SubscribeItem(IEasyDAClient,String,String,String,Int32,EasyDAItemChangedEventHandler) Method

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

Visual Basic
C#
C++/CLI

'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)

[Extension()]
public static int SubscribeItem( 
   IEasyDAClient client,
   string machineName,
   string serverClass,
   string itemId,
   int requestedUpdateRate,
   EasyDAItemChangedEventHandler callback
)

[Extension()]
public:
static int SubscribeItem( 
   IEasyDAClient^ client,
   String^ machineName,
   String^ serverClass,
   String^ itemId,
   int requestedUpdateRate,
   EasyDAItemChangedEventHandler^ 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

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

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

CSharp
VB.NET

// 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.

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 callback method specified using lambda expression.

Imports OpcLabs.EasyOpc.DataAccess

Namespace DocExamples.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

CSharp
VB.NET

// 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.

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

namespace DocExamples.DataAccess.Xml
{
    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 specified using lambda expression.

Imports OpcLabs.EasyOpc.DataAccess

Namespace DocExamples.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)
        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