Overview

PipeReader manages memory on the caller’s behalf.

Always call PipeReader.AdvanceTo after calling PipeReader.ReadAsync:

ReadAsync returns a ReadOnlySequence<byte> that is only valid until the call to AdvanceTo.
Using the return value after the call to AdvanceTo throws an exception.

PipeReader.AdvanceTo takes two SequencePosition arguments: one that marks how much data was consumed, and one that marks how much of the buffer was observed.

Consumed — When data is marked as consumed, the pipe returns the memory to the underlying buffer pool.
Observed — When data is marked as observed, the next call to ReadAsync either:

won’t return until there’s more data written to the pipe (assuming all day was marked as observed).
return immediately with the observed and unobserved data (but not data already consumed).

Patterns for Reading Data with PipeReader

For examples below, consider this helper method:

bool TryParseLines(ref ReadOnlySequence<byte> buffer, out Message message);

Reading a Single Message

Read a single message from a PipeReader and return it to the caller:

async ValueTask<Message?> ReadSingleMessageAsync(PipeReader reader, CancellationToken cancellationToken = default)
{
    while (true)
    {
        ReadResult result = await reader.ReadAsync(cancellationToken);
        ReadOnlySequence<byte> buffer = result.Buffer;

        // In the event that no message is parsed successfully, mark consumed as nothing and examined as the entire buffer.
        SequencePosition consumed = buffer.Start;
        SequencePosition examined = buffer.End;

        try
        {
            if (TryParseLines(ref buffer, out Message message))
            {
                // A single message was successfully parsed so mark the start of the parsed buffer as consumed. 
                // TryParseLines trims the buffer to point to the data after the message was parsed.
                consumed = buffer.Start;

                // Examined is marked the same as consumed here, so the next call to ReadSingleMessageAsync will 
                // process the next message if there is one.
                examined = consumed;

                return message;
            }

            // There's no more data to be processed.
            if (result.IsCompleted)
            {
                if (buffer.Length > 0)
                {
                    // The message is incomplete and there's no more data to process.
                    throw new InvalidDataException("Incomplete message.");
                }

                break;
            }
        }
        finally
        {
            reader.AdvanceTo(consumed, examined);
        }
    }

    return null;
}

Reading Multiple Messages

Read all messages from a PipeReader and call ProcessMessageAsync on each:

async Task ProcessMessagesAsync(PipeReader reader, CancellationToken cancellationToken = default)
{
    try
    {
        while (true)
        {
            ReadResult result = await reader.ReadAsync(cancellationToken);
            ReadOnlySequence<byte> buffer = result.Buffer;

            try
            {
                // Process all messages from the buffer, modifying the input buffer on each iteration.
                while (TryParseLines(ref buffer, out Message message))
                {
                    await ProcessMessageAsync(message);
                }

                // There's no more data to be processed.
                if (result.IsCompleted)
                {
                    if (buffer.Length > 0)
                    {
                        // The message is incomplete and there's no more data to process.
                        throw new InvalidDataException("Incomplete message.");
                    }
                    break;
                }
            }
            finally
            {
                // Since all messages in the buffer are being processed, you can use the
                // remaining buffer's Start and End position to determine consumed and examined.
                reader.AdvanceTo(buffer.Start, buffer.End);
            }
        }
    }
    finally
    {
        await reader.CompleteAsync();
    }
}

Cancellation with PipeReader

PipeReader.ReadAsync accepts a CancellationToken. However, PipeReader.CancelPendingRead cancels the current read operation without throwing an exception. It causes the current or next call to ReadAsync to return a ReadResult with IsCanceled set to true. to This approach may be favorable in some cases.

Common Problems with PipeReader

See this page for more information.