Waits until the OPC-DA item quality becomes "good". Specify machine name, server class, and item descriptor.
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).
- 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 cannot be null
(Nothing
in Visual Basic).
- serverClass
- Contains ProgID of the OPC server to be used in the operation.
The value of this parameter cannot be null
(Nothing
in Visual Basic).
- itemDescriptor
- The OPC-DA item involved in the operation.
Because the OpcLabs.EasyOpc.DataAccess.DAItemDescriptor has an implicit conversion from System.String and OpcLabs.EasyOpc.DataAccess.AddressSpace.DANodeElement, in languages that support implicit conversion operators (such as C# or VB.NET), you can simply use a string (representing the qualified name of the node), or a OpcLabs.EasyOpc.DataAccess.AddressSpace.DANodeElement object (result from OPC browsing), in place of this parameter, and the corresponding OPC DA item descriptor will be constructed automatically. When the implicit conversion operators are not supported (such as with Python.NET), you can use the FromString or FromDANodeElement static method 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