OPC Studio User's Guide and Reference
ItemChanged Event (EasyDAClientCore)
Example 



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OpcLabs.EasyOpcClassic Assembly > OpcLabs.EasyOpc.DataAccess Namespace > EasyDAClientCore Class : ItemChanged Event
Raised for every significant change in an OPC item subscribed to by the SubscribeItem method.
Syntax
'Declaration
 
Public Event ItemChanged As EasyDAItemChangedEventHandler
 
'Usage
 
Dim instance As EasyDAClientCore
Dim handler As EasyDAItemChangedEventHandler
 
AddHandler instance.ItemChanged, handler
Event Data

The event handler receives an argument of type EasyDAItemChangedEventArgs containing data related to this event. The following EasyDAItemChangedEventArgs properties provide information specific to this event.

PropertyDescription
Holds arguments that were used to subscribe to an item in an OPC-DA server.  
Diagnostics information (such as warnings) assembled during the operation. (Inherited from OpcLabs.BaseLib.OperationModel.OperationEventArgs)
Count of diagnostic information elements assembled during the operation. (Inherited from OpcLabs.BaseLib.OperationModel.OperationEventArgs)
Textual summary of diagnostics information, one message per line. (Inherited from OpcLabs.BaseLib.OperationModel.OperationEventArgs)
Gets or sets the error ID of the error. (Inherited from OpcLabs.BaseLib.OperationModel.OperationEventArgs)
Gets or sets a message that describes the current exception. (Inherited from OpcLabs.BaseLib.OperationModel.OperationEventArgs)
The first line of the error message. (Inherited from OpcLabs.BaseLib.OperationModel.OperationEventArgs)
Gets the current exception. Contains null reference when no exception. (Inherited from OpcLabs.BaseLib.OperationModel.OperationEventArgs)
A normalized OpcLabs.BaseLib.OperationModel.OperationEventArgs.Diagnostics collection. (Inherited from OpcLabs.BaseLib.OperationModel.OperationEventArgs)
A normalized OpcLabs.BaseLib.OperationModel.OperationEventArgs.Exception object, or null if there was no error. (Inherited from OpcLabs.BaseLib.OperationModel.OperationEventArgs)
Status information corresponding to the contents of the event arguments.  
Gets indication whether the operation has succeeded. (Inherited from OpcLabs.BaseLib.OperationModel.OperationEventArgs)
Gets the current (new) OpcLabs.EasyOpc.DataAccess.DAVtq (value, timestamp and quality) of the OPC item. Null reference when System.Exception is not null.  
Example
// This example shows how subscribe to changes of multiple items and display each change, identifying the different
// subscriptions by an integer.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
// OPC client and subscriber examples in C# on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-CSharp .
// Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
// a commercial license in order to use Online Forums, and we reply to every post.

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

namespace DocExamples.DataAccess._EasyDAClient
{
    partial class SubscribeMultipleItems
    {
        public static void StateAsInteger()
        {
            // Instantiate the client object.
            using (var client = new EasyDAClient())
            {
                // Hook events
                client.ItemChanged += client_StateAsInteger_ItemChanged;

                Console.WriteLine("Subscribing...");
                int[] handleArray = client.SubscribeMultipleItems(new[]
                {
                    new DAItemGroupArguments("", "OPCLabs.KitServer.2", "Simulation.Random", 1000,
                        state: 1), // An integer we have chosen to identify the subscription
                    new DAItemGroupArguments("", "OPCLabs.KitServer.2", "Trends.Ramp (1 min)", 1000,
                        state: 2), // An integer we have chosen to identify the subscription
                    new DAItemGroupArguments("", "OPCLabs.KitServer.2", "Trends.Sine (1 min)", 1000,
                        state: 3), // An integer we have chosen to identify the subscription
                    new DAItemGroupArguments("", "OPCLabs.KitServer.2", "Simulation.Register_I4", 1000,
                        state: 4) // An integer we have chosen to identify the subscription
                });

                for (int i = 0; i < handleArray.Length; i++)
                    Console.WriteLine($"handleArray[{i}]: {handleArray[i]}");

                Console.WriteLine("Processing item changed events for 10 seconds...");
                Thread.Sleep(10 * 1000);

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

            Console.WriteLine("Waiting for 5 seconds...");
            Thread.Sleep(5 * 1000);

            Console.WriteLine("Finished.");
        }

        // Item changed event handler
        static void client_StateAsInteger_ItemChanged(object sender, EasyDAItemChangedEventArgs eventArgs)
        {
            // Obtain the integer state we have passed in.
            var stateAsInteger = (int)eventArgs.Arguments.State;

            // Display the data
            if (eventArgs.Succeeded)
                Console.WriteLine($"{stateAsInteger}: {eventArgs.Vtq}");
            else
                Console.WriteLine($"{stateAsInteger} *** Failure: {eventArgs.ErrorMessageBrief}");
        }
    }
}
// This example shows how subscribe to changes of multiple items and display each change, identifying the different
// subscriptions by an object.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
// OPC client and subscriber examples in C# on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-CSharp .
// Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
// a commercial license in order to use Online Forums, and we reply to every post.

