An example of an extension method for an interface I control.

ExtensionExample.cs

// We are keeping the interface super simple to implement.
// It turns out that there are a number of situations where I want to
// send a message after a delay of a given period.  Rather than force
// every implementation to provide that I add via the PublishAfterDelay
// extension method.

public interface ISimulatorBus
{
    void Publish(object message);
}

public static class SimulatorBusExtensions
{
    
    public static Task PublishAfterDelay(this ISimulatorBus bus, object message, TimeSpan timeUntilSend)
    {
        return PublishAfterDelay(bus, message, timeUntilSend, CancellationToken.None);
    }

    public static async Task PublishAfterDelay(
        this ISimulatorBus bus,
        object message,
        TimeSpan timeUntilSend,
        CancellationToken cancellationToken)
    {
        await Task.Delay(timeUntilSend, cancellationToken);
        bus.Publish(message);
    }
}