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Build beautiful native apps in record time 🚀
Build Mobile, Desktop and WebAssembly apps with C# and XAML.
I'd go with C# + Uno (https://platform.uno/). Disclaimer: I haven't used Uno before, but from their presentations and what I've seen online, they are top-notch. I plan to test them in the mid-term future.
It is Both Under the GPL and it is also Proprietary if using the GPL version the software you make...
Take a very serious look at: https://livecode.com They have an open source version but be careful of the licensing. Basically if you start it on the open source version you need to keep the app open source (if I read the license right). But it's an absolutely wonderful prototyping environment.
A multi-platform .NET UI framework