ReadValue(IEasyUAClient,UAEndpointDescriptor,UANodeDescriptor,UAAttributeId) Method

Reads an attribute's value using an endpoint descriptor, node Id and attribute Id.

Syntax

Visual Basic
C#
C++/CLI

'Declaration
 
<ExtensionAttribute()>
<CanBeNullAttribute()>
Public Overloads Shared Function ReadValue( _
   ByVal client As IEasyUAClient, _
   ByVal endpointDescriptor As UAEndpointDescriptor, _
   ByVal nodeDescriptor As UANodeDescriptor, _
   ByVal attributeId As UAAttributeId _
) As Object

'Usage
 
Dim client As IEasyUAClient
Dim endpointDescriptor As UAEndpointDescriptor
Dim nodeDescriptor As UANodeDescriptor
Dim attributeId As UAAttributeId
Dim value As Object
 
value = IEasyUAClientExtension.ReadValue(client, endpointDescriptor, nodeDescriptor, attributeId)

[Extension()]
[CanBeNull()]
public static object ReadValue( 
   IEasyUAClient client,
   UAEndpointDescriptor endpointDescriptor,
   UANodeDescriptor nodeDescriptor,
   UAAttributeId attributeId
)

[Extension()]
[CanBeNull()]
public:
static Object^ ReadValue( 
   IEasyUAClient^ client,
   UAEndpointDescriptor^ endpointDescriptor,
   UANodeDescriptor^ nodeDescriptor,
   UAAttributeId attributeId
)

Parameters

client

The client object that will perform the operation.

This is typically the EasyUAClient object.

The value of this parameter cannot be null (Nothing in Visual Basic).

endpointDescriptor

Endpoint descriptor. Identifies the OPC-UA server.

Because the UAEndpointDescriptor has implicit conversions from System.String and System.Uri, in languages that support implicit conversion operators (such as C# or VB.NET), you can simply use a string (representing the endpoint URL, or a so-called OPC UA endpoint descriptor string), or a System.Uri object, in place of this parameter, and the corresponding endpoint descriptor will be constructed automatically. When the implicit conversion operators are not supported (such as with Python.NET), you can use the UAEndpointDescriptor.FromString or UAEndpointDescriptor.FromUri static method instead.

Also, because the OpcLabs.EasyOpc.UA.Discovery.UAApplicationElement and OpcLabs.EasyOpc.UA.Discovery.UADiscoveryElement have an implicit conversion to UAEndpointDescriptor, in languages that support implicit conversion operators (such as C# or VB.NET), you can simply use a OpcLabs.EasyOpc.UA.Discovery.UAApplicationElement or OpcLabs.EasyOpc.UA.Discovery.UADiscoveryElement (results from OPC UA discovery) in place of this parameter, and the corresponding endpoint descriptor will be constructed automatically. When the implicit conversion operators are not supported (such as with Python.NET), you can convert a (non-null) OpcLabs.EasyOpc.UA.Discovery.UAApplicationElement or OpcLabs.EasyOpc.UA.Discovery.UADiscoveryElement to UAEndpointDescriptor using the OpcLabs.EasyOpc.UA.Discovery.UAApplicationElement.ToUAEndpointDescriptor or OpcLabs.EasyOpc.UA.Discovery.UADiscoveryElement.ToUAEndpointDescriptor method instead.

If you are using OPC Wizard (for server development), an implicit conversion from EasyUAServerCore can be used in the same way to simply pass the server object in place of this parameter, which will use its OpcLabs.EasyOpc.UA.EasyUAServerCore.EffectiveServerDescriptor property for the connection.

The value of this parameter cannot be null (Nothing in Visual Basic).

nodeDescriptor

Node descriptor. Identifies the node in OPC server's address space.

Because the UANodeDescriptor has implicit conversions from OpcLabs.EasyOpc.UA.AddressSpace.UANodeId, OpcLabs.EasyOpc.UA.AddressSpace.UANodeElement, OpcLabs.EasyOpc.UA.Navigation.UABrowsePath and System.String, in languages that support implicit conversion operators (such as C# or VB.NET), you can simply use a OpcLabs.EasyOpc.UA.AddressSpace.UANodeId object (representing the Id of the OPC UA node), a node element object (from OPC UA browsing), OpcLabs.EasyOpc.UA.Navigation.UABrowsePath object (representing OPC UA absolute browse path), or a string (with expanded node Id text) in place of this parameter, and the corresponding node descriptor will be constructed automatically. When the implicit conversion operators are not supported (such as with Python.NET), you can use the UANodeDescriptor.FromString, UANodeDescriptor.FromUABrowsePath, UANodeDescriptor.FromUANodeElement or UANodeDescriptor.FromUANodeId static method instead.

If you are using OPC Wizard (for server development), an implicit conversion from OpcLabs.EasyOpc.UA.NodeSpace.UAServerNode can be used in the same way to simply pass the server node in place of this parameter, which will use its OpcLabs.EasyOpc.UA.NodeSpace.UAServerNode.EffectiveNodeDescriptor property for the operation.

The value of this parameter cannot be null (Nothing in Visual Basic).

attributeId

Attribute Id. Identifies an attribute of a node.

Return Value

If successful, the method returns the actual value of OPC attribute 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.ComponentModel.InvalidEnumArgumentException	An invalid enumeration value was used. 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.UA.OperationModel.UAException	The OPC UA 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.ComponentModel.InvalidEnumArgumentException

An invalid enumeration value was used.

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.UA.OperationModel.UAException

The OPC UA 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

You can obtain nodeDescriptor e.g. by calling one of the browsing methods on EasyUAClientCore object.

