OPC Studio User's Guide and Reference
SubscribeItem(IEasyDAClient,String,String,String,Int32,EasyDAItemChangedEventHandler) Method
Example 



OpcLabs.EasyOpcClassicCore Assembly > OpcLabs.EasyOpc.DataAccess Namespace > IEasyDAClientExtension Class > SubscribeItem Method : SubscribeItem(IEasyDAClient,String,String,String,Int32,EasyDAItemChangedEventHandler) Method
The client object that will perform the operation.
Name of the machine (empty string for local computer).
Contains ProgID of the OPC server.
ID of the item that will be subscribed to.
How often should the updates be received (number of milliseconds)
A callback method to be invoked for each significant item change.
Subscribe to changes of a particular OPC item. The ItemChanged event is generated for each significant item change.Subscribe to OPC item. Specify machine name, server class, item ID, requested update rate, and a callback method.
Syntax
'Declaration
 
<ExtensionAttribute()>
Public Overloads Shared Function SubscribeItem( _
   ByVal client As IEasyDAClient, _
   ByVal machineName As String, _
   ByVal serverClass As String, _
   ByVal itemId As String, _
   ByVal requestedUpdateRate As Integer, _
   ByVal callback As EasyDAItemChangedEventHandler _
) As Integer
'Usage
 
Dim client As IEasyDAClient
Dim machineName As String
Dim serverClass As String
Dim itemId As String
Dim requestedUpdateRate As Integer
Dim callback As EasyDAItemChangedEventHandler
Dim value As Integer
 
value = IEasyDAClientExtension.SubscribeItem(client, machineName, serverClass, itemId, requestedUpdateRate, callback)

Parameters

client
The client object that will perform the operation.
machineName
Name of the machine (empty string for local computer).
serverClass
Contains ProgID of the OPC server.
itemId
ID of the item that will be subscribed to.
requestedUpdateRate
How often should the updates be received (number of milliseconds)
callback
A callback method to be invoked for each significant item change.

Return Value

The method returns an integer handle that uniquely identifies the item subscription.
Exceptions
ExceptionDescription

A null reference (Nothing in Visual Basic) is passed to a method that does not accept it as a valid argument.

This is a usage error, i.e. it will never occur (the exception will not be thrown) in a correctly written program. Your code should not catch this exception.

The value of an argument is outside the allowable range of values as defined by the invoked method.

This is a usage error, i.e. it will never occur (the exception will not be thrown) in a correctly written program. Your code should not catch this exception.

Remarks

It is more efficient to subscribe to multiple items using the SubscribeMultipleItems method.

If callback is a null reference, only the ItemChanged events are generated, but no callback method is invoked.
Example

.NET

.NET

.NET

// Hooking up events and receiving OPC item changes.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .

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

namespace DocExamples.DataAccess._EasyDAClient
{
    partial class SubscribeItem
    {
        public static void Main1()
        {
            // Instantiate the client object.
            using (var client = new EasyDAClient())
            {
                var eventHandler = new EasyDAItemChangedEventHandler(client_ItemChanged);
                client.ItemChanged += eventHandler;

                Console.WriteLine("Subscribing item...");
                client.SubscribeItem("", "OPCLabs.KitServer.2", "Demo.Ramp", 200);

                Console.WriteLine("Process item change notifications for 30 seconds...");
                Thread.Sleep(30 * 1000);

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

                client.ItemChanged -= eventHandler;
            }

            Console.WriteLine("Finished.");
        }

        static void client_ItemChanged(object sender, EasyDAItemChangedEventArgs e)
        {
            if (e.Succeeded)
                Console.WriteLine(e.Vtq);
            else
                Console.WriteLine($"*** Failure: {e.ErrorMessageBrief}");
        }
    }
}
# Hooking up events and receiving OPC item changes.
#
# Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .

#requires -Version 5.1
using namespace OpcLabs.EasyOpc.DataAccess

# The path below assumes that the current directory is [ProductDir]/Examples-NET/PowerShell/Windows .
Add-Type -Path "../../../Components/Opclabs.QuickOpc/net472/OpcLabs.EasyOpcClassicCore.dll"
Add-Type -Path "../../../Components/Opclabs.QuickOpc/net472/OpcLabs.EasyOpcClassic.dll"
Add-Type -Path "../../../Components/Opclabs.QuickOpc/net472/OpcLabs.EasyOpcClassicComponents.dll"

# Instantiate the client object.
$client = New-Object EasyDAClient

# Item changed event handler
Register-ObjectEvent -InputObject $client -EventName ItemChanged -Action { 
    if ($EventArgs.Succeeded) {
        Write-Host $EventArgs.Vtq
    }
    else {
        Write-Host "*** Failure: $($EventArgs.ErrorMessageBrief)"
    }
}

Write-Host "Subscribing item..."
[IEasyDAClientExtension]::SubscribeItem($client, "", "OPCLabs.KitServer.2", "Demo.Ramp", 200)

