using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
/// <summary>   
    /// 用於取得一個文本文件的編碼方式(Encoding)。   
    /// </summary>   
    public class TxtFileEncoder
    {
        public TxtFileEncoder()
        {
            
 
//   
            // TODO: 在此處添加構造函數邏輯   
            //   
        }
        /// <summary>   
        /// 取得一個文本文件的編碼方式。如果無法在文件頭部找到有效的前導符，Encoding.Default將被返回。   
        /// </summary>   
        /// <param name="fileName">文件名。</param>   
        /// <returns></returns>   
        public
 
 static Encoding GetEncoding(string fileName)
        {
            return GetEncoding(fileName, Encoding.Default);
        }
        /// <summary>   
        /// 取得一個文本文件流的編碼方式。   
        /// </summary>   
        /// <param name="stream">文本文件流。</param>   
        /// <returns></returns>
 
   
        public static Encoding GetEncoding(FileStream stream)
        {
            return GetEncoding(stream, Encoding.Default);
        }
        /// <summary>   
        /// 取得一個文本文件的編碼方式。   
        /// </summary>   
        /// <param name="fileName">文件名。</param>   
        /// <param name="defaultEncoding">默認編碼方式。當該方法無法從文件的頭部取得有效的前導符時，將返回該編碼方式。</param>   
        /// <returns></returns>   
        public static Encoding GetEncoding(string fileName, Encoding defaultEncoding)
        {
            FileStream fs = new FileStream(fileName, FileMode.Open);
            Encoding targetEncoding = GetEncoding(fs, defaultEncoding);
            fs.Close();
            return targetEncoding;
        }
        /// <summary>   
        /// 取得一個文本文件流的編碼方式。   
        /// </summary>   
        /// <param name="stream">文本文件流。</param>   
        /// <param name="defaultEncoding">默認編碼方式。當該方法無法從文件的頭部取得有效的前導符時，將返回該編碼方式。</param>   
        /// <returns></returns>   
        public static Encoding GetEncoding(FileStream stream, Encoding defaultEncoding)
        {
            Encoding targetEncoding = defaultEncoding;
            if (stream != null && stream.Length >= 2)
            {
                //保存文件流的前4個字節   
                byte byte1 = 0;
                byte byte2 = 0;
                byte byte3 = 0;
                byte byte4 = 0;
                //保存當前Seek位置   
                long origPos = stream.Seek(0, SeekOrigin.Begin);
                stream.Seek(0, SeekOrigin.Begin);

                int nByte = stream.ReadByte();
                byte1 = Convert.ToByte(nByte);
                byte2 = Convert.ToByte(stream.ReadByte());
                if (stream.Length >= 3)
                {
                    byte3 = Convert.ToByte(stream.ReadByte());
                }
                if (stream.Length >= 4)
                {
                    byte4 = Convert.ToByte(stream.ReadByte());
                }
                //根據文件流的前4個字節判斷Encoding   
                //Unicode {0xFF, 0xFE};   
                //BE-Unicode {0xFE, 0xFF};   
                //UTF8 = {0xEF, 0xBB, 0xBF};   
                if (byte1 == 0xFE && byte2 == 0xFF)//UnicodeBe   
                {
                    targetEncoding = Encoding.BigEndianUnicode;
                }
                if (byte1 == 0xFF && byte2 == 0xFE && byte3 != 0xFF)//Unicode   
                {
                    targetEncoding = Encoding.Unicode;
                }
                if (byte1 == 0xEF && byte2 == 0xBB && byte3 == 0xBF)//UTF8   
                {
                    targetEncoding = Encoding.UTF8;
                }
                //恢復Seek位置         
                stream.Seek(origPos, SeekOrigin.Begin);
            }
            return targetEncoding;
        }



        // 新增加一個方法，解決了不帶BOM的 UTF8 編碼問題   

        /// <summary>   
        /// 通過給定的文件流，判斷文件的編碼類型   
        /// </summary>   
        /// <param name="fs">文件流</param>   
        /// <returns>文件的編碼類型</returns>   
        public static System.Text.Encoding GetEncoding(Stream fs)
        {
            byte[] Unicode = new byte[] { 0xFF, 0xFE, 0x41 };
            byte[] UnicodeBIG = new byte[] { 0xFE, 0xFF, 0x00 };
            byte[] UTF8 = new byte[] { 0xEF, 0xBB, 0xBF }; //帶BOM   
            Encoding reVal = Encoding.Default;

            BinaryReader r = new BinaryReader(fs, System.Text.Encoding.Default);
            byte[] ss = r.ReadBytes(4);
            if (ss[0] == 0xFE && ss[1] == 0xFF && ss[2] == 0x00)
            {
                reVal = Encoding.BigEndianUnicode;
            }
            else if (ss[0] == 0xFF && ss[1] == 0xFE && ss[2] == 0x41)
            {
                reVal = Encoding.Unicode;
            }
            else
            {
                if (ss[0] == 0xEF && ss[1] == 0xBB && ss[2] == 0xBF)
                {
                    reVal = Encoding.UTF8;
                }
                else
                {
                    int i;
                    int.TryParse(fs.Length.ToString(), out i);
                    ss = r.ReadBytes(i);

                    if (IsUTF8Bytes(ss))
                        reVal = Encoding.UTF8;
                }
            }
            r.Close();
            return reVal;

        }

        /// <summary>   
        /// 判斷是否是不帶 BOM 的 UTF8 格式   
        /// </summary>   
        /// <param name="data"></param>   
        /// <returns></returns>   
        private static bool IsUTF8Bytes(byte[] data)
        {
            int charByteCounter = 1;　 //計算當前正分析的字符應還有的字節數   
            byte curByte; //當前分析的字節.   
            for (int i = 0; i < data.Length; i++)
            {
                curByte = data[i];
                if (charByteCounter == 1)
                {
                    if (curByte >= 0x80)
                    {
                        //判斷當前   
                        while (((curByte <<= 1) & 0x80) != 0)
                        {
                            charByteCounter++;
                        }
                        //標記位首位若為非0 則至少以2個1開始 如:110XXXXX...........1111110X　   
                        if (charByteCounter == 1 || charByteCounter > 6)
                        {
                            return false;
                        }
                    }
                }
                else
                {
                    //若是UTF-8 此時第一位必須為1   
                    if ((curByte & 0xC0) != 0x80)
                    {
                        return false;
                    }
                    charByteCounter--;
                }
            }
            if (charByteCounter > 1)
            {
                throw new Exception("非預期的byte格式!");
            }
            return true;
        }
    }
}

C# 獲取文件編碼