Microsoft.IdentityServer.ServiceHost.exe in and of itself is an unmanaged Windows executable that is generated when the high-level AD FS managed code is compiled. When executed, the unmanaged code inside Microsoft.IdentityServer.ServiceHost.exe leverages Common Language Runtime (CLR) to run the managed AD FS code within a virtual runtime environment. This virtual runtime environment is comprised of one or more application domains, which provide a unit of isolation for the runtime environment and allow different applications to run inside separate containers within a process. The managed AD FS code is executed within an application domain inside the virtual runtime environment