Development does not happen on one single platform in today’s environment. Microsoft .NET has evolved in the last two decades to allow for a full cross platform development. Microsoft has introduced .NET Core to its .NET ecosystem, a fork from existing .Net Framework to develop a new framework that is very much portable across various platforms.
Applications targeting .NET Core can easily run on Windows, Linus and macOS, at the same time Microsoft is committed to keep as an open source platform model. Microsoft .NET Core framework should not be viewed as a replacement to .NET framework. Applications using Windows Presentation Foundation (WPF), Workflow Foundation (WF), Windows Forms and other libraries not currently supported by .Net core will continue to use .NET Framework.