concurrent upload in chunks

Can you clarify what you mean when you say "code is fine, and doesn't have any errors"? How have you verified that the code is uploading the correct data?

For instance, I don't see in this code where you're using "memStream", so it doesn't look like you're sending the actual file data.

Refer to the UploadSessionAppendV2Async documentation for information on the parameters it takes and how to use it.

Note that Dropbox will only save the exact bytes you send, so as verification I suggest comparing comparing the original file with the file uploaded on Dropbox to see if they differ. For example, you can take a hash of both files and see if the hashes match.

Oh, Sorry I didn't see your reply.
Can I use UploadSessionAppendV2Async to run in parallel like this?

I tried it and my file was corrupted. Can I just do UploadSessionAppendV2Async sequentially?

Yes, the UploadSessionAppendV2Async method supports both "concurrent" and "sequential" upload session types. Which one you use is up to you, but in either case you'll need to make sure you're sending the correct data at the correct offsets.

I used "concurrent" and my file was uploaded at the correct size. But it cannot be opened and used

And this is when I do the upload with "sequential", it can be opened

Can you tell me why this is? Thanks

From that description, it sounds like the code you're using for the "concurrent" type is sending the wrong pieces of data and/or sending the data with the wrong offsets. You'll need to debug the code you've written for that to determine why it's doing so and how to correct it.