MemoryMappedViewStream.ReadAsync() reading beyond the end of the memory mapped file

Question

I am working on a sample to show how to read a large file and process it in chunks. It seems to be misbehaving however, returning an invalid number of bytes read.

For example, imagine we have a test file with a length of 8320 bytes and we use the following code to read it in chunks of 4096 bytes:

public static async IAsyncEnumerable<byte[]> ConvertInChunks(string filePath, int chunkSize,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
{
    byte[]? buffer = null;
    try
    {
        buffer = ArrayPool<byte>.Shared.Rent(chunkSize);
        var memory = buffer.AsMemory(0, chunkSize);

        var fileLength = new FileInfo(filePath).Length;
        using var mm = MemoryMappedFile.CreateFromFile(filePath);
        await using var accessor = mm.CreateViewStream();

        var bytesRead = 0L;
        while (bytesRead < fileLength)
        {
            var read = await accessor.ReadAsync(memory, cancellationToken);
            bytesRead += read;

            yield return memory[..read].ToArray();
        }
    }
    finally
    {
        if (buffer is not null)
            ArrayPool<byte>.Shared.Return(buffer);
    }
}

What I am finding is that the final iteration is still returning a read value of 4096. This seems like a bug to me, but maybe I am doing something incorrectly in my setup. I can work around this by explicitly splitting my memory buffer so that it is the minimum between bytes remaining and 4096, but I really expected the call to ReadAsync to read either 4096 or less when there is less data remaining in the stream. Note, I also tried the other ReadAsync overload with a buffer and explicit offset and count and it had the same behavior.

Answer 1

All credit to @MarkGravell for this answer:

The solution is simple, set the view stream size explicitly in the create call:

var fileLength = new FileInfo(filePath).Length;
using var mm = MemoryMappedFile.CreateFromFile(filePath);
await using var accessor = mm.CreateViewStream(0, fileLength);