WaitForItemValue Method (

Waits until the OPC-DA item quality becomes "good". Specify machine name, server class, item descriptor, group parameters, and timeout in milliseconds.

Syntax

Visual Basic
C#
C++/CLI

'Declaration
 
<CanBeNullAttribute()>
Function WaitForItemValue( _
   ByVal machineName As String, _
   ByVal serverClass As String, _
   ByVal itemId As String, _
   ByVal requestedUpdateRate As Integer, _
   ByVal millisecondsTimeout As Integer _
) As Object

'Usage
 
Dim instance As _EasyDAClient
Dim machineName As String
Dim serverClass As String
Dim itemId As String
Dim requestedUpdateRate As Integer
Dim millisecondsTimeout As Integer
Dim value As Object
 
value = instance.WaitForItemValue(machineName, serverClass, itemId, requestedUpdateRate, millisecondsTimeout)

[CanBeNull()]
object WaitForItemValue( 
   string machineName,
   string serverClass,
   string itemId,
   int requestedUpdateRate,
   int millisecondsTimeout
)

[CanBeNull()]
Object^ WaitForItemValue( 
   String^ machineName,
   String^ serverClass,
   String^ itemId,
   int requestedUpdateRate,
   int millisecondsTimeout
)

Parameters

machineName

Name of the machine. Determines the computer on which the OPC server is located. It 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.

The value represents a UNC or DNS computer name. Any string can be passed to this parameter (i.e. will not cause System.ArgumentException), but not all values make sense and will work when an operation using them is attempted. IPv6 addresses are normally enclosed between '[' and ']'.

The value of this parameter can be null (Nothing in Visual Basic).

serverClass

Contains ProgID of the OPC server to be used in the operation.

The value of this parameter can be null (Nothing in Visual Basic).

itemId

The value of this parameter can be null (Nothing in Visual Basic).

requestedUpdateRate

millisecondsTimeout

The number of milliseconds to wait, or System.Threading.Timeout.Infinite to wait indefinitely.

Valid values of this parameter are in the range from -1 to 2147483647 (Int32.MaxValue).

Return Value

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

This method can return null (Nothing in Visual Basic).

Exceptions

Exception Description

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. 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.
System.ArgumentOutOfRangeException	The value of an argument is outside the allowable range of values as defined by the invoked method. 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.
OpcLabs.EasyOpc.OperationModel.OpcException	The OPC "Classic" (or OPC XML-DA) operation has failed. This operation exception in uniformly used to allow common handling of various kinds of errors. The System.Exception.InnerException always contains information about the actual error cause. This is an operation error that depends on factors external to your program, and thus cannot be always avoided. Your code must handle it appropriately.

System.ArgumentNullException

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.

System.ArgumentOutOfRangeException

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

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.

OpcLabs.EasyOpc.OperationModel.OpcException

The OPC "Classic" (or OPC XML-DA) operation has failed. This operation exception in uniformly used to allow common handling of various kinds of errors. The System.Exception.InnerException always contains information about the actual error cause.

This is an operation error that depends on factors external to your program, and thus cannot be always avoided. Your code must handle it appropriately.

Remarks

This member or type is for use from COM. It is not meant to be used from .NET or Python. Refer to the corresponding .NET member or type instead, if you are developing in .NET or Python.

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

CSharp
VB.NET
Python

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

' This example shows how to wait on an item until a value with "good" quality becomes available.
'
' Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
' OPC client and subscriber examples in VB.NET on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-VBNET .
' 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.

Imports OpcLabs.EasyOpc.DataAccess
Imports OpcLabs.EasyOpc.DataAccess.Extensions
Imports OpcLabs.EasyOpc.OperationModel

Namespace DataAccess._EasyDAClientExtension
Friend Class WaitForItemValue
Public Shared Sub Main1()
' Instantiate the client object.
Dim client = New EasyDAClient()

Console.WriteLine("Waiting until an item value with ""good"" quality becomes available...")
Dim value As Object
Try
value = client.WaitForItemValue("", "OPCLabs.KitServer.2", "Demo.Unreliable",
groupParameters:=100, ' this Is the requested update rate
millisecondsTimeout:=60 * 1000)
Catch opcException As OpcException
Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message)
Exit Sub
End Try

' Display the obtained item value.
Console.WriteLine($"value: {value}")
End Sub
End Class
End Namespace

# This example shows how to wait on an item until a value with "good" quality becomes available.
#
# 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 .
# 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.
# The QuickOPC package is needed. Install it using "pip install opclabs_quickopc".
import opclabs_quickopc

# Import .NET namespaces.
from OpcLabs.EasyOpc.DataAccess import *
from OpcLabs.EasyOpc.DataAccess.Extensions import *
from OpcLabs.EasyOpc.OperationModel import *


# Instantiate the client object
client = EasyDAClient()

print('Waiting until an item value with "good" quality becomes available...')
try:
    value = IEasyDAClientExtension2.WaitForItemValue(client, '', 'OPCLabs.KitServer.2',
                                                     DAItemDescriptor('Demo.Unreliable'),
                                                     DAGroupParameters(100),    # this is the requested update rate
                                                     60*1000)    # timeout in milliseconds
except OpcException as opcException:
    print('*** Failure: ' + opcException.GetBaseException().Message)
    exit()

# Display the obtained item value.
print('value: ', value, sep='')

Requirements

Target Platforms: .NET Framework: Windows 10 (selected versions), Windows 11 (selected versions), Windows Server 2016, Windows Server 2022; .NET: Linux, macOS, Microsoft Windows

Reference

_EasyDAClient Interface
_EasyDAClient Members

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Missing some example? Ask us for it on our Online Forums! You do not have to own a commercial license in order to use Online Forums, and we reply to every post.