Just in. Microsoft’s next version of .Net, designated.Net 5, will support Swift interop. Official Windows support for Swift is key to making Swift a first class ML language. .Net 5 is to be released in 2020. Note that Microsoft is only commiting to interop. It’s not clear if there will be official support for a Windows Swift compiler or even official support for a Windows run-time.
Objective-C and Swift interoperability will be supported on multiple operating systems.
Source: Introducing .Net 5