ReadValue Method (_EasyUAClient)

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

Syntax

Visual Basic
C#
C++/CLI

'Declaration
 
<CanBeNullAttribute()>
Function ReadValue( _
   ByVal endpointDescriptorString As String, _
   ByVal nodeDescriptorString As String _
) As Object

'Usage
 
Dim instance As _EasyUAClient
Dim endpointDescriptorString As String
Dim nodeDescriptorString As String
Dim value As Object
 
value = instance.ReadValue(endpointDescriptorString, nodeDescriptorString)

[CanBeNull()]
object ReadValue( 
   string endpointDescriptorString,
   string nodeDescriptorString
)

[CanBeNull()]
Object^ ReadValue( 
   String^ endpointDescriptorString,
   String^ nodeDescriptorString
)

Parameters

endpointDescriptorString

Endpoint descriptor. Identifies the OPC-UA server.

The value represents an OPC UA endpoint descriptor string. 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. The OPC UA endpoint descriptor string is typically the URL of the server endpoint.

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

nodeDescriptorString

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

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

Return Value

If successful, the method returns the actual value of OPC attribute requested.

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

Remarks

You can obtain nodeDescriptorString e.g. by calling one of the browsing methods on OpcLabs.EasyOpc.UA.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 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.

Example

// This example shows how to read value of a single node, and display it.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
// 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.

#include "stdafx.h"    // Includes "QuickOpc.h", and other commonly used files
#include "ReadValue.h"

namespace _EasyUAClient
{
    void ReadValue::Main()
    {
        // Initialize the COM library
        CoInitializeEx(NULL, COINIT_MULTITHREADED);
        {
            // Instantiate the client object
            _EasyUAClientPtr ClientPtr(__uuidof(EasyUAClient));

            // Perform the operation
            _variant_t value = ClientPtr->ReadValue(
                //L"http://opcua.demo-this.com:51211/UA/SampleServer", 
                L"opc.tcp://opcua.demo-this.com:51210/UA/SampleServer",
                L"nsu=http://test.org/UA/Data/ ;i=10853");
    
            // Display results
            _variant_t vString;
            vString.ChangeType(VT_BSTR, &value);
            _tprintf(_T("value: %s\n"), (LPCTSTR)CW2CT((_bstr_t)vString));
        }
         // Release all interface pointers BEFORE calling CoUninitialize()
        CoUninitialize();
    }
}

// This example shows how to read value of a single node, and display it.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
// 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.

class procedure ReadValue.Main;
var
  Client: EasyUAClient;
  Value: OleVariant;
begin
  // Instantiate the client object
  Client := CoEasyUAClient.Create;

  Value := Client.ReadValue(
    //'http://opcua.demo-this.com:51211/UA/SampleServer',
    'opc.tcp://opcua.demo-this.com:51210/UA/SampleServer',
    'nsu=http://test.org/UA/Data/ ;i=10853');
  WriteLn('value: ', Value);
end;

// This example shows how to read value of a single node, and display it.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
// 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.

var Client = new ActiveXObject("OpcLabs.EasyOpc.UA.EasyUAClient");
var value = Client.ReadValue(
    //"http://opcua.demo-this.com:51211/UA/SampleServer",
    "opc.tcp://opcua.demo-this.com:51210/UA/SampleServer",
    "nsu=http://test.org/UA/Data/ ;i=10853");
WScript.Echo("value: " + value);

// This example shows how to read value 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 Object Pascal (Delphi) on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-OP .
// 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.

class procedure ReadValue.Main;
var
  Client: OpcLabs_EasyOpcUA_TLB._EasyUAClient;
  Value: OleVariant;
begin
  // Instantiate the client object
  Client := CoEasyUAClient.Create;

  // Obtain value of a node
  try
    Value := Client.ReadValue(
      //'http://opcua.demo-this.com:51211/UA/SampleServer',
      //'https://opcua.demo-this.com:51212/UA/SampleServer/',
      'opc.tcp://opcua.demo-this.com:51210/UA/SampleServer',
      'nsu=http://test.org/UA/Data/ ;i=10853');
  except
    on E: EOleException do
    begin
      WriteLn(Format('*** Failure: %s', [E.GetBaseException.Message]));
      Exit;
    end;
  end;

  // Display results
  WriteLn('value: ', Value);
end;

// This example shows how to read value 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 PHP on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-PHP .
// 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.

// Instantiate the client object
$Client = new COM("OpcLabs.EasyOpc.UA.EasyUAClient");

// Perform the operation
try
{
    $value = $Client->ReadValue(
        //"http://opcua.demo-this.com:51211/UA/SampleServer", 
        "opc.tcp://opcua.demo-this.com:51210/UA/SampleServer", 
        "nsu=http://test.org/UA/Data/ ;i=10853");
}
catch (com_exception $e)
{
    printf("*** Failure: %s\n", $e->getMessage());
    Exit();
}


// Display results
printf("Value: %s\n", $value);

<!-- This example shows how to read value 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 PHP on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-PHP .-->
<!-- 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.-->
<html>
<body>
<?php

    // Instantiate the client object
    $Client = new COM("OpcLabs.EasyOpc.UA.EasyUAClient");

