reading specific part of FileStream as byte array

Question

I'm trying to do something like pagination on a FileStream, function get FileStream and numberOfChunks and currentChunk, it slices the FileStream to numberOfChunks and return only byte of currentChunk but it only works when the currentChunk is 0

public async Task<byte[]> GetStreamChunkAsync(Stream inputStream, int numberOfChunks, int currentChunk)
{
    if (numberOfChunks <= 0 || currentChunk < 0 || currentChunk >= numberOfChunks)
    {
        throw new ArgumentOutOfRangeException("Invalid numberOfChunks or currentChunk values");
    }

    int bufferSize = (int)Math.Ceiling((double)inputStream.Length / numberOfChunks);
    
    int startPosition = currentChunk * bufferSize;
    
    int remainingBytes = (int)Math.Min(bufferSize, inputStream.Length - startPosition);
    
    byte[] buffer =new byte[remainingBytes];

    int bytesRead = await inputStream.ReadAsync(buffer, startPosition, buffer.Length);

    if (bytesRead > 0)
    {
        return buffer;
    }

    // Return null if no bytes were read (unexpected end of stream)
    return null;
}

Answer 1

You shouldn't set the position manually. The stream will advance anyway.

You do have another issue: you need a read loop, as the buffer may not be filled fully. A read loop would look like this

    int bytesRead
    while ((bytesRead = await inputStream.ReadAsync(buffer, startPosition, buffer.Length) > 0)
    {
        startPosition += bytesRead;
        if (startPosition == buffer.Length)
            break;
    }

Or you can use ReadAtLeastAsync

var bytesRead = await inputStream.ReadAtLeastAsync(buffer, buffer.Length, throwOnEndOfStream: false);

Having said that, I would recommend you instead use the new Async Iterator feature, which means you can do a await foreach over this.

You can also return Memory<byte> which will be more efficient in the case when you return the last partial chunk, which you would otherwise have to copy to a new array.

public async IAsyncEnumerable<Memory<byte>> GetStreamChunksAsync(Stream inputStream, int numberOfChunks)
{
    if (numberOfChunks <= 0)
    {
        throw new ArgumentOutOfRangeException(nameof(numberOfChunks), numberOfChunks, "Invalid numberOfChunks or currentChunk values");
    }

    int bufferSize = (int)Math.Ceiling((double)inputStream.Length / numberOfChunks);

    while (true)
    {
        byte[] buffer = new byte[bufferSize];
        var bytesRead = await inputStream.ReadAtLeastAsync(buffer, buffer.Length, throwOnEndOfStream: false);
        if (bytesRead > 0)
            yield return buffer.AsMemory(0, bytesRead);
        if (bytesRead < buffer.Length)
            break;
    }
}

Then use it like this

await using var fs = File.Open("SomeFile");
await foreach (var chunk in GetStreamChunksAsync(fs, 10)
{
    // do stuff with chunk
}