Subscribe to alarms and events in C#

The code

var filter = new FilterDefinition
{
    AreaId          = ObjectIds.Server,
    Severity        = EventSeverity.Min,
    IgnoreSuppressedOrShelved = true,
    EventTypes      = new NodeIdCollection { ObjectTypeIds.BaseEventType },
};
filter.SelectClauses = filter.ConstructSelectClauses(
    session, ObjectTypeIds.BaseEventType);

var subscription = new Subscription(session.DefaultSubscription)
{
    PublishingInterval = 500, KeepAliveCount = 10, LifetimeCount = 100,
};
session.AddSubscription(subscription);
subscription.Create();

var item = filter.CreateMonitoredItem(session);
item.Notification += (mi, e) =>
{
    if (e.NotificationValue is not EventFieldList fields) return;
    var ev = filter.GetFilterTargetValues(fields.EventFields);
    Console.WriteLine($"[{ev.Severity}] {ev.SourceName}: {ev.Message?.Text}");
};

subscription.AddItem(item);
subscription.ApplyChanges();
await Task.Delay(10000);

What it prints

[500] MachineTool_1: Tool life below threshold
[800] MachineTool_1: Tool breakage detected
[100] Server: Session activated

Notes

• The select clause order determines the field order in every notification. Use the helper that maps them back by name rather than indexing positionally.
• Narrow EventTypes to what you care about. Subscribing to BaseEventType on a busy server delivers a lot of noise you then pay to discard.
• Call ConditionRefresh after a reconnect, or your client believes the plant is healthy while alarms are still active.
• Add ConditionClassId to the select clause for OEE work: it is what makes downtime reasons portable across machine vendors.

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.