QuickOPC User's Guide and Reference
Examples - OPC XML-DA - Change update rate of a single subscription
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// This example shows how change the update rate of an existing OPC XML-DA subscription.

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

namespace DocExamples.DataAccess.Xml
{
    class ChangeItemSubscription
    {
        public static void Main1Xml()
        {
            // Instantiate the client object.
            using (var client = new EasyDAClient())
            {
                client.ItemChanged += client_ItemChanged;

                Console.WriteLine("Subscribing...");
                int handle = client.SubscribeItem(
                    "http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx",
                    "Dynamic/Analog Types/Int",
                    2000,
                    state: null);

                Console.WriteLine("Waiting for 20 seconds...");
                Thread.Sleep(20 * 1000);

                Console.WriteLine("Changing subscription...");
                client.ChangeItemSubscription(handle, new DAGroupParameters(500));

                Console.WriteLine("Waiting for 10 seconds...");
                Thread.Sleep(10 * 1000);

                Console.WriteLine("Unsubscribing...");
                client.UnsubscribeAllItems();

                Console.WriteLine("Waiting for 10 seconds...");
                Thread.Sleep(10 * 1000);
            }
        }

        // Item changed event handler
        static void client_ItemChanged(object sender, EasyDAItemChangedEventArgs e)
        {
            if (e.Succeeded)
                Console.WriteLine(e.Vtq);
            else
                Console.WriteLine("*** Failure: {0}", e.ErrorMessageBrief);
        }
    }
}
QuickOPC supports OPC XML-DA also on Linux and macOS.
See Also

Examples - OPC Data Access

Conceptual