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

namespace DocExamples.DataAccess._EasyDAClient
{
    partial class SubscribeMultipleItems
    {
        class CustomObject
        {
            public CustomObject(string name)
            {
                Name = name;
            }

            public string Name { get; }
        }


        public static void StateAsObject()
        {
            // Instantiate the client object.
            using (var client = new EasyDAClient())
            {
                // Hook events
                client.ItemChanged += client_StateAsObject_ItemChanged;

                Console.WriteLine("Subscribing...");
                int[] handleArray = client.SubscribeMultipleItems(new[]
                {
                    new DAItemGroupArguments("", "OPCLabs.KitServer.2", "Simulation.Random", 1000,
                        new CustomObject("First")), // A custom object that corresponds to the subscription
                    new DAItemGroupArguments("", "OPCLabs.KitServer.2", "Trends.Ramp (1 min)", 1000,
                        new CustomObject("Second")), // A custom object that corresponds to the subscription
                    new DAItemGroupArguments("", "OPCLabs.KitServer.2", "Trends.Sine (1 min)", 1000,
                        new CustomObject("Third")), // A custom object that corresponds to the subscription
                    new DAItemGroupArguments("", "OPCLabs.KitServer.2", "Simulation.Register_I4", 1000,
                        new CustomObject("Fourth")) // A custom object that corresponds to the subscription
                });

                for (int i = 0; i < handleArray.Length; i++)
                    Console.WriteLine($"handleArray[{i}]: {handleArray[i]}");

                Console.WriteLine("Processing item changed events for 10 seconds...");
                Thread.Sleep(10 * 1000);

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

            Console.WriteLine("Waiting for 5 seconds...");
            Thread.Sleep(5 * 1000);

            Console.WriteLine("Finished.");
        }

        // Item changed event handler
        static void client_StateAsObject_ItemChanged(object sender, EasyDAItemChangedEventArgs eventArgs)
        {
            // Obtain the custom object we have passed in.
            var stateAsObject = (CustomObject)eventArgs.Arguments.State;

            // Display the data
            if (eventArgs.Succeeded)
                Console.WriteLine($"{stateAsObject.Name}: {eventArgs.Vtq}");
            else
                Console.WriteLine($"{stateAsObject.Name} *** Failure: {eventArgs.ErrorMessageBrief}");
        }
    }
}
// This example shows how to store current state of the subscribed items in a dictionary.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
// OPC client and subscriber examples in C# on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-CSharp .
// Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
// a commercial license in order to use Online Forums, and we reply to every post.

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

namespace DocExamples.DataAccess._EasyDAClient
{
    partial class SubscribeMultipleItems
    {
        public static void StoreInDictionary()
        {
            // Instantiate the client object.
            using (var client = new EasyDAClient())
            {
                client.ItemChanged += client_ItemChanged_StoreInDictionary;

                Console.WriteLine("Subscribing item changes...");
                client.SubscribeMultipleItems(
                    new[] {
                            new DAItemGroupArguments("", "OPCLabs.KitServer.2", "Simulation.Random", 1000, null), 
                            new DAItemGroupArguments("", "OPCLabs.KitServer.2", "Trends.Ramp (1 min)", 1000, null), 
                            new DAItemGroupArguments("", "OPCLabs.KitServer.2", "Trends.Sine (1 min)", 1000, null),  
                            new DAItemGroupArguments("", "OPCLabs.KitServer.2", "Simulation.Register_I4", 1000, null)
                        });

                Console.WriteLine("Processing item changed events for 1 minute...");
                int startTickCount = Environment.TickCount;
                do
                {
                    Thread.Sleep(5*1000);

                    // Each 5 seconds, display the current state of the items we have subscribed to.
                    lock (_serialize)
                    {
                        Console.WriteLine();
                        foreach (KeyValuePair<DAItemDescriptor, DAVtqResult> pair in _vtqResultDictionary)
                        {
                            DAItemDescriptor itemDescriptor = pair.Key;
                            DAVtqResult vtqResult = pair.Value;
                            Console.WriteLine($"{itemDescriptor}: {vtqResult}");
                        }

                        // The code above shows how you can process the complete contents of the dictionary. In other
                        // scenarios, you may want to access just a specific entry in the dictionary. You can achieve that
                        // by indexing the dictionary by the item descriptor of the item you are interested in.
                    }
                } while (Environment.TickCount < startTickCount + 60*1000);

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

            Console.WriteLine("Finished.");
        }

        // Item changed event handler
        static void client_ItemChanged_StoreInDictionary(object sender, EasyDAItemChangedEventArgs e)
        {
            lock (_serialize)
                // Convert the event arguments to a DAVtq result object, and store it in the dictionary under the key which
                // is the item descriptor of the item this item changed event is for.
                _vtqResultDictionary[e.Arguments.ItemDescriptor] = (DAVtqResult)e;
        }

        // Holds last known state of each subscribed item.
        private static readonly Dictionary<DAItemDescriptor, DAVtqResult> _vtqResultDictionary = 
            new Dictionary<DAItemDescriptor, DAVtqResult>();

        // Synchronization object used to prevent simultaneous access to the dictionary.
        private static readonly object _serialize = new object();
    }
}
Requirements

Target Platforms: .NET Framework: Windows 10 (selected versions), Windows 11 (selected versions), Windows Server 2016, Windows Server 2022; .NET: Linux, macOS, Microsoft Windows

See Also