iOS Xamarin.iOS corrupted network downloads - xamarin.ios

Question

I have an application written with Xamarin.iOS that causes files to be corrupted after they are downloaded into the application.
I am comparing checksums by download the file within the application, and then uploading the downloaded file to dropbox. I am then comparing the checksum of the dropbox file to the original file on the server.
Normally, the checksums match. Occasionally, the checksums do not match. These files are really videos, and it causes the videos to not be playable.
The Question
How can downloads (using HttpWebRequest) occasionally cause corrupted files, but continue to download successfully?
The Code
public class DownloadService : IDownloadService
{
private const int BufferSize = 1024*1024;
/// <summary>
/// Download a remote file to a local location.
/// </summary>
/// <param name="url">The url of the file to download.</param>
/// <param name="localPath">The local path to save the file.</param>
/// <param name="progress">The progress off the download (percentage|bytesread|totalbytes).</param>
/// <param name="cancel">A reference to a boolean that can be set to true if you want to cancel the download.</param>
/// <returns>
/// Returns the local file that was downloaded.
/// Use this instead of the localPath parameter because you may have passed no extension asking for us to autodetermine the extension.
/// </returns>
public string DownloadFile(string url, string localPath, Action<int, long, long> progress, ref bool cancel)
{
cancel = false;
var didCancel = false;
HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
webRequest.ReadWriteTimeout = 3000;
var buffer = new byte[BufferSize];
var realPath = localPath;
try
{
using (var response = webRequest.GetResponse())
{
// get the total size of this request
var totalSize = 0;
try { totalSize = int.Parse(response.Headers["MediaItemLength"]);
}catch(Exception ex) { throw new NotImplementedException("Get the total size some other way!"); }
// ensure there is a proper extension
if (string.IsNullOrEmpty(Path.GetExtension(localPath)))
{
// the localpath given didn't have an extension.
// we need to try to determine the exention through custom HTTP headers.
string httpHeaderExtension = null;
try
{
httpHeaderExtension = response.Headers["MediaExtension"];
if (string.IsNullOrEmpty(httpHeaderExtension))
throw new Exception("The header was present, but it was empty.");
}
catch (Exception ex)
{
throw new Exception("localPath didn't provide an extension and we couldn't infer would through custom HTTP headers.");
}
// add the correct extension to our local file path
realPath += httpHeaderExtension;
}
if(File.Exists(realPath))
File.Delete(realPath);
using (var responseStream = response.GetResponseStream())
{
using (var dest = new FileStream(realPath, FileMode.CreateNew, FileAccess.Write))
{
long totalBytes = 0;
int currentBlockSize;
while ((currentBlockSize = responseStream.Read(buffer, 0, buffer.Length)) > 0)
{
totalBytes += currentBlockSize;
var percentage = (int)(totalBytes * 100.0 / totalSize);
dest.Write(buffer, 0, currentBlockSize);
if (progress != null)
progress(percentage, totalBytes, totalSize);
if (cancel)
{
didCancel = true;
break;
}
}
}
}
}
}catch(Exception ex)
{
// delete the file
if (File.Exists(realPath))
File.Delete(realPath);
throw;
}
// delete the file
if (didCancel && File.Exists(realPath))
File.Delete(realPath);
return realPath;
}
}
The code is pretty straight forward. I do include some additional/custom headers for getting the total file size so that I can avoid a HEAD request to get the value. I need the total file size for download progress.