CSharp SDK await dbx.Files.UploadAsync(...) object gets disposed before upload

Below is example code about how to do chunk upload.

 private async Task Upload(string file)
 {
 const string TargetPath = "/test.zip";
 const int ChunkSize = 4096 * 1024;
 using (var fileStream = File.Open(file, FileMode.Open))
 {
 if (fileStream.Length <= ChunkSize)
 {
 await this.client.Files.UploadAsync(TargetPath, body: fileStream);
 }
 else
 {
 Console.WriteLine("Start chunk uploading");
 await this.ChunkUpload(TargetPath, fileStream, ChunkSize);
 }
 } 
 }

 private async Task ChunkUpload(String path, FileStream stream, int chunkSize)
 {
 int numChunks = (int)Math.Ceiling((double)stream.Length / chunkSize);
 byte[] buffer = new byte[chunkSize];
 string sessionId = null;
 for (var idx = 0; idx < numChunks; idx++)
 {
 var byteRead = stream.Read(buffer, 0, chunkSize);

 using (var memSream = new MemoryStream(buffer, 0, byteRead))
 {
 if (idx == 0)
 {
 var result = await this.client.Files.UploadSessionStartAsync(memSream);
 sessionId = result.SessionId;
 }
 else
 {
 var cursor = new UploadSessionCursor(sessionId, (ulong)(chunkSize * idx));

 if (idx == numChunks - 1)
 {
 await this.client.Files.UploadSessionFinishAsync(cursor, new CommitInfo(path), memSream);
 }
 else
 {
 await this.client.Files.UploadSessionAppendAsync(cursor, memSream);
 }
 }
 }
 }
 }