Write-Host "Processing item changes for 30 seconds..."
$stopwatch =  [System.Diagnostics.Stopwatch]::StartNew() 
while ($stopwatch.Elapsed.TotalSeconds -lt 30) {    
    Start-Sleep -Seconds 1
}

Write-Host "Unsubscribing items..."
$client.UnsubscribeAllItems()

Write-Host "Finished."
# Hooking up events and receiving OPC item changes.
#
# Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
# OPC client and subscriber examples in Python on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-Python .
# The QuickOPC package is needed. Install it using "pip install opclabs_quickopc".
import opclabs_quickopc
import time

# Import .NET namespaces.
from OpcLabs.EasyOpc.DataAccess import *


# Item changed event handler.
def itemChanged(sender, e):
    if e.Succeeded:
        print(e.Vtq)
    else:
        print('*** Failure: ', e.ErrorMessageBrief, sep='')


# Instantiate the client object.
client = EasyDAClient()

client.ItemChanged += itemChanged

print('Subscribing item changes...')
IEasyDAClientExtension.SubscribeItem(client, '', 'OPCLabs.KitServer.2', 'Demo.Ramp', 200)

print('Processing item change notifications for 30 seconds...')
time.sleep(30)

print('Unsubscribing all items...')
client.UnsubscribeAllItems()

client.ItemChanged -= itemChanged

print('Finished.')
' Hooking up events and receiving OPC item changes.
'
' Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .

Imports System.Threading
Imports OpcLabs.EasyOpc.DataAccess
Imports OpcLabs.EasyOpc.DataAccess.OperationModel

Namespace DataAccess._EasyDAClient
    Partial Friend Class SubscribeItem
        Shared Sub Main1()
            Using client = New EasyDAClient()
                Dim eventHandler = New EasyDAItemChangedEventHandler(AddressOf client_ItemChanged)
                AddHandler client.ItemChanged, eventHandler

                Console.WriteLine("Subscribing item...")
                client.SubscribeItem("", "OPCLabs.KitServer.2", "Demo.Ramp", 200)
                Thread.Sleep(30 * 1000)
                client.UnsubscribeAllItems()
                RemoveHandler client.ItemChanged, eventHandler
            End Using
        End Sub

        Private Shared Sub client_ItemChanged(sender As Object, e As EasyDAItemChangedEventArgs)
            If e.Succeeded Then
                Console.WriteLine(e.Vtq)
            Else
                Console.WriteLine("*** Failure: {0}", e.ErrorMessageBrief)
            End If
        End Sub
    End Class
End Namespace
// This example shows how subscribe to changes of a single item and display the value of the item with each change,
// using a callback method specified using lambda expression.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .

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

namespace DocExamples.DataAccess._EasyDAClient
{
    partial class SubscribeItem
    {
        public static void CallbackLambda()
        {
            // Instantiate the client object.
            var client = new EasyDAClient();

            Console.WriteLine("Subscribing...");
            // The callback is a lambda expression the displays the value
            client.SubscribeItem("", "OPCLabs.KitServer.2", "Simulation.Random", 1000,
                (sender, eventArgs) =>
                {
                    Debug.Assert(eventArgs != null);

                    if (eventArgs.Succeeded)
                    {
                        Debug.Assert(eventArgs.Vtq != null);
                        Console.WriteLine(eventArgs.Vtq.ToString());
                    }
                    else
                        Console.WriteLine("*** Failure: {0}", eventArgs.ErrorMessageBrief);
                });

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

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

            Console.WriteLine("Waiting for 2 seconds...");
            Thread.Sleep(2 * 1000);
        }
    }
}
# This example shows how subscribe to changes of a single item and display the value of the item with each change,
# using a regular callback method.
#
# Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
# OPC client and subscriber examples in Python on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-Python .
# The QuickOPC package is needed. Install it using "pip install opclabs_quickopc".
import opclabs_quickopc
import time

# Import .NET namespaces.
from OpcLabs.EasyOpc.DataAccess import *


# Item changed callback.
def itemChangedCallback(sender, e):
    assert e is not None
    if e.Succeeded:
        assert e.Vtq is not None
        print(e.Vtq)
    else:
        print('*** Failure: ', e.ErrorMessageBrief, sep='')


# Instantiate the client object.
client = EasyDAClient()

print('Subscribing item changes...')
# The callback is a regular method that displays the value.
IEasyDAClientExtension.SubscribeItem(client,
                                     '', 'OPCLabs.KitServer.2', 'Simulation.Random', 1000,
                                     EasyDAItemChangedEventHandler(itemChangedCallback))

print('Processing item change callbacks for 10 seconds...')
time.sleep(10)

print('Unsubscribing all items...')
client.UnsubscribeAllItems()

print('Waiting for 2 seconds...')
time.sleep(2)

print('Finished.')
' This example shows how subscribe to changes of a single item and display the value of the item with each change,
' using a callback method specified using lambda expression.
'
' Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .

