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I have been using this code to capture the webcam and I have been trying to learn from it and make it better. Rider IDE suggested I should use an async variant of MemoryMappedViewStream.Read but it doesn't work at all. It produces all-black images suggesting the async and sync methods are totally different. I am wondering why that's the case?

// Working:
sourceStream.Read(MemoryMarshal.AsBytes(image.GetPixelMemoryGroup().Single().Span));

// NOT Working:
var bytes = MemoryMarshal.AsBytes(image.GetPixelMemoryGroup().Single().Span).ToArray();
await sourceStream.ReadAsync(bytes, 0, bytes.Length, token);

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Node.JS
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  • The code is different - in second (async) case you make a copy into the _new_ array, with `ToArray`. Then you read bytes from stream into this copy, but that doesn't affect image, so it remains black. – Evk Oct 09 '21 at 08:35
  • @Evk yes. ToArray creates a copy but it should correctly copy the byte array. Can you please elaborate further – Node.JS Oct 09 '21 at 08:41
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    In your first example, you read bytes from `sourceStream` directly into memory location of `image.GetPixelMemoryGroup`, so directly into image. In your second example, you create a _copy_ of (probably black) image memory and then read bytes into that _copy_. So image bytes are now in `bytes` variable, not in image. Image is unaffected and remains black. – Evk Oct 09 '21 at 08:47
  • Makes sense. Thank you. Is there a way to use the async method correctly? – Node.JS Oct 09 '21 at 08:52

2 Answers2

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Those two versions are not the same. In "sync" version you obtain a reference to memory location of an image via image.GetPixelMemoryGroup(). Then you read data from sourceStream directly into that location.

In "async" version you again obtain reference to memory location via image.GetPixelMemoryGroup but then you do something different - you call ToArray. This extension method copies bytes from image memory location into new array, the one you hold in bytes variable. You then read data from sourceStream into that bytes array, NOT directly into image memory locaiton. Then you discard bytes array, so you read them to nowhere basically.

Now,MemoryMappedViewStream inherits from UnmanagedMemoryStream and all read\write operations are implemented in UnmanagedMemoryStream. This kind of stream represents data in memory and there is nothing async it can do. The only reason it even has ReadAsync is because base stream class (Stream) has those methods. Even if you manage to make ReadAsync work - in this case it will not be asynchornous anyway. As far as I know - MemoryMappedViewStream does now allow real asynchronous access, even though it could make sense, since it has underlying file.

In short - I'd just continue with sync version, because there is no benefit in this case to use "async" one. Static analyzer of course doesn't know that, it only sees that there is Async-named analog of the method you use.

Evk
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await sourceStream.ReadAsync(bytes, 0, bytes.Length, token).ConfigureAwait(false);

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Kamran Mammadov
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