WaitForItemValue(IEasyDAClient,String,String,DAItemDescriptor) Method

Subscribes to the specified OPC-DA item, and monitors its data until it has "good" quality, an error occurs, or the alloted time elapses.

Syntax

Visual Basic
C#
C++/CLI

'Declaration
 
<ExtensionAttribute()>
<CanBeNullAttribute()>
Public Overloads Shared Function WaitForItemValue( _
   ByVal client As IEasyDAClient, _
   ByVal machineName As String, _
   ByVal serverClass As String, _
   ByVal itemDescriptor As DAItemDescriptor _
) As Object

'Usage
 
Dim client As IEasyDAClient
Dim machineName As String
Dim serverClass As String
Dim itemDescriptor As DAItemDescriptor
Dim value As Object
 
value = IEasyDAClientExtension2.WaitForItemValue(client, machineName, serverClass, itemDescriptor)

[Extension()]
[CanBeNull()]
public static object WaitForItemValue( 
   IEasyDAClient client,
   string machineName,
   string serverClass,
   DAItemDescriptor itemDescriptor
)

[Extension()]
[CanBeNull()]
public:
static Object^ WaitForItemValue( 
   IEasyDAClient^ client,
   String^ machineName,
   String^ serverClass,
   DAItemDescriptor^ itemDescriptor
)

Parameters

client: The client object that will perform the operation.
machineName: Name of the machine. Determines the computer on which the OPC server is located. May be an empty string, in which case the OPC server is assumed to exist on the local computer or at the computer specified for it by DCOM configuration.
serverClass: Contains ProgID of the OPC server to be used in the operation.
itemDescriptor: The OPC-DA item involved in the operation.

Return Value

If successful, the function returns the actual value of OPC item requested.

Exceptions

Exception	Description
OpcLabs.EasyOpc.OperationModel.OpcException	The OPC "Classic" (or OPC XML-DA) operation has failed.
System.ArgumentNullException	A `null` reference (`Nothing` in Visual Basic) is passed to a method that does not accept it as a valid argument.

Example

CSharp

// This example shows how to wait on an item until a value with "good" quality becomes available.

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

namespace DocExamples.DataAccess._EasyDAClientExtension
{
    class WaitForItemValue
    {
        public static void Main1()
        {
            // Instantiate the client object.
            var client = new EasyDAClient();

            Console.WriteLine("Waiting until an item value with \"good\" quality becomes available...");
            object value;
            try
            {
                value = client.WaitForItemValue("", "OPCLabs.KitServer.2", "Demo.Unreliable", 
                    groupParameters: 100,   // this is the requested update rate
                    millisecondsTimeout: 60*1000);
            }
            catch (OpcException opcException)
            {
                Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message);
                return;
            }

            // Display the obtained item value.
            Console.WriteLine($"value: {value}");
        }
    }
}

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

IEasyDAClientExtension2 Class
IEasyDAClientExtension2 Members
Overload List