Is there a way of creating a Photo Projects extension in Xamarin as only see Today, Finder and Share available?
Not at this time: https://developer.xamarin.com/guides/mac/platform-features/extensions/#Xamarin.Mac_Extension_Support
So this does not solve your problem directly, but if you have a large C# codebase you could use a technique like this:
to create a thin "objective-c/C" layer and project and handle most of the work in C#.
That sample is quicklook, but the idea should hold for photos.
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