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int utf8_probability(byte[] rawtext)

  {

   int score = 0;

   int i, rawtextlen = 0;

   int goodbytes = 0, asciibytes = 0;

   // Maybe also use UTF8 Byte Order Mark:  EF BB BF

   // Check to see if characters fit into acceptable ranges

   rawtextlen = rawtext.Length;

   for (i = 0; i < rawtextlen; i++)

   {

    if ((rawtext[i] & (byte)0x7F) == rawtext[i])

    {  // One byte

     asciibytes++;

     // Ignore ASCII, can throw off count

    }

    else

    {

     int m_rawInt0 = Convert.ToInt16(rawtext[i]);

     int m_rawInt1 = Convert.ToInt16(rawtext[i+1]);

     int m_rawInt2 = Convert.ToInt16(rawtext[i+2]);

     if (256-64 <= m_rawInt0 && m_rawInt0 <= 256-33 && // Two bytes

      i+1 < rawtextlen &&

      256-128 <= m_rawInt1 && m_rawInt1 <= 256-65)

     {

      goodbytes += 2;

      i++;

     }

     else if (256-32 <= m_rawInt0 && m_rawInt0 <= 256-17 && // Three bytes

      i+2 < rawtextlen &&

      256-128 <= m_rawInt1 && m_rawInt1 <= 256-65 &&

      256-128 <= m_rawInt2 && m_rawInt2 <= 256-65)

     {

      goodbytes += 3;

      i+=2;

     }

    }

   }

   if (asciibytes == rawtextlen) { return 0; }

   score = (int)(100 * ((float)goodbytes/(float)(rawtextlen-asciibytes)));

   // If not above 98, reduce to zero to prevent coincidental matches

   // Allows for some (few) bad formed sequences

   if (score > 98)

   {

    return score;

   }

   else if (score > 95 && goodbytes > 30)

   {

    return score;

   }

   else

   {

    return 0;

   }

  }

Encoding encode;  

StreamReader srtest = new StreamReader(file.FullName,Encoding.Default);  

int p = utf8_probability(Encoding.Default.GetBytes(srtest.ReadToEnd()));  

if( p>80 )  

    encode = Encoding.GetEncoding(65001);//utf8  

else  

    encode = Encoding.Default;  

srtest.Close();