ProgressableStreamContent.cs

// Source: https://forums.xamarin.com/discussion/180009/how-to-work-with-custom-progressable-stream-content

using System;
using System.IO;
using System.Net;
using System.Net.Http;
using System.Threading.Tasks;

namespace Misc
{
    internal class ProgressableStreamContent : HttpContent
    {
        /// <summary>
        /// Lets keep buffer of 20kb
        /// </summary>
        private const int defaultBufferSize = 5 * 4096;

        private HttpContent content;
        private int bufferSize;
        private Action<long, long> progress;

        public ProgressableStreamContent(HttpContent content, Action<long, long> progress) : this(content, defaultBufferSize, progress) { }

        public ProgressableStreamContent(HttpContent content, int bufferSize, Action<long, long> progress)
        {
            if (bufferSize <= 0)
                throw new ArgumentOutOfRangeException(nameof(bufferSize));

            this.content = content ?? throw new ArgumentNullException(nameof(content));
            this.bufferSize = bufferSize;
            this.progress = progress;

            foreach (var h in content.Headers)
                Headers.Add(h.Key, h.Value);
        }

        protected override Task SerializeToStreamAsync(Stream stream, TransportContext context)
        {

            return Task.Run(async () =>
            {
                var buffer = new byte[bufferSize];
                TryComputeLength(out var size);
                var uploaded = 0;

                using (var sinput = await content.ReadAsStreamAsync())
                {
                    while (true)
                    {
                        var length = sinput.Read(buffer, 0, buffer.Length);
                        if (length <= 0) break;

                        uploaded += length;
                        progress?.Invoke(uploaded, size);

                        stream.Write(buffer, 0, length);
                        stream.Flush();
                    }
                }
                stream.Flush();
            });
        }

        protected override bool TryComputeLength(out long length)
        {
            length = content.Headers.ContentLength.GetValueOrDefault();
            return true;
        }

        protected override void Dispose(bool disposing)
        {
            if (disposing)
                content.Dispose();

            base.Dispose(disposing);
        }
    }
}