Stream.ReadBlockAsync(Memory<byte>, CancellationToken)

This offers to Stream for reading bytes what StreamReader.ReadBlockAsync already offers for reading characters: the convenient ability to read exactly the requested number of characters.

Two variants are offered:

1. ReadBlockAsync will return the number of bytes read, which may be less than the length of the buffer if the end of the stream is reached.
2. ReadBlockOrThrowAsync will always fill the entire buffer or it will throw EndOfStreamException if there isn't enough bytes on the stream.

The existing Stream.ReadAsync methods do not guarantee to fill a given buffer. Rather, they will initialize the given buffer with whatever they have in their existing buffer or the first set of bytes they receive, up to the specified number of bytes. So if you know you need 5 bytes, you have to create a read loop to ensure you actually get all 5 bytes before continuing. This is extremely inconvenient, and the ReadBlockAsync method changes the semantic.

Example

When you absolutely want the buffer full or it's an error condition:

var buffer = new byte[5];
await stream.ReadBlockOrThrowAsync(buffer);
// If no EndOfStreamException is thrown we know that buffer is fully initialized.

When you want the buffer full, but consider a premature end of stream to be acceptable:

var buffer = new byte[5];
int bytesRead = await stream.ReadBlockAsync(buffer);
if (bytesRead < 5) {
  // We encountered an end of stream before we got 5 bytes. Work with what we have.
} else {
  // We got all 5 bytes
}