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



OpcLabs.EasyOpcClassicCore Assembly > OpcLabs.EasyOpc.DataAccess Namespace > IEasyDAClientExtension Class > SubscribeItem Method : SubscribeItem(IEasyDAClient,DAItemGroupArguments,EasyDAItemChangedEventHandler) Method
The client object that will perform the operation.
Arguments specifying how to subscribe to an OPC DA item.
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 using an object that holds all necessary arguments, and an optional callback.
Syntax
'Declaration
 
<ExtensionAttribute()>
Public Overloads Shared Function SubscribeItem( _
   ByVal client As IEasyDAClient, _
   ByVal itemGroupArguments As DAItemGroupArguments, _
   ByVal callback As EasyDAItemChangedEventHandler _
) As Integer
'Usage
 
Dim client As IEasyDAClient
Dim itemGroupArguments As DAItemGroupArguments
Dim callback As EasyDAItemChangedEventHandler
Dim value As Integer
 
value = IEasyDAClientExtension.SubscribeItem(client, itemGroupArguments, callback)

Parameters

client
The client object that will perform the operation.
itemGroupArguments
Arguments specifying how to subscribe to an OPC DA item.
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.

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

// 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
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