QuickOPC User's Guide and Reference
Examples - Reactive Programming - Observe a range of OPC UA array elements
// Shows how to create an observable for a range of OPC UA array elements, and subscribe to it.

using OpcLabs.EasyOpc.UA.Reactive;
using System;
using System.Threading;
using OpcLabs.EasyOpc.UA;
using OpcLabs.EasyOpc.UA.OperationModel;

namespace ReactiveDocExamples
{
    namespace _UADataChangeNotificationObservable
    {
        partial class Subscribe
        {
            public static void IndexRangeList()
            {
                // Define which server we will work with.
                UAEndpointDescriptor endpointDescriptor =
                    "opc.tcp://opcua.demo-this.com:51210/UA/SampleServer";
                // or "http://opcua.demo-this.com:51211/UA/SampleServer" (currently not supported)
                // or "https://opcua.demo-this.com:51212/UA/SampleServer/"

                Console.WriteLine("Creating observable...");
                var attributeArguments = new UAAttributeArguments(endpointDescriptor, "nsu=http://test.org/UA/Data/ ;i=10933")
                {
                    IndexRangeList = UAIndexRangeList.OneDimension(2, 4)
                };
                var monitoredItemArguments = new UAMonitoredItemArguments(attributeArguments, monitoringParameters: 1000);
                UADataChangeNotificationObservable<Int32[]> observable =
                    UADataChangeNotificationObservable.Create<Int32[]>(monitoredItemArguments);

                Console.WriteLine("Subscribing...");
                using (observable.Subscribe(e => Console.WriteLine(
                    (e.Exception is null) ? e.AttributeData.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

Examples - OPC Unified Architecture