如何判断文件夹下的文件编码？

    /// <summary>
    /// 验证文件里是否有不是utf8的文件
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void button2_Click(object sender, EventArgs e)
    {
        // 文件格式
        string fileStyle = "*.*";
        DirectoryInfo folder = new DirectoryInfo(@"C:\Users\sunjj\Desktop\textfile");
        Encoding encodingUTF8 = Encoding.UTF8;
        foreach (FileInfo file in folder.GetFiles(fileStyle))
        {
            if (!encodingUTF8.Equals(GetEncoding(file.FullName, Encoding.ASCII)))
            {
                MessageBox.Show(file.Name);
            }
        }
    }
    public static Encoding GetEncoding(string fileName, Encoding defaultEncoding)
    {
          FileStream fs = new FileStream(fileName, FileMode.Open);
          Encoding targetEncoding = GetEncoding(fs, defaultEncoding);
          fs.Close();
          return targetEncoding;
     }

     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个字节   
              byte byte1 = 0;
              byte byte2 = 0;
              byte byte3 = 0;
              byte byte4 = 0;
              //保存当前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());
              }
              //根据文件流的前4个字节判断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;
              }
              //恢复Seek位置         
              stream.Seek(origPos, SeekOrigin.Begin);
          }
          return targetEncoding;
      }