Multiple Async File Uploads with chunking to ASP.Net Web API

I'm not familiar with that kind of chunked upload, but I believe this should work:

• Use flowTotalSize to pre-allocate the file when the first chunk comes in.
• Have one SemaphoreSlim per file to serialize the asynchronous writes for that file.
• Each chunk will write to its own offset (flowChunkSize * (flowChunkNumber - 1)) within the file.

This doesn't handle situations where the uploads are unexpectedly terminated. That kind of solution usually involves allocating/writing a temporary file (with a special extension) and then moving/renaming that file once the last chunk arrives.

Don't forget to ensure that your file writing is actually asynchronous.

Using @Stephen Cleary's answer, and this thread: https://github.com/flowjs/ng-flow/issues/41 I was able to make an ASP.NET Web Api Implementation and uploaded it for those still wondering about this question such as @Herb Caudill

https://github.com/samhowes/NgFlowSample/tree/master.

The original answer is the real answer to this question, but I don't have enough reputation yet to comment. I did not use a SemaphoreSlim, but instead enabled file Write sharing. But did in fact pre-allocate and make sure that each chunk is getting written to the right location by calculating an offset.

I will be contributing this to the Flow samples at: https://github.com/flowjs/flow.js/tree/master/samples