPosition+=8;
                    Debug.WriteLine("  Added signed 64-bit integer.");
                    break;
                case 'Q':
                    outputList.Add((object)(ulong)BitConverter.ToUInt64(bytes,byteArrayPosition));
                    byteArrayPosition+=8;
                    Debug.WriteLine("  Added unsigned 64-bit integer.");
                    break;
                case 'l':
                    outputList.Add((object)(int)BitConverter.ToInt32(bytes, byteArrayPosition));
                    byteArrayPosition+=4;
                    Debug.WriteLine("  Added signed 32-bit integer.");
                    break;
                case 'L':
                    outputList.Add((object)(uint)BitConverter.ToUInt32(bytes, byteArrayPosition));
                    byteArrayPosition+=4;
                    Debug.WriteLine("  Added unsignedsigned 32-bit integer.");
                    break;
                case 'h':
                    outputList.Add((object)(short)BitConverter.ToInt16(bytes, byteArrayPosition));
                    byteArrayPosition += 2;
                    Debug.WriteLine("  Added signed 16-bit integer.");
                    break;
                case 'H':
                    outputList.Add((object)(ushort)BitConverter.ToUInt16(bytes, byteArrayPosition));
                    byteArrayPosition += 2;
                    Debug.WriteLine("  Added unsigned 16-bit integer.");
                    break;
                case 'b':
                    buf = new byte[1];
                    Array.Copy(bytes,byteArrayPosition,buf,0,1);
                    outputList.Add((object)(sbyte)buf[0]);
                    byteArrayPosition++;
                    Debug.WriteLine("  Added signed byte");
                    break;
                case 'B':
                    buf = new byte[1];
                    Array.Copy(bytes, byteArrayPosition, buf, 0, 1);
                    outputList.Add((object)(byte)buf[0]);
                    byteArrayPosition++;
                    Debug.WriteLine("  Added unsigned byte");
                    break;
                case 'x':
                    byteArrayPosition++;
                    Debug.WriteLine("  Ignoring a byte");
                    break;
                default:
                    throw new ArgumentException("You should not be here.");
            }
        }
        return outputList.ToArray();
    }

    /// <summary>
    /// Convert an array of objects to a byte array, along with a string that can be used with Unpack.
    /// </summary>
    /// <param name="items">An object array of items to convert</param>
    /// <param name="LittleEndian">Set to False if you want to use big endian output.</param>
    /// <param name="NeededFormatStringToRecover">Variable to place an 'Unpack'-compatible format string into.</param>
    /// <returns>A Byte array containing the objects provided in binary format.</returns>
    public static byte[] Pack(object[] items, bool LittleEndian, out string NeededFormatStringToRecover)
    {

        // make a byte list to hold the bytes of output
        List<byte> outputBytes = new List<byte>();

        // should we be flipping bits for proper endinanness?
        bool endianFlip = (LittleEndian != BitConverter.IsLittleEndian);

        // start working on the output string
        string outString = (LittleEndian == false ? ">" : "<");

        // convert each item in the objects to the representative bytes
        foreach (object o in items)
        {
            byte[] theseBytes = TypeAgnosticGetBytes(o);
            if (endianFlip == true) theseBytes = (byte[])theseBytes.Reverse();
            outString += GetFormatSpecifierFor(o);
            outputBytes.AddRange(theseBytes);
        }

        NeededFormatStringToRecover = outString;

        return outputBytes.ToArray();

    }

    public static byte[] Pack(object[] items)
    {
        string dummy = "";
        return Pack(items, true, out dummy);
    }
}

Answer 2

BinaryWriter and BinaryReader will send arbitrary items to a byte array or read arbitrary items from a byte array

var str = new MemoryStream();
var bw = new BinaryWriter(str);
bw.Write(42);
bw.Write("hello");
...
var bytes = str.ToArray();

Answer 3

.NET (and thus, C#) has the Marshal.StructureToPtr and Marshal.PtrToStructure methods.

You can abuse these to cast raw memory to a struct like you would in C, not that I'd recommend doing it this way (as it isn't exactly portable). You also need to get your Byte[] array buffer into the native heap in order to perform the operation on it:

T FromBuffer<T>(Byte[] buffer) where T : struct {

    T temp = new T();
    int size = Marshal.SizeOf(temp);
    IntPtr ptr = Marshal.AllocHGlobal(size);

    Marshal.Copy(buffer, 0, ptr, size);

    T ret = (T)Marshal.PtrToStructure(ptr, temp.GetType());
    Marshal.FreeHGlobal(ptr);

    return ret;
}