Question

What is the most efficient way to replace one sequence of the bytes (eg 67 67 67) with some other sequence of the bytes (eg 90).
The sequences can have different length.

Answer 1

Here is a short app which does what you need:

    static void Main(string[] args)
    {
        byte [] bArray = new byte[] {11, 67, 67, 67, 33, 34, 67, 67, 11, 33, 67, 67, 67, 67};

        byte[] result = Replace(bArray, new byte[] {67, 67, 67}, new byte[] {90});

        foreach (byte b in result)
        {
            Console.WriteLine(b);
        }
    }

    private static byte [] Replace(byte[] input, byte[] pattern, byte[] replacement)
    {
        if (pattern.Length == 0)
        {
            return input;
        }

        List<byte> result = new List<byte>();

        int i;

        for (i = 0; i <= input.Length - pattern.Length; i++)
        {
            bool foundMatch = true;
            for (int j = 0; j < pattern.Length; j++)
            {
                if (input[i + j] != pattern[j])
                {
                    foundMatch = false;
                    break;
                }
            }

            if (foundMatch)
            {
                result.AddRange(replacement);
                i += pattern.Length - 1;
            }
            else
            {
                result.Add(input[i]);
            }
        }

        for (; i < input.Length; i++ )
        {
            result.Add(input[i]);
        }

        return result.ToArray();
    }