How to create a multi-platform application?

Basically you need .Net Core which is the new implementation of .Net. It works on all three major desktop operating systems.

For mobile operating systems you can use Xamarin . Works on all 3 major platforms. See more at Is Xamarin a viable option for mobile development? and link .

Both .Net Core and Xamarin are fully supported by Microsoft. And compatible in essence. If you do the basics, whatever you do with one will run on the other.

You also have the Mono that works on the desktop but today you have little reason to use it, you need to have a specific need to compensate for its adoption. On mobile devices it even works but so limited and with additional effort that I doubt there is a case that compensates its use. It tends to stay in the background with the .Net Core adoption.

There is no way to create 100% cross platform applications. You will always have to use specific libraries, you will have to write code thinking about differences in operating systems.

Java sells this idea that you make a code and run it anywhere. But it does not tell you that it takes effort and that in some cases it gets bad on all operating systems.

The specifics of what you need to do to ensure that you run well on all platforms is best to ask specifically when you're having problems.

It would be interesting to know the .Net Standard that is the portable way to develop applications .

The version of .NET that enables this cross-platform feature is Mono. Mono has a tool that checks if your project is fit to migrate .

Once this is done, it may be necessary to change something, graphically speaking. The Mono Toolkits are here .