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## PR Type What kind of change does this PR introduce? <!-- Please uncomment one or more that apply to this PR. --> - Feature ## What is the current behavior? <!-- Please describe the current behavior that you are modifying, or link to a relevant issue. --> Right now there is no (easy) way to cast a `Memory<TFrom>` instance to a `Memory<TTo>` instance. There are APIs to to do that for `Span<T>` instances, but not for `Memory<T>`. The reason for that is that with a `Span<T>` it's just a matter of retrieving the wrapped reference, reinterpreting it and then adjusting the size, then creating a new `Span<T>` instance. But a `Memory<T>` instance is completely different: it wraps an object which could be either a `T[]` array, a `MemoryManager<T>` instance, etc. The result is that currently there are no APIs in the BCL nor in the toolkit to just "cast" a `Memory<T>`. This feature has been requested by a number of developers, including in a well known library such as `ImageSharp`: > Yes, that's exactly what I would need. But I'm wondering how would you implement it. > It's certainly non trivial to cast a `Memory<byte>` to a `Memory<TPixel>` and if there's an API for that I would gladly want to know... > So I pressume `ImageSharp` would need to do some work under the hood. (_`ImageSharp` issue, [here](https://github.com/SixLabors/ImageSharp/issues/1097#issuecomment-580639914)_) To solve that, I created a very simplified version of the code included in this PR, into a PR [here](https://github.com/SixLabors/ImageSharp/pull/1314). Having this available right out of the box in the `HighPerformance` package would be helpful in a number of similar situations, especially with `Memory<T>` APIs becoming more and more common across libraries now (as they've been out for a while). ## What is the new behavior? <!-- Describe how was this issue resolved or changed? --> This PR includes 4 new extensions for the `Memory<T>` and `ReadOnlyMemory<T>` types that enable the following: ```csharp // Cast between two Memory<T> instances... Memory<byte> memoryOfBytes = new byte[128].AsMemory(); Memory<float> memoryOfFloats = memoryOfBytes.Cast<byte, float>(); // ...any number of times is needed Memory<int> memoryOfInts = memoryOfFloats.Cast<float, int>(); Memory<byte> backToBytesMemory = memoryOfInts.Cast<int, byte>(); // Or just convert into bytes directly Memory<int> sourceAsInts = new int[128].AsMemory(); Memory<byte> sourceAsBytes = sourceAsInts.AsBytes(); // Want to get a stream from a string? Why not! 😄 using (Stream stream = "Hello world".AsMemory().AsBytes().AsStream()) { // Use the stream here, which reads *directly* from the string data! } ``` Here is the full list of the new APIs introduced in this PR: ```csharp namespace Microsoft.Toolkit.HighPerformance.Extensions { public static class MemoryExtensions { public static Memory<byte> AsBytes<T>(this Memory<T> memory) where T : unmanaged; public static Memory<TTo> Cast<TFrom, TTo>(this Memory<TFrom> memory) where TFrom : unmanaged where TTo : unmanaged; } public static class ReadOnlyMemoryExtensions { public static ReadOnlyMemory<byte> AsBytes<T>(this ReadOnlyMemory<T> memory) where T : unmanaged; public static ReadOnlyMemory<TTo> Cast<TFrom, TTo>(this ReadOnlyMemory<TFrom> memory) where TFrom : unmanaged where TTo : unmanaged; } } ``` ## Notes Marking as draft as this is still being worked on, but feedbacks and reviews are welcome! 😄 ## PR Checklist Please check if your PR fulfills the following requirements: - [X] Tested code with current [supported SDKs](../readme.md#supported) - [ ] ~~Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->~~ - [ ] ~~Sample in sample app has been added / updated (for bug fixes / features)~~ - [ ] ~~Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)~~ - [X] Tests for the changes have been added (for bug fixes / features) (if applicable) - [X] Header has been added to all new source files (run *build/UpdateHeaders.bat*) - [X] Contains **NO** breaking changes