AddJson and AddJsonObject extension methods for IConfigurationBuilder

InMemoryConfiguration.cs

    public sealed class InMemoryFileProvider : IFileProvider
    {
        private readonly string _json;

        public InMemoryFileProvider(string json)
        {
            _json = json;
        }

        public IFileInfo GetFileInfo(string subpath) => new FileInfo(_json);

        IDirectoryContents IFileProvider.GetDirectoryContents(string subpath) => throw new NotImplementedException();

        IChangeToken IFileProvider.Watch(string filter) => throw new NotImplementedException();

        public sealed class FileInfo : IFileInfo
        {
            private readonly string _json;

            public FileInfo(string json)
            {
                _json = json;
            }

            public Stream CreateReadStream()
            {
                var result = new MemoryStream();
                using (var writer = new StreamWriter(result, new UTF8Encoding(encoderShouldEmitUTF8Identifier: false), 512, leaveOpen: true))
                    writer.Write(_json);
                result.Seek(0, SeekOrigin.Begin);
                return result;
            }

            public bool Exists => true;
            public bool IsDirectory => false;
            public long Length => throw new NotImplementedException();
            public string PhysicalPath => throw new NotImplementedException();
            public string Name => throw new NotImplementedException();
            public DateTimeOffset LastModified => throw new NotImplementedException();
        }
    }

    public static class InMemoryExtensions
    {
        public static IConfigurationBuilder AddJson(this IConfigurationBuilder builder, string json) =>
            builder.AddJsonFile(new InMemoryFileProvider(json), "path", optional: false, reloadOnChange: false);

        public static IConfigurationBuilder AddJsonObject<T>(this IConfigurationBuilder builder, T instance) =>
            builder.AddJson(JsonConvert.SerializeObject(instance));
    }