Base64 encode/decode large file

转载：http://www.cnblogs.com/jzywh/archive/2008/04/20/base64_encode_large_file.html

The class System.Convert provide two basic methods "ToBase64String()" and "Convert.FromBase64String()" to encode a byte array to a base64 string and decode a base64 string to a byte array.

public string Encode(byte[] data)
{
    return Convert.ToBase64String(data);
}
        
public byte[] Decode(string strBase64)
{
    return Convert.FromBase64String(strBase64);
}

It is very good to use them to encode and decode base64. But in some case, it is a disaster.

For example, if you want to encode a 4 gb file to base64, the code above must throw an OutOfMemory exception., because you must read the file into a byte array. So we need to look for another way to encode and decode by base64.

Long days ago, a man have posted an article about how to deal with it.

http://blogs.microsoft.co.il/blogs/kim/archive/2007/10/09/base64-encode-large-files-very-large-files.aspx

This man use XmlWriter to work around it.

By researching the basis of the Base64 encoding in rfc, I found another more directly way to deal with it.

According rfc3548, base64 encode data in the unit of 3 bytes to 4 bytes, if the last part's length is less than 3,
the char '=' will be padded. So we can encode file in small chunks whose size is 3, then we can get the encoding data of the file by combiling encoding data of every chunks.

So I have below code:

public void EncodeFile(string inputFile, string outputFile)
{
       using(FileStream inputStream = File.Open(inputFile, FileMode.Open, FileAccess.Read, FileShare.Read))
          {
                using(StreamWriter outputWriter = new StreamWriter(outputFile, false, Encoding.ASCII))
              {
                  byte[] data = new byte[3 * 1024]; //Chunk size is 3k
                  int read    = inputStream.Read(data, 0, data.Length);
                  
               while(read > 0)
                    {
                      outputWriter.Write(Convert.ToBase64String(data, 0, read));
                      read = inputStream.Read(data, 0, data.Length);
                  }
                  
                  outputWriter.Close();                    
              }
              
              inputStream.Close();
          }
      }

    public void DecodeFile(string inputFile, string outputFile)
        {
          using (FileStream inputStream = File.Open(inputFile, FileMode.Open, FileAccess.Read, FileShare.Read))
            {
              using (FileStream outputStream = File.Create(outputFile))
                {
                  byte[] data = new byte[4 * 1024]; //Chunk size is 4k
                  int read = inputStream.Read(data, 0, data.Length);

                  byte[] chunk    = new byte[3 * 1024];
          
                  while (read > 0)
                   {
                      chunk = Convert.FromBase64String(Encoding.ASCII.GetString(data, 0, read));
                      outputStream.Write(chunk, 0, chunk.Length);
                      read = inputStream.Read(data, 0, data.Length);
                  }

                  outputStream.Close();
              }

              inputStream.Close();
          }
      }

The methods also can be improved to support mime format (76 chars per line).

public static void EncodeFile(string inputFile, string outputFile)
       {
         using(FileStream inputStream = File.Open(inputFile, FileMode.Open, FileAccess.Read, FileShare.Read))
           {
              using(StreamWriter outputWriter = new StreamWriter(outputFile, false, Encoding.ASCII))                {
                  byte[] data = new byte[57 * 1024]; //Chunk size is 57k
                  int read    = inputStream.Read(data, 0, data.Length);
                  
                 while(read > 0)
                   {
                      outputWriter.WriteLine(Convert.ToBase64String(data, 0, read, Base64FormattingOptions.InsertLineBreaks));
                      read = inputStream.Read(data, 0, data.Length);
                  }
                  
                  outputWriter.Close();                    
              }
              
              inputStream.Close();
          }
      }

      public static void DecodeFile(string inputFile, string outputFile)
       {
      using (StreamReader reader = new StreamReader(inputFile, Encoding.ASCII, true))
           {
          using (FileStream outputStream = File.Create(outputFile))
               {                
              string line = reader.ReadLine();

              while (!string.IsNullOrEmpty(line))
                   {
                  if (line.Length > 76)
                      throw new InvalidDataException("Invalid mime-format base64 file");

                  byte[] chunk = Convert.FromBase64String(line);
                  outputStream.Write(chunk, 0, chunk.Length);
                  line = reader.ReadLine();
              }

              outputStream.Close();
          }

          reader.Close();
      }
  }