C# - Asynchroner Download von Dateien

public static async Task<Stream> DownloadFileAsync(this HttpClient client, string url, CancellationToken token, IProgress<double> progress = null)
        {
            var response = await client.GetAsync(url, HttpCompletionOption.ResponseHeadersRead, token);

            if (!response.IsSuccessStatusCode)
            {
                throw new Exception(string.Format("The request returned with HTTP status code {0}", response.StatusCode));
            }

            var length = response.Content.Headers.ContentLength.HasValue ? response.Content.Headers.ContentLength.Value : -1L;

            using (var stream = await response.Content.ReadAsStreamAsync())
            {
                MemoryStream memoryStream = null;

                bool nolength = (length == -1);

                int size = ((nolength) ? 8192 : (int)length);

                if (nolength)
                    memoryStream = new MemoryStream();

                long total = 0;
                int nread = 0;
                int offset = 0;

                byte[] buffer = new byte[size];

                token.ThrowIfCancellationRequested();

                while ((nread = await stream.ReadAsync(buffer, offset, size, token)) != 0)
                {
                    if (nolength)
                    {
                        memoryStream.Write(buffer, 0, nread);
                    }
                    else
                    {
                        offset += nread;
                        size -= nread;
                    }
                    total += nread;

                    if (progress != null)
                        progress.Report((double)offset);

                    token.ThrowIfCancellationRequested();
                }

                return nolength ? memoryStream : new MemoryStream(buffer);
            }