How to write/read bits from/to a Stream? (C#)

Question

How can I write bits to a stream (System.IO.Stream) or read in C#? thanks.

Answer 1

You could create an extension method on Stream that enumerates the bits, like this:

public static class StreamExtensions
{
    public static IEnumerable<bool> ReadBits(this Stream input)
    {
        if (input == null) throw new ArgumentNullException("input");
        if (!input.CanRead) throw new ArgumentException("Cannot read from input", "input");
        return ReadBitsCore(input);
    }

    private static IEnumerable<bool> ReadBitsCore(Stream input)
    {
        int readByte;
        while((readByte = input.ReadByte()) >= 0)
        {
            for(int i = 7; i >= 0; i--)
                yield return ((readByte >> i) & 1) == 1;
        }
    }
}

Using this extension method is easy:

foreach(bool bit in stream.ReadBits())
{
    // do something with the bit
}

Attention: you should not call ReadBits multiple times on the same Stream, otherwise the subsequent calls will forget the current bit position and will just start reading the next byte.

Answer 2

This is not possible with the default stream class. The C# (BCL) Stream class operates on the granularity of bytes at it's lowest level. What you can do is write a wrapper class which reads bytes and partititions them out to bits.

For example:

class BitStream : IDisposable {
  private Stream m__stream;
  private byte? m_current;
  private int m_index;
  public byte ReadNextBit() { 
    if ( !m_current.HasValue ) {
      m_current = ReadNextByte();
      m_index = 0;
    }
    var value = (m_byte.Value >> m_index) & 0x1;
    m_index++;
    if (m_index == 8) {
      m_current = null;
    }
    return value;
  }
  private byte ReadNextByte() {
    ...
  }
  // Dispose implementation omitted
}

Note: This will read the bits in right to left fashion which may or may not be what you're intending.