Subscribe to data changes in C#

The code

var subscription = new Subscription(session.DefaultSubscription)
{
    PublishingInterval = 1000,       // delivery rate the consumer needs
    KeepAliveCount     = 10,
    LifetimeCount      = 100,
    MaxNotificationsPerPublish = 1000,
    PublishingEnabled  = true,
    Priority           = 10,
};
session.AddSubscription(subscription);
subscription.Create();

Console.WriteLine($"revised publishing interval: {subscription.CurrentPublishingInterval}");

var item = new MonitoredItem(subscription.DefaultItem)
{
    StartNodeId      = new NodeId(2258),
    AttributeId      = Attributes.Value,
    SamplingInterval = 250,          // how often the server looks
    QueueSize        = 10,
    DiscardOldest    = true,
    Filter = new DataChangeFilter
    {
        Trigger       = DataChangeTrigger.StatusValue,
        DeadbandType  = (uint)DeadbandType.Absolute,
        DeadbandValue = 0.5,
    },
};

item.Notification += (mi, e) =>
{
    foreach (var dv in mi.DequeueValues())
        Console.WriteLine($"{dv.Value} [{dv.StatusCode}] {dv.SourceTimestamp:O}");
};

subscription.AddItem(item);
subscription.ApplyChanges();

Console.WriteLine($"revised sampling interval: {item.Status.SamplingInterval}");
await Task.Delay(5000);
subscription.Delete(true);

What it prints

revised publishing interval: 1000
revised sampling interval: 250
08/09/2026 03:45:00 [Good] 2026-09-08T03:45:00.1020000Z
08/09/2026 03:45:01 [Good] 2026-09-08T03:45:01.1040000Z

Notes

• Nothing happens until ApplyChanges. Adding items without it is a common first mistake.
• Read CurrentPublishingInterval and item.Status.SamplingInterval after creation. You requested; the server revised, and believing your request is how clients end up wrong about their own sample rate.
• DequeueValues drains the item's queue. If it returns more values than you expect per notification, your sampling interval is faster than your publishing interval, which is fine, but size the queue for it.
• Percent deadband needs EURange on the node. Absolute is the safe default.
• Keep the number of distinct publishing intervals small; each one is another subscription server-side.

What breaks first

Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.