33. Streams, StreamReader/StreamWriter and async file I/O

// Text: StreamWriter / StreamReader wrap a stream and handle encoding (UTF-8 by default)
using (var writer = new StreamWriter("log.txt", append: true))
{
    writer.WriteLine($"{DateTime.UtcNow:O} Started");
}   // Dispose flushes and closes the file

using var reader = new StreamReader("big.csv");
string? line;
long count = 0;
while ((line = await reader.ReadLineAsync()) != null)   // streams line by line
    if (line.Contains("ERROR")) count++;

// Bytes: FileStream with explicit options
await using var fs = new FileStream("photo.jpg", FileMode.Open, FileAccess.Read, FileShare.Read, bufferSize: 81920, useAsync: true);
var buffer = new byte[81920];
int read;
while ((read = await fs.ReadAsync(buffer)) > 0) { /* process buffer[..read] */ }

// Copy one stream into another (e.g. upload → Blob storage, file → HTTP response)
await using var source = File.OpenRead("report.pdf");
await using var target = File.Create("copy.pdf");
await source.CopyToAsync(target);

// MemoryStream: a stream over a byte[] in memory — handy for tests and in-memory files
using var ms = new MemoryStream();
await JsonSerializer.SerializeAsync(ms, new { Id = 1 });
byte[] json = ms.ToArray();

// Compression by chaining streams
await using var zip = new GZipStream(File.Create("data.gz"), CompressionLevel.Optimal);
await using var zipWriter = new StreamWriter(zip);
await zipWriter.WriteAsync("compressed text");