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CSharp · Console

/Program.cs

using System;
using System.Linq;

public static class Program
{
    public static void Main()
    {
        // this is my string
        string s = "7C04048404048C04049404059C0405";

        // this is my byte array to which I want to append the string, keeping the same format of the array
        var bufferByte = new byte[] { 0x40, 0x03, 0xE8, 0x03, 0xE8, 0x49 };

        bufferByte = bufferByte.Concat(StringToByteArray(s)).ToArray();

        Console.WriteLine(ByteArrayToString(bufferByte)); // 4003E803E8497C04048404048C04049404059C0405
    }

    public static string ByteArrayToString(byte[] ba)
    {
        return BitConverter.ToString(ba).Replace("-", "");
    }

    public static byte[] StringToByteArray(string hex)
    {
        int    numberChars = hex.Length;
        byte[] bytes       = new byte[numberChars / 2];

        for (int i = 0; i < numberChars; i += 2)
            bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);

        return bytes;
    }
}