string url = "http://file.cshelloworld.com/images/1771477326069108736.jpg";
long totalSize = 0;//文件總大小
long downLoadingSize = 0;//當(dāng)前已經(jīng)下載了多少

private void Button_Click(object sender, RoutedEventArgs e)
{
    Task.Run(async () =>
    {
        //獲取到文件總大小 通過head請求
        using HttpClient client = new HttpClient();
        HttpRequestMessage requestMessage = new HttpRequestMessage(HttpMethod.Head, url);
        var response = await client.SendAsync(requestMessage);
        totalSize = response.Content.Headers.ContentLength.Value;
        using FileStream fileStream = new FileStream("d:\a.jpj", FileMode.Create, FileAccess.Write, FileShare.Read);

        //開始分片下載
        while (downLoadingSize < totalSize)
        {
            //組裝range 0,1000 1000,2000 0,9999
            long start = downLoadingSize;
            long end = start + 1000;
            if (end > (totalSize - 1))
            {
                end = totalSize - 1;
            }
            client.DefaultRequestHeaders.Range = new System.Net.Http.Headers.RangeHeaderValue(start, end);
            var res = await client.GetAsync(url);
            byte[] bytes = await res.Content.ReadAsByteArrayAsync();
            await fileStream.WriteAsync(bytes, 0, bytes.Length);
            //更新UI的進(jìn)度
            downLoadingSize += bytes.Length;
            int process = (int)((downLoadingSize / (decimal)totalSize) * 100);
            this.Dispatcher.Invoke(() =>
            {
                cont.Text = process + "%";
                this.processBar.Value = process;
            });
        }
        fileStream.Close();
    });
}

HttpRequestMessage requestMessage = new HttpRequestMessage(HttpMethod.Head, url);

client.DefaultRequestHeaders.Range = new System.Net.Http.Headers.RangeHeaderValue(start, end);