<PackageReference Include="System.Reactive" Version="4.3.1" />

ImmediateScheduler

public sealed class ImmediateScheduler : LocalScheduler
Represents an object that schedules units of work to run immediately on the current thread.
/* Error decompiling System.IDisposable System.Reactive.Concurrency.ImmediateScheduler/AsyncLockScheduler::Schedule(TState,System.TimeSpan,System.Func`3) */