Encode a FileStream to base64 with c#

I know how to encode / decode a simple string to / from base64.

But how would I do that if the data is already been written to a FileStream object. Let's say I have only access to the FileStream object not to the previously stored original data in it. How would I encode a FileStream to base64 before I flush the FileStream to a file.

Ofc I could just open my file and encode / decode it after I have written the FileStream to the file, but I would like to do this all in one single step without doing two file operations one after another. The file could be larger and it would also take double time to load, encode and save it again after it was just saved a short time before.

Maybe someone of you knows a better solution? Can I convert the FileStream to a string, encode the string and then convert the string back to a FileStream for example or what would I do and how would such a code look like?


You can also encode bytes to Base64. How to get this from a stream see here: How to convert an Stream into a byte[] in C#?

Or I think it should be also possible to use the .ToString() method and encode this.

An easy one as an extension method

public static class Extensions
    public static Stream ConvertToBase64(this Stream stream)
        byte[] bytes;
        using (var memoryStream = new MemoryStream())
            bytes = memoryStream.ToArray();

        string base64 = Convert.ToBase64String(bytes);
        return new MemoryStream(Encoding.UTF8.GetBytes(base64));

You may try something like that:

    public Stream ConvertToBase64(Stream stream)
        Byte[] inArray = new Byte[(int)stream.Length];
        Char[] outArray = new Char[(int)(stream.Length * 1.34)];
        stream.Read(inArray, 0, (int)stream.Length);
        Convert.ToBase64CharArray(inArray, 0, inArray.Length, outArray, 0);
        return new MemoryStream(Encoding.UTF8.GetBytes(outArray));

Since the file will be larger, you don't have very much choice in how to do this. You cannot process the file in place since that will destroy the information you need to use. You have two options that I can see:

  1. Read in the entire file, base64 encode, re-write the encoded data.
  2. Read the file in smaller pieces, encoding as you go along. Encode to a temporary file in the same directory. When you are finished, delete the original file, and rename the temporary file.

Of course, the whole point of streams is to avoid this sort of scenario. Instead of creating the content and stuffing it into a file stream, stuff it into a memory stream. Then encode that and only then save to disk.

When dealing with large streams, like a file sized over 4GB - you don't want to load the file into memory (as a Byte[]) because not only is it very slow, but also may cause a crash as even in 64-bit processes a Byte[] cannot exceed 2GB (or 4GB with gcAllowVeryLargeObjects).

Fortunately there's a neat helper in .NET called ToBase64Transform which processes a stream in chunks.

You use it with CryptoStream like so:

using( FileStream   inputFile    = new FileStream( @"C:\VeryLargeFile.bin", FileMode.Open, FileAccess.Read ) )
using( CryptoStream base64Stream = new CryptoStream( inputFile, new ToBase64Transform(), CryptoStreamMode.Read ) )
using( FileStream   outputFile   = new FileStream( @"C:\VeryLargeBase64File.txt", FileMode.CreateNew, FileAccess.Write ) )
    await base64Stream.CopyToAsync( outputFile ).ConfigureAwait(false);

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