SubscribeItem(IEasyDAClient,ServerDescriptor,DAItemDescriptor,DAGroupParameters,EasyDAItemChangedEventHandler,Object) Method

Subscribe to changes of a particular OPC item. The ItemChanged event is generated for each significant item change.Subscribe to OPC item. Specify server and item descriptors, group parameters object, callback method, and a user-defined state object.

Syntax

Visual Basic
C#
C++/CLI

'Declaration
 
<ExtensionAttribute()>
Public Overloads Shared Function SubscribeItem( _
   ByVal client As IEasyDAClient, _
   ByVal serverDescriptor As ServerDescriptor, _
   ByVal itemDescriptor As DAItemDescriptor, _
   ByVal groupParameters As DAGroupParameters, _
   ByVal callback As EasyDAItemChangedEventHandler, _
   ByVal state As Object _
) As Integer

'Usage
 
Dim client As IEasyDAClient
Dim serverDescriptor As ServerDescriptor
Dim itemDescriptor As DAItemDescriptor
Dim groupParameters As DAGroupParameters
Dim callback As EasyDAItemChangedEventHandler
Dim state As Object
Dim value As Integer
 
value = IEasyDAClientExtension.SubscribeItem(client, serverDescriptor, itemDescriptor, groupParameters, callback, state)

[Extension()]
public static int SubscribeItem( 
   IEasyDAClient client,
   ServerDescriptor serverDescriptor,
   DAItemDescriptor itemDescriptor,
   DAGroupParameters groupParameters,
   EasyDAItemChangedEventHandler callback,
   object state
)

[Extension()]
public:
static int SubscribeItem( 
   IEasyDAClient^ client,
   ServerDescriptor^ serverDescriptor,
   DAItemDescriptor^ itemDescriptor,
   DAGroupParameters^ groupParameters,
   EasyDAItemChangedEventHandler^ callback,
   Object^ state
)

Parameters

client: The client object that will perform the operation.
serverDescriptor: The OPC server involved in the operation.
itemDescriptor: The OPC item to be subscribed to.
groupParameters: An object containing subscription parameters, such as the requested update rate.
callback: A callback method to be invoked for each significant item change.
state: The state object (can be any object supplied by your code); available in event notifications.

Return Value

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

Exceptions

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

Remarks

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

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

Example

CSharp
PowerShell
VB.NET

// Hooking up events and receiving OPC item changes.

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.

#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 "../../../Assemblies/net47/OpcLabs.EasyOpcClassic.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.

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

Namespace DocExamples.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 2012, Windows Server 2016; .NET Core, .NET 5, .NET 6: Linux, macOS, Microsoft Windows

Reference

IEasyDAClientExtension Class
IEasyDAClientExtension Members
Overload List