Only the attribute's value is returned (status code and timestamps are not returned). The status code must be 'good' for this method to succeed; otherwise, a OpcLabs.EasyOpc.UA.OperationModel.UAException is thrown.

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
PowerShell
VB.NET
Python

// This example shows how to read a value of a specific attribute of a single node, and display it.
//
// 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.UA;
using OpcLabs.EasyOpc.UA.OperationModel;

namespace UADocExamples._EasyUAClient
{
    partial class ReadValue
    {
        public static void Overload2()
        {
            UAEndpointDescriptor endpointDescriptor =
                "opc.tcp://opcua.demo-this.com:51210/UA/SampleServer";
            // or "http://opcua.demo-this.com:51211/UA/SampleServer" (currently not supported)
            // or "https://opcua.demo-this.com:51212/UA/SampleServer/"

            UANodeDescriptor nodeDescriptor = "nsu=http://test.org/UA/Data/ ;i=10853";

            // Instantiate the client object
            var client = new EasyUAClient();

            // Obtain value of a DataType attribute
            object value;
            try
            {
                value = client.ReadValue(endpointDescriptor, nodeDescriptor, UAAttributeId.DataType);
            }
            catch (UAException uaException)
            {
                Console.WriteLine($"*** Failure: {uaException.GetBaseException().Message}");
                return;
            }

            // Display results
            Console.WriteLine($"value type: {value?.GetType()}");
            Console.WriteLine($"value: {value}");
        }
    }
}

# This example shows how to read a value of a specific attribute of a single node, and display it.
#
# Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
# OPC client and subscriber examples in PowerShell on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-PowerShell .
# 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.

#requires -Version 5.1
using namespace OpcLabs.EasyOpc.UA
using namespace OpcLabs.EasyOpc.UA.OperationModel

# The path below assumes that the current directory is [ProductDir]/Examples-NET/PowerShell/Windows .
Add-Type -Path "../../../Components/Opclabs.QuickOpc/net472/OpcLabs.EasyOpcUA.dll"
Add-Type -Path "../../../Components/Opclabs.QuickOpc/net472/OpcLabs.EasyOpcUAComponents.dll"

[UAEndpointDescriptor]$endpointDescriptor =
    "opc.tcp://opcua.demo-this.com:51210/UA/SampleServer"
# or "http://opcua.demo-this.com:51211/UA/SampleServer" (currently not supported)
# or "https://opcua.demo-this.com:51212/UA/SampleServer/"

# Instantiate the client object.
$client = New-Object EasyUAClient

# Obtain value of a DataType attribute
try {
    $value = [OpcLabs.EasyOpc.UA.IEasyUAClientExtension]::ReadValue($client,
        $endpointDescriptor, "nsu=http://test.org/UA/Data/ ;i=10853",
        [OpcLabs.EasyOpc.UA.UAAttributeId]::DataType)
}
catch [UAException] {
    Write-Host "*** Failure: $($PSItem.Exception.GetBaseException().Message)"
    return
}

# Display results
Write-Host "value type: $($value.GetType())"
Write-Host "value: $($value)"

' This example shows how to read a value of a specific attribute of a single node, and display it.
'
' 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.UA
Imports OpcLabs.EasyOpc.UA.OperationModel

Namespace _EasyUAClient
    Partial Friend Class ReadValue
        Public Shared Sub Overload2()

            ' Define which server we will work with.
            Dim endpointDescriptor As UAEndpointDescriptor =
                    "opc.tcp://opcua.demo-this.com:51210/UA/SampleServer"
            ' or "http://opcua.demo-this.com:51211/UA/SampleServer" (currently not supported)
            ' or "https://opcua.demo-this.com:51212/UA/SampleServer/"

            Dim nodeDescriptor As UANodeDescriptor = "nsu=http://test.org/UA/Data/ ;i=10853"

            ' Instantiate the client object.
            Dim client = New EasyUAClient()

            ' Obtain value of a DataType attribute.
            Dim value As Object
            Try
                value = client.ReadValue(endpointDescriptor, nodeDescriptor, UAAttributeId.DataType)
            Catch uaException As UAException
                Console.WriteLine("*** Failure: {0}", uaException.GetBaseException.Message)
                Exit Sub
            End Try

            ' Display results
            Console.WriteLine("value type: {0}", value.GetType())
            Console.WriteLine("value: {0}", value)
        End Sub
    End Class
End Namespace

# This example shows how to read a value of a specific attribute of a single node, and display it.
#
# 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.UA import *
from OpcLabs.EasyOpc.UA.OperationModel import *


endpointDescriptor = UAEndpointDescriptor('opc.tcp://opcua.demo-this.com:51210/UA/SampleServer')
# or 'http://opcua.demo-this.com:51211/UA/SampleServer' (currently not supported)
# or 'https://opcua.demo-this.com:51212/UA/SampleServer/'

nodeDescriptor = UANodeDescriptor('nsu=http://test.org/UA/Data/ ;i=10853')

# Instantiate the client object.
client = EasyUAClient()

# Obtain value of a DataType attribute.
try:
    value = IEasyUAClientExtension.ReadValue(client, endpointDescriptor, nodeDescriptor, UAAttributeId.DataType)
except UAException as uaException:
    print('*** Failure: ' + uaException.GetBaseException().Message)
    exit()

# Display results.
print('value type: ', type(value), sep='')  # The output will differ from pure .NET languages such as C# or VB.NET.
print('value: ', value, sep='')

print()
print('Finished.')

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

IEasyUAClientExtension Class
IEasyUAClientExtension Members
Overload List

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