Waits until the OPC-DA item quality becomes "good". Specify item arguments object and group parameters.
Syntax
Parameters
- client
- The client object that will perform the operation.
This is typically the OpcLabs.EasyOpc.DataAccess.EasyDAClient object.
The value of this parameter cannot be null
(Nothing
in Visual Basic).
- itemArguments
- Holds an OPC server and item descriptor.
The value of this parameter cannot be null
(Nothing
in Visual Basic).
- groupParameters
- Contains subscription parameters, such as the requested update rate.
Because the OpcLabs.EasyOpc.DataAccess.DAGroupParameters has an implicit conversion from System.Int32, in languages that support implicit conversion operators (such as C# or VB.NET), you can simply use an integer (representing the requested update rate in milliseconds) in place of this parameter, and the corresponding OPC DA group parameters object will be constructed automatically. When the implicit conversion operators are not supported (such as with Python.NET), you can use the DAGroupParameters Constructor(Int32) constructor instead.
The value of this parameter cannot be null
(Nothing
in Visual Basic).
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 |
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. |
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. |
Example
// 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
See Also