C# Replace bytes in Byte[]

Question

What is the best way to replace some bytes in a byte array??

For instance i have bytesFromServer = listener.Receive(ref groupEP); and i can do BitConverter.ToString(bytesFromServer) to convert it into a readable format to return something along the lines of

48 65 6c 6c 6f 20 
74 68 65 72 65 20 
68 65 6c 70 66 75 
6c 20 70 65 6f 70 
6c 65   

I would like to replace something inside of that making "68 65 6c" to something like "68 00 00" (just as an example). There is not .Replace() on a byte[].

Would there be an easy way to convert that back into a byte[]?

Any help appreciated. Thank you!

Answer 1

You could program it.... try this for a start... this is however not robust not production like code yet...beaware of off-by-one errors I didn't fully test this...

    public int FindBytes(byte[] src, byte[] find)
    {
        int index = -1;
        int matchIndex = 0;
        // handle the complete source array
        for(int i=0; i<src.Length; i++)
        {
            if(src[i] == find[matchIndex])
            {
                if (matchIndex==(find.Length-1))
                {
                    index = i - matchIndex;
                    break;
                }
                matchIndex++;
            }
            else if (src[i] == find[0])
            {
                matchIndex = 1;
            }
            else
            {
                matchIndex = 0;
            }

        }
        return index;
    }

    public byte[] ReplaceBytes(byte[] src, byte[] search, byte[] repl)
    {
        byte[] dst = null;
        int index = FindBytes(src, search);
        if (index>=0)
        {
            dst = new byte[src.Length - search.Length + repl.Length];
            // before found array
            Buffer.BlockCopy(src,0,dst,0, index);
            // repl copy
            Buffer.BlockCopy(repl,0,dst,index,repl.Length);
            // rest of src array
            Buffer.BlockCopy(
                src, 
                index+search.Length , 
                dst, 
                index+repl.Length, 
                src.Length-(index+search.Length));
        }
        return dst;
    }

Implement as an extension method

public void Replace(this byte[] src, byte[] search, byte[] repl)
{
      ReplaceBytes(src, search, repl);
}

usage normal method:

ReplaceBytes(bytesfromServer, 
             new byte[] {0x75, 0x83 } , 
             new byte[]{ 0x68, 0x65, 0x6c});

Extension method usage:

bytesfromServer.Replace(
             new byte[] {0x75, 0x83 }, 
             new byte[]{ 0x68, 0x65, 0x6c});

Answer 2

Improving on rene's code, I created a while loop for it to replace all occurences:

public static byte[] ReplaceBytes(byte[] src, byte[] search, byte[] repl)
{
    byte[] dst = null;
    byte[] temp = null;
    int index = FindBytes(src, search);
    while (index >= 0)
    {
        if (temp == null)
            temp = src;
        else
            temp = dst;

        dst = new byte[temp.Length - search.Length + repl.Length];

        // before found array
        Buffer.BlockCopy(temp, 0, dst, 0, index);
        // repl copy
        Buffer.BlockCopy(repl, 0, dst, index, repl.Length);
        // rest of src array
        Buffer.BlockCopy(
            temp,
            index + search.Length,
            dst,
            index + repl.Length,
            temp.Length - (index + search.Length));


        index = FindBytes(dst, search);
    }
    return dst;
}

This method will work, but if the source bytes is too huge, I prefer to have a "windowing" function to process the bytes chunk by chunk. Else it will take a huge amount of memory.

Answer 3

How about Array.Copy?

Answer 4

Unfortunately there are issues with all of the posts (as already pointed out in comments). There is a correct answer in this other question

I needed a solution so for myself and wrote the following code. This is also more flexible in using enumerable and multiple search replace terms.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;


public class ByteTools
{
    static void ByteReplaceTests()
    {
        var examples = new(string source, string search, string replace)[]
            {
                ("bababanana", "babanana", "apple"),
                ("hello guys", "hello", "hello world"),
                ("apple", "peach", "pear"),
                ("aaaa", "a", "abc"),
                ("pear", "pear", ""),
                ("good morning world", "morning", "morning"),
                ("ababab", "ab", "ababab"),
                ("ababab", "abab", "ab"),
                ("", "aa", "bb"),
            };

        int i = 0;
        foreach (var (source, search, replace) in examples)
        {
            var stringReplaceResults = source.Replace(search, replace);

            var sourceByte = Encoding.ASCII.GetBytes(source);
            var searchByte = Encoding.ASCII.GetBytes(search);
            var replaceByte = Encoding.ASCII.GetBytes(replace);
            //converts string values to bytes, does the replace, then converts back to string
            var byteReplaceResults = Encoding.ASCII.GetString(
                ByteReplace(sourceByte, (searchByte, replaceByte)).ToArray());

            Console.WriteLine($"{i}: {source}, {search}, {replace}");
            Console.WriteLine($"       String.Replace() => {stringReplaceResults}");
            Console.WriteLine($"       BytesReplace()   => {byteReplaceResults}");
            i++;
        }
    }

    static IEnumerable<byte> ByteReplace(IEnumerable<byte> source, params (byte[] search, byte[] replace)[] replacements)
    {
        if (source == null)
            throw new ArgumentNullException(nameof(source));
        if (replacements == null)
            throw new ArgumentNullException(nameof(replacements));
        if (replacements.Any(r => r.search == null || r.search.Length == 0))
            throw new ArgumentOutOfRangeException(nameof(replacements), "Search parameter cannot be null or empty");
        if (replacements.Any(r => r.replace == null))
            throw new ArgumentOutOfRangeException(nameof(replacements), "Replace parameter cannot be null");

        var maxMatchSize = replacements.Select(r => r.search.Length).Max();
        var bufferSize = maxMatchSize * 2;
        var buffer = new byte[bufferSize];
        int bufferStart = 0;
        int bufferPosition = 0;

        byte[] nextBytes()
        {
            foreach ((byte[] search, byte[] replace) in replacements)
            {
                if (ByteStartsWith(buffer, bufferStart, bufferPosition - bufferStart, search))
                {
                    bufferStart += search.Length;
                    return replace;
                }
            }

            var returnBytes = new byte[] { buffer[bufferStart] };
            bufferStart++;
            return returnBytes;
        }

        foreach (var dataByte in source)
        {
            buffer[bufferPosition] = dataByte;
            bufferPosition++;

            if (bufferPosition - bufferStart >= maxMatchSize)
            {
                foreach (var resultByte in nextBytes())
                    yield return resultByte;
            }

            if (bufferPosition == bufferSize - 1)
            {
                Buffer.BlockCopy(buffer, bufferStart, buffer, 0, bufferPosition - bufferStart);
                bufferPosition -= bufferStart;
                bufferStart = 0;
            }
        }

        while (bufferStart < bufferPosition)
        {
            foreach (var resultByte in nextBytes())
                yield return resultByte;
        }
    }
    static bool ByteStartsWith(byte[] data, int dataOffset, int dataLength, byte[] startsWith)
    {
        if (data == null)
            throw new ArgumentNullException(nameof(data));

        if (startsWith == null)
            throw new ArgumentNullException(nameof(startsWith));

        if (dataLength < startsWith.Length)
            return false;

        for (int i = 0; i < startsWith.Length; i++)
        {
            if (data[i + dataOffset] != startsWith[i])
                return false;
        }

        return true;
    }
}