SharePoint Online REST API chunked file upload

FileUploader.cs

using System;
using System.Net;
using Newtonsoft.Json.Linq;

namespace SharePoint.FileOperations
{
    class FileUploader
    {
        public static void ChunkedFileUpload(string webUrl, ICredentials credentials, string sourcePath, string targetFolderUrl, int chunkSizeBytes, Action<long, long> chunkUploaded)
        {
            using (var client = new WebClient())
            {
                client.BaseAddress = webUrl;
                client.Credentials = credentials;
                client.Headers.Add("X-FORMS_BASED_AUTH_ACCEPTED", "f");
                var formDigest = RequestFormDigest(webUrl, credentials);
                client.Headers.Add("X-RequestDigest", formDigest);

                //create an empty file first
                var fileName = System.IO.Path.GetFileName(sourcePath);
                var createFileRequestUrl = string.Format("/_api/web/getfolderbyserverrelativeurl('{0}')/files/add(url='{1}',overwrite=true)", targetFolderUrl, fileName);
                client.UploadString(createFileRequestUrl, "POST");

                var targetUrl = System.IO.Path.Combine(targetFolderUrl, fileName);
                var firstChunk = true;
                var uploadId = Guid.NewGuid();
                var offset = 0L;

                using (var inputStream = System.IO.File.OpenRead(sourcePath))
                {
                    var buffer = new byte[chunkSizeBytes];
                    int bytesRead;
                    while ((bytesRead = inputStream.Read(buffer, 0, buffer.Length)) > 0)
                    {
                        if (firstChunk)
                        {
                            var endpointUrl = string.Format("/_api/web/getfilebyserverrelativeurl('{0}')/startupload(uploadId=guid'{1}')", targetUrl, uploadId);
                            client.UploadData(endpointUrl, buffer);
                            firstChunk = false;
                        }
                        else if (inputStream.Position == inputStream.Length)
                        {
                            var endpointUrl = string.Format("/_api/web/getfilebyserverrelativeurl('{0}')/finishupload(uploadId=guid'{1}',fileOffset={2})", targetUrl, uploadId, offset);
                            var finalBuffer = new byte[bytesRead];
                            Array.Copy(buffer, finalBuffer, finalBuffer.Length);
                            client.UploadData(endpointUrl, finalBuffer);
                        }
                        else
                        {
                            var endpointUrl = string.Format("/_api/web/getfilebyserverrelativeurl('{0}')/continueupload(uploadId=guid'{1}',fileOffset={2})", targetUrl, uploadId, offset);
                            client.UploadData(endpointUrl, buffer);
                        }
                        offset += bytesRead;
                        chunkUploaded(offset, inputStream.Length);
                    }
                }
            }
        }

        public static string RequestFormDigest(string webUrl, ICredentials credentials)
        {
            using (var client = new WebClient())
            {
                client.BaseAddress = webUrl;
                client.Credentials = credentials;
                client.Headers.Add("X-FORMS_BASED_AUTH_ACCEPTED", "f");
                client.Headers.Add("Accept", "application/json; odata=verbose");
                var endpointUrl = "/_api/contextinfo";
                var content = client.UploadString(endpointUrl, "POST");
                var data = JObject.Parse(content);
                return data["d"]["GetContextWebInformation"]["FormDigestValue"].ToString();
            }
        }
    }
}