QuickOPC User's Guide and Reference
WaitForItemValue(IEasyDAClient,DAItemGroupArguments,Int32) Method
Example 



OpcLabs.EasyOpcClassicCore Assembly > OpcLabs.EasyOpc.DataAccess.Extensions Namespace > IEasyDAClientExtension2 Class > WaitForItemValue Method : WaitForItemValue(IEasyDAClient,DAItemGroupArguments,Int32) Method
The client object that will perform the operation.
Holds an OPC server and item descriptor, and OPC group parameters.
The number of milliseconds to wait, or System.Threading.Timeout.Infinite to wait indefinitely.
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
'Declaration
 
<ExtensionAttribute()>
<CanBeNullAttribute()>
Public Overloads Shared Function WaitForItemValue( _
   ByVal client As IEasyDAClient, _
   ByVal itemGroupArguments As DAItemGroupArguments, _
   ByVal millisecondsTimeout As Integer _
) As Object
'Usage
 
Dim client As IEasyDAClient
Dim itemGroupArguments As DAItemGroupArguments
Dim millisecondsTimeout As Integer
Dim value As Object
 
value = IEasyDAClientExtension2.WaitForItemValue(client, itemGroupArguments, millisecondsTimeout)
[Extension()]
[CanBeNull()]
public static object WaitForItemValue( 
   IEasyDAClient client,
   DAItemGroupArguments itemGroupArguments,
   int millisecondsTimeout
)

Parameters

client
The client object that will perform the operation.
itemGroupArguments
Holds an OPC server and item descriptor, and OPC group parameters.
millisecondsTimeout
The number of milliseconds to wait, or System.Threading.Timeout.Infinite to wait indefinitely.

Return Value

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

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.

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.

Example

.NET

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

# 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

See Also