1// THIS DEADLOCKS in ASP.NET / WPF / WinForms
2public byte[] GetData()
3{
4    var ms = new MemoryStream();
5    // .Result blocks the current thread, which holds the SynchronizationContext
6    // CopyToAsync's continuation needs that same thread → deadlock
7    sourceStream.CopyToAsync(ms).Result;
8    return ms.ToArray();
9}

1// FIX: Use async all the way up
2public async Task<byte[]> GetDataAsync()
3{
4    var ms = new MemoryStream();
5    await sourceStream.CopyToAsync(ms);
6    return ms.ToArray();
7}

1// If you cannot make the caller async, use ConfigureAwait(false)
2public byte[] GetData()
3{
4    var ms = new MemoryStream();
5    sourceStream.CopyToAsync(ms).ConfigureAwait(false).GetAwaiter().GetResult();
6    return ms.ToArray();
7}
8
9// Better: use async throughout (preferred approach)
10public async Task<byte[]> GetDataAsync()
11{
12    var ms = new MemoryStream();
13    await sourceStream.CopyToAsync(ms).ConfigureAwait(false);
14    return ms.ToArray();
15}

1// THIS MAY HANG: HttpClient buffers the entire response by default
2var response = await client.GetAsync(url);
3var ms = new MemoryStream();
4await response.Content.CopyToAsync(ms);  // Hangs if response is huge
5
6// FIX: Use ResponseHeadersRead to stream the response
7var response = await client.GetAsync(url, HttpCompletionOption.ResponseHeadersRead);
8using var stream = await response.Content.ReadAsStreamAsync();
9var ms = new MemoryStream();
10await stream.CopyToAsync(ms);

1// CopyToAsync reads until the source returns 0 bytes
2// If the source never signals end-of-stream, CopyToAsync never returns
3
4// Problem: NetworkStream stays open
5var networkStream = tcpClient.GetStream();
6var ms = new MemoryStream();
7await networkStream.CopyToAsync(ms);  // Hangs until the remote side closes
8
9// Fix 1: Use a CancellationToken with timeout
10var cts = new CancellationTokenSource(TimeSpan.FromSeconds(30));
11try
12{
13    await networkStream.CopyToAsync(ms, cts.Token);
14}
15catch (OperationCanceledException)
16{
17    Console.WriteLine("Copy timed out");
18}
19
20// Fix 2: Read a known number of bytes instead
21var buffer = new byte[contentLength];
22int totalRead = 0;
23while (totalRead < contentLength)
24{
25    int read = await networkStream.ReadAsync(
26        buffer, totalRead, contentLength - totalRead);
27    if (read == 0) break;
28    totalRead += read;
29}

1// Problem: GZipStream must be closed/flushed before reading the output
2var compressed = new MemoryStream();
3using (var gzip = new GZipStream(compressed, CompressionMode.Compress, leaveOpen: true))
4{
5    await sourceStream.CopyToAsync(gzip);
6}  // GZipStream writes final bytes on Dispose
7
8compressed.Position = 0;
9// Now compressed contains the full gzip data
10
11// Common mistake: reading compressed before closing GZipStream
12var gzip = new GZipStream(compressed, CompressionMode.Compress);
13await sourceStream.CopyToAsync(gzip);
14compressed.Position = 0;  // Missing final bytes — output is truncated or corrupt

1public async Task CopyWithProgressAsync(
2    Stream source, Stream destination,
3    IProgress<long> progress = null,
4    CancellationToken ct = default,
5    int bufferSize = 81920)
6{
7    var buffer = new byte[bufferSize];
8    long totalBytes = 0;
9    int bytesRead;
10
11    while ((bytesRead = await source.ReadAsync(buffer, 0, buffer.Length, ct)) > 0)
12    {
13        await destination.WriteAsync(buffer, 0, bytesRead, ct);
14        totalBytes += bytesRead;
15        progress?.Report(totalBytes);
16    }
17}
18
19// Usage
20var cts = new CancellationTokenSource(TimeSpan.FromMinutes(5));
21var progress = new Progress<long>(bytes => Console.WriteLine($"Copied {bytes} bytes"));
22await CopyWithProgressAsync(source, dest, progress, cts.Token);