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



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OpcLabs.EasyOpcClassicCore Assembly > OpcLabs.EasyOpc.DataAccess Namespace > IEasyDAClientExtension Class > SubscribeItem Method : SubscribeItem(IEasyDAClient,EasyDAItemSubscriptionArguments) Method
The client object that will perform the operation.

This is typically the EasyDAClient object.

The value of this parameter cannot be null (Nothing in Visual Basic).

Arguments specifying how to subscribe to an OPC DA item.

The value of this parameter cannot be null (Nothing in Visual Basic).

Subscribe to OPC item using an object that holds all necessary arguments, including an optional callback.
Syntax
'Declaration
 
<ExtensionAttribute()>
Public Overloads Shared Function SubscribeItem( _
   ByVal client As IEasyDAClient, _
   ByVal easyItemSubscriptionArguments As EasyDAItemSubscriptionArguments _
) As Integer
 
'Usage
 
Dim client As IEasyDAClient
Dim easyItemSubscriptionArguments As EasyDAItemSubscriptionArguments
Dim value As Integer
 
value = IEasyDAClientExtension.SubscribeItem(client, easyItemSubscriptionArguments)

Parameters

client
The client object that will perform the operation.

This is typically the EasyDAClient object.

The value of this parameter cannot be null (Nothing in Visual Basic).

easyItemSubscriptionArguments
Arguments specifying how to subscribe to an OPC DA item.

The value of this parameter cannot be null (Nothing in Visual Basic).

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

The ItemChanged event is generated for each significant item change.

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

The server(s) can be local or can be remotely accessed via DCOM.

This is an extension method (info: C#, VB.NET). In languages that have support for extensions methods (such as C# and VB.NET), you can use the extension method as if it were a regular method on the object that is its first parameter. In other languages (such as with Python.NET), you will call the extension as a static method, and pass it the object on which it acts as its first parameter.

Example
// 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 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 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}");
        }
    }
}
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