QuickOPC User's Guide and Reference
Examples - Reactive Programming - Create OPC DA observable and subscribe
// Shows how to create an observable for OPC-DA item changes, and subscribe to it.

using System;
using System.Threading;
using OpcLabs.EasyOpc.DataAccess.Reactive;

namespace ReactiveDocExamples
{
    namespace _DAItemChangedObservable
    {
        partial class Subscribe
        {
            public static void Main1()
            {
                Console.WriteLine("Creating observable...");
                DAItemChangedObservable<double> ramp = 
                    DAItemChangedObservable.Create<double>("", "OPCLabs.KitServer.2", "Demo.Ramp", 1000);

                Console.WriteLine("Subscribing...");
                using (ramp.Subscribe(e => Console.WriteLine(
                    (e.Exception is null) ? e.Vtq.ToString() : e.Exception.GetBaseException().ToString())))
                {
                    Console.WriteLine("Waiting for 10 seconds...");
                    Thread.Sleep(10*1000);

                    Console.WriteLine("Unsubscribing...");
                }

                Console.WriteLine("Waiting for 2 seconds...");
                Thread.Sleep(2 * 1000);
            }
        }
    }
}
See Also

Conceptual