You can use C# APIs in C# Desktop applications! I have a sample here in GitHub.
In general, to get access to the C# APIS, add two references to your project:
C:\Program Files (x86)\Windows Kits\10\UnionMetadata\Windows.winmd
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETCore\v4.5\System.Runtime.WindowsRuntime.dll
Note that the #2 version depends on the version of .NET that you’re using.
Background tasks will not be supported, but all other Bluetooth C#
features should be there.