[API Proposal]: Add possibility to create ReadOnlySequence<T> with multiple segments.

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
30/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
csharp
Domain
backend-api-design

Research direction

Start by reviewing the existing ReadOnlySequence and ReadOnlySequenceSegment APIs, along with the BufferSegment example from ASP.NET Core. The proposal presents multiple possible APIs, so first determine the intended public design; done means an agreed API for constructing multiple-segment sequences with its behavior specified.

Written by the indexing model from the issue text.

Description

api-suggestion area-System.Memory needs-further-triage
Background and motivation

ReadOnlySequence<T> has APIs to create empty or single-segment instances, but there's no on-the-box help for creating multiple-segment instances.

The first thing a user needs to do is create a ReadOnlySequenceSegment<T> derived class.

API Proposal

ASP.NET Core has a few implementations of this:

internal sealed class BufferSegment : ReadOnlySequenceSegment<byte>
{
    public BufferSegment(Memory<byte> memory)
    {
        Memory = memory;
    }
 
    public BufferSegment Append(ReadOnlyMemory<byte> memory)
    {
        var segment = new BufferSegment(memory)
        {
            RunningIndex = RunningIndex + Memory.Length
        };
        Next = segment;
        return segment;
    }
}

Just having a public implementation would go a long way helping users.

I know there are a few more elaborated implementations, but just having this would help a lot.

Adding to that, simple APIs to create from IEnumerable<IEnumerable<T>>

new ReadOnlySequence<T>(IEnumerable<IEnumerable<T>> source);
new ReadOnlySequence<T,U>(U source, Func<U, IEnumerable<IEnumerable<T>> sourceSelector, Func<IEnumerable<T>,ReadOnlyMemory<T>> segmentSelector);
API Usage
ReadOnlySequence<byte> Transform(ReadOnlySequence<byte> source, Func<ReadOnlyMemory<byte>, ReadOnlyMemory<byte>> transform)
{
    var e = source.GetEnumerable();

    if (!e.MoveNext())
    {
        return ReadOnlySequence<byte>.Empty;
    }

    ReadOnlySequenceSegment<T>? first = null;
    ReadOnlySequenceSegment<T> last;

    do
    {
        if (first is null)
        {
            last = first = new BufferSegment(transform(e.Current));
        }
        else
        {
            last = last.Append(transform(e.Current));
        }
    }
    while (e.MoveNext());

    return new ReadOnlySequence<byte>(first, 0, last, last.Memory.Length);
}
Alternative Designs

No response

Risks

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Dominant language
C#
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First steps

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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