Line delimited json serializing and de-serializing

To implement with .NET 5 (C# 9) and the System.Text.Json.JsonSerializer class, and for "big" data, I wrote code for streaming processing.

Using the System.IO.Pipelines extension package, this is quite efficient.

using System;
using System.Buffers;
using System.Collections.Generic;
using System.IO;
using System.IO.Pipelines;
using System.Text;
using System.Text.Json;
using System.Threading.Tasks;

class Program
{
    static readonly byte[] NewLineChars = {(byte)'\r', (byte)'\n'};
    static readonly byte[] WhiteSpaceChars = {(byte)'\r', (byte)'\n', (byte)' ', (byte)'\t'};

    private static async Task Main()
    {
        JsonSerializerOptions jsonOptions = new(JsonSerializerDefaults.Web);
        var json = "{\"some\":\"thing1\"}\r\n{\"some\":\"thing2\"}\r\n{\"some\":\"thing3\"}";
        var contentStream = new MemoryStream(Encoding.UTF8.GetBytes(json));
        var pipeReader = PipeReader.Create(contentStream);
        await foreach (var foo in ReadItemsAsync<Foo>(pipeReader, jsonOptions))
        {
            Console.WriteLine($"foo: {foo.Some}");
        }
    }

    static async IAsyncEnumerable<TValue> ReadItemsAsync<TValue>(PipeReader pipeReader, JsonSerializerOptions jsonOptions = null)
    {
        while (true)
        {
            var result = await pipeReader.ReadAsync();
            var buffer = result.Buffer;
            bool isCompleted = result.IsCompleted;
            SequencePosition bufferPosition = buffer.Start;
            while (true)
            {
                var(value, advanceSequence) = TryReadNextItem<TValue>(buffer, ref bufferPosition, isCompleted, jsonOptions);
                if (value != null)
                {
                    yield return value;
                }

                if (advanceSequence)
                {
                    pipeReader.AdvanceTo(bufferPosition, buffer.End); //advance our position in the pipe
                    break;
                }
            }

            if (isCompleted)
                yield break;
        }
    }

    static (TValue, bool) TryReadNextItem<TValue>(ReadOnlySequence<byte> sequence, ref SequencePosition sequencePosition, bool isCompleted, JsonSerializerOptions jsonOptions)
    {
        var reader = new SequenceReader<byte>(sequence.Slice(sequencePosition));
        while (!reader.End) // loop until we've come to the end or read an item
        {
            if (reader.TryReadToAny(out ReadOnlySpan<byte> itemBytes, NewLineChars, advancePastDelimiter: true))
            {
                sequencePosition = reader.Position;
                if (itemBytes.TrimStart(WhiteSpaceChars).IsEmpty)
                {
                    continue;
                }

                return (JsonSerializer.Deserialize<TValue>(itemBytes, jsonOptions), false);
            }
            else if (isCompleted)
            {
                // read last item
                var remainingReader = sequence.Slice(reader.Position);
                ReadOnlySpan<byte> remainingSpan = remainingReader.IsSingleSegment ? remainingReader.First.Span : remainingReader.ToArray();
                reader.Advance(remainingReader.Length); // advance reader to the end
                sequencePosition = reader.Position;
                if (!remainingSpan.TrimStart(WhiteSpaceChars).IsEmpty)
                {
                    return (JsonSerializer.Deserialize<TValue>(remainingSpan, jsonOptions), true);
                }
                else
                {
                    return (default, true);
                }
            }
            else
            {
                // no more items in sequence
                break;
            }
        }

        // PipeReader needs to read more
        return (default, true);
    }
}

public class Foo
{
    public string Some
    {
        get;
        set;
    }
}

Run at https://dotnetfiddle.net/M5cNo1

You can do so by manually parsing your JSON using JsonTextReader and setting the SupportMultipleContent flag to true.

If we look at your first example, and create a POCO called Foo:

public class Foo
{
    [JsonProperty("some")]
    public string Some { get; set; }
}

This is how we parse it:

var json = "{\"some\":\"thing1\"}\r\n{\"some\":\"thing2\"}\r\n{\"some\":\"thing3\"}";
var jsonReader = new JsonTextReader(new StringReader(json))
{
    SupportMultipleContent = true // This is important!
};

var jsonSerializer = new JsonSerializer();
while (jsonReader.Read())
{
    Foo foo = jsonSerializer.Deserialize<Foo>(jsonReader);
}

If you want list of items as result simply add each item to a list inside the while loop to your list.

listOfFoo.Add(jsonSerializer.Deserialize<Foo>(jsonReader));

Note: with Json.Net 10.0.4 and later same code also supports comma separated JSON entries see How to deserialize dodgy JSON (with improperly quoted strings, and missing brackets)?)