파일이나 배열의 문자 인코딩을 찾아주는 클래스 http://blog.ryuanerin.kr/31

EncodingFinder.cs

// RyuaNerin
// 2013-03-08
// 2016-02-18

using System.Collections.Generic;
using System.IO;
using System.Text;

namespace RyuaNerin
{
    public static class EncodingFinder
    {
        private struct PreambleInfo
        {
            public Encoding Encoding;
            public byte[] Preamble;
        }

        private static PreambleInfo[] m_preambles = null;

        private static int MaxPreambleLength { get; private set; }

        static EncodingFinder()
        {
            var lst = new List<PreambleInfo>();

            byte[] preamble;
            foreach (EncodingInfo oEncodingInfo in Encoding.GetEncodings())
            {
                preamble = oEncodingInfo.GetEncoding().GetPreamble();

                if (preamble.Length > 0)
                {
                    lst.Add(new PreambleInfo { Encoding = oEncodingInfo.GetEncoding(), Preamble = preamble });

                    if (preamble.Length > MaxPreambleLength) MaxPreambleLength = preamble.Length;
                }
            }

            lst.Sort((x, y) => x.Preamble.Length.CompareTo(y.Preamble.Length));

            m_preambles = lst.ToArray();
        }

        public static Encoding DetectEncoding(string FileName)
        {
            using (var file = File.OpenRead(FileName))
            {
                int n = (int)file.Length;
                if (n == 0) return null;
                if (n > MaxPreambleLength) n = MaxPreambleLength;

                var bytes = new byte[n];
                file.Read(bytes, 0, n);

                return DetectEncoding(bytes);
            }
        }

        public static Encoding DetectEncoding(byte[] bytes)
        {
            int i;
            bool match;

            foreach (var preamble in m_preambles)
            {
                if (bytes.Length >= preamble.Preamble.Length)
                {
                    match = true;

                    for (i = 0; i < preamble.Preamble.Length; i++)
                    {
                        if (bytes[i] != preamble.Preamble[i])
                        {
                            match = false;
                            break;
                        }
                    }

                    if (match) return preamble.Encoding;
                }
            }
            return null;
        }

        public static string ReadAllText(string fileName)
        {
            Encoding usedEncoding;
            return ReadAllText(fileName, Encoding.Default, out usedEncoding);
        }
        public static string ReadAllText(string fileName, Encoding defaultEncoding)
        {
            Encoding usedEncoding;
            return ReadAllText(fileName, defaultEncoding, out usedEncoding);
        }
        public static string ReadAllText(string fileName, Encoding defaultEncoding, out Encoding usedEncoding)
        {
            byte[] bytes = File.ReadAllBytes(fileName);

            usedEncoding = DetectEncoding(bytes);

            int offset = 0;
            if (usedEncoding == null)
            {
                offset = 0;
                usedEncoding = defaultEncoding;
            }
            else
            {
                offset = usedEncoding.GetPreamble().Length;
            }
            return usedEncoding.GetString(bytes, offset, bytes.Length - offset);
        }
    }
}