Imports OpcLabs.EasyOpc.DataAccess

Namespace DataAccess._EasyDAClient
    Partial Friend Class SubscribeItem
        Shared Sub CallbackLambda()
            ' Instantiate the client object
            Dim client = New EasyDAClient()

            Console.WriteLine("Subscribing...")
            ' The callback is a lambda expression the displays the value
            client.SubscribeItem("", "OPCLabs.KitServer.2", "Simulation.Random", 1000,
                    Sub(sender, eventArgs)
                        Debug.Assert(eventArgs IsNot Nothing)
                        If eventArgs.Succeeded Then
                            Debug.Assert(eventArgs.Vtq IsNot Nothing)
                            Console.WriteLine(eventArgs.Vtq.ToString())
                        Else
                            Console.WriteLine("*** Failure: {0}", eventArgs.ErrorMessageBrief)
                        End If
                    End Sub)

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

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

            Console.WriteLine("Waiting for 2 seconds...")
            Threading.Thread.Sleep(2 * 1000)
        End Sub
    End Class
End Namespace
// This example shows how subscribe to changes of a single item in an OPC XML-DA server and display the value of the item 
// with each change, using a callback method specified using lambda expression.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .

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

namespace DocExamples.DataAccess.Xml
{
    partial class SubscribeItem
    {
        public static void CallbackLambdaXml()
        {
            // Instantiate the client object.
            var client = new EasyDAClient();

            Console.WriteLine("Subscribing item...");
            // The callback is a lambda expression the displays the value
            client.SubscribeItem(
                "http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx",
                "Dynamic/Analog Types/Int",
                1000,
                (sender, eventArgs) =>
                {
                    Debug.Assert(eventArgs != null);

                    if (eventArgs.Succeeded)
                    {
                        Debug.Assert(eventArgs.Vtq != null);
                        Console.WriteLine(eventArgs.Vtq.ToString());
                    }
                    else
                        Console.WriteLine($"*** Failure: {eventArgs.ErrorMessageBrief}");
                },
                state: null);

            Console.WriteLine("Processing item changed events for 30 seconds...");
            Thread.Sleep(30 * 1000);

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

            Console.WriteLine("Waiting for 2 seconds...");
            Thread.Sleep(2 * 1000);

            Console.WriteLine("Finished.");
        }
    }
}
# This example shows how subscribe to changes of a single item in an OPC XML-DA server and display the value of the item
# with each change, using a callback method.
#
# Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
# OPC client and subscriber examples in Python on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-Python .
# The QuickOPC package is needed. Install it using "pip install opclabs_quickopc".
import opclabs_quickopc
import time

# Import .NET namespaces.
from OpcLabs.EasyOpc import *
from OpcLabs.EasyOpc.DataAccess import *
from OpcLabs.EasyOpc.DataAccess.OperationModel import *


# Item changed callback
def itemChanged(sender, e):
    assert e is not None
    if e.Succeeded:
        assert e.Vtq is not None
        print(e.Vtq)
    else:
        print('*** Failure: ', e.ErrorMessageBrief, sep='')


# Instantiate the client object
client = EasyDAClient()

print('Subscribing item changes...')
IEasyDAClientExtension.SubscribeItem(client,
    ServerDescriptor('http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx'),
    DAItemDescriptor('Dynamic/Analog Types/Int'),
    DAGroupParameters(1000),
    EasyDAItemChangedEventHandler(itemChanged),
    None)

print('Processing item change callbacks for 10 seconds...')
time.sleep(10)

print('Unsubscribing item changes...')
client.UnsubscribeAllItems()

print('Waiting for 2 seconds...')
time.sleep(2)

print('Finished.')
' This example shows how subscribe to changes of a single item in an OPC XML-DA server and display the value of the item 
' with each change, using a callback method specified using lambda expression.
'
' Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .

Imports OpcLabs.EasyOpc.DataAccess

Namespace DataAccess.Xml
    Partial Friend Class SubscribeItem
        Shared Sub CallbackLambdaXml()
            ' Instantiate the client object
            Dim client = New EasyDAClient()

            Console.WriteLine("Subscribing...")
            ' The callback is a lambda expression the displays the value
            client.SubscribeItem(
                    "http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx",
                    "Dynamic/Analog Types/Int",
                    1000,
                    Sub(sender, eventArgs)
                        Debug.Assert(eventArgs IsNot Nothing)
                        If eventArgs.Succeeded Then
                            Debug.Assert(eventArgs.Vtq IsNot Nothing)
                            Console.WriteLine(eventArgs.Vtq.ToString())
                        Else
                            Console.WriteLine("*** Failure: {0}", eventArgs.ErrorMessageBrief)
                        End If
                    End Sub,
                    state:=Nothing)

            Console.WriteLine("Processing item changed events for 30 seconds...")
            Threading.Thread.Sleep(30 * 1000)

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

            Console.WriteLine("Waiting for 2 seconds...")
            Threading.Thread.Sleep(2 * 1000)

            Console.WriteLine("Finished.")
        End Sub
    End Class
End Namespace
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