    // Perform the operation
    $value = $Client->ReadValue(
        //"http://opcua.demo-this.com:51211/UA/SampleServer", 
        "opc.tcp://opcua.demo-this.com:51210/UA/SampleServer", 
        "nsu=http://test.org/UA/Data/ ;i=10853");

    // Display results
    // Read item value and display it
    printf("Value: %s<br>", $value);

?>
</body>
</html>

// This example shows how to read value of a single node, and display it.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .

mle_outputtext.Text = ""

// Instantiate the client object
OLEObject client
client = CREATE OLEObject
client.ConnectToNewObject("OpcLabs.EasyOpc.UA.EasyUAClient")

// Obtain value of a node
mle_outputtext.Text = mle_outputtext.Text + "Reading node value..." + "~r~n"
Any value
TRY
    value = client.ReadValue("http://opcua.demo-this.com:51211/UA/SampleServer", "nsu=http://test.org/UA/Data/ ;i=10853")
CATCH (OLERuntimeError oleRuntimeError)
    mle_outputtext.Text = mle_outputtext.Text + "*** Failure: " + oleRuntimeError.Description + "~r~n"
    RETURN
END TRY

// Display results
mle_outputtext.Text = mle_outputtext.Text + String(value) + "~r~n"

# This example shows how to read value of a single node, and display it.
#
# The Python for Windows (pywin32) extensions package is needed. Install it using "pip install pypiwin32".
# CAUTION: We now recommend using Python.NET package instead. Full set of examples with Python.NET is available!
#
# Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
import win32com.client
from pywintypes import com_error

# Instantiate the client object
client = win32com.client.Dispatch('OpcLabs.EasyOpc.UA.EasyUAClient') 

# Perform the operation
try:
    value = client.ReadValue('opc.tcp://opcua.demo-this.com:51210/UA/SampleServer', 'nsu=http://test.org/UA/Data/ ;i=10853')
except com_error as e:
    print('*** Failure: ' + e.args[2][1] + ': ' + e.args[2][2])
    exit()

# Display results
print('value: ', value)

REM This example shows how to read value of a single node, and display it.
REM
REM Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
REM OPC client and subscriber examples in Visual Basic on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-VB .
REM Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
REM a commercial license in order to use Online Forums, and we reply to every post.

Public Sub ReadValue_Main_Command_Click()
    OutputText = ""

    ' Instantiate the client object
    Dim Client As New EasyUAClient

    ' Perform the operation
    On Error Resume Next
    Dim value As Variant
    value = Client.ReadValue("opc.tcp://opcua.demo-this.com:51210/UA/SampleServer", "nsu=http://test.org/UA/Data/ ;i=10853")
    If Err.Number <> 0 Then
        OutputText = OutputText & "*** Failure: " & Err.Source & ": " & Err.Description & vbCrLf
        Exit Sub
    End If
    On Error GoTo 0
    
    ' Display results
    OutputText = OutputText & "value: " & value & vbCrLf
End Sub

Rem This example shows how to read value of a single node, and display it.
Rem
Rem Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
Rem OPC client and subscriber examples in VBScript on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-VBScript .
Rem Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
Rem a commercial license in order to use Online Forums, and we reply to every post.

Option Explicit

' Instantiate the client object
Dim Client: Set Client = CreateObject("OpcLabs.EasyOpc.UA.EasyUAClient")

' Perform the operation
On Error Resume Next
Dim value: value = Client.ReadValue("opc.tcp://opcua.demo-this.com:51210/UA/SampleServer", "nsu=http://test.org/UA/Data/ ;i=10853")
If Err.Number <> 0 Then
    WScript.Echo "*** Failure: " & Err.Source & ": " & Err.Description
    WScript.Quit
End If
On Error Goto 0

' Display results
WScript.Echo "value: " & value

Object Pascal

// This example shows how to repeatedly read value 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 Object Pascal (Delphi) on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-OP .
// 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.

class procedure ReadValue.Repeated;
const
  EndpointDescriptorUrlString = 
    //'http://opcua.demo-this.com:51211/UA/SampleServer';
    //'https://opcua.demo-this.com:51212/UA/SampleServer/';
    'opc.tcp://opcua.demo-this.com:51210/UA/SampleServer';
  NodeIdExpandedText = 'nsu=http://test.org/UA/Data/ ;i=10221';
// Example settings with Softing dataFEED OPC Suite:
//  EndpointDescriptorUrlString = 'opc.tcp://localhost:4980/Softing_dataFEED_OPC_Suite_Configuration1';
//  NodeIdExpandedText = 'nsu=Local%20Items ;s=Local Items.EAK_Test1.EAK_Testwert1_I4';
var
  Client: OpcLabs_EasyOpcUA_TLB._EasyUAClient;
  Value: OleVariant;
  I: Integer;
begin
  // Instantiate the client object
  Client := CoEasyUAClient.Create;

  for I := 1 to 60 do
  begin
    Write('@', TimeToStr(Now), ': ');

    // Obtain value of a node
    try
      Value := Client.ReadValue(EndpointDescriptorUrlString, NodeIdExpandedText);
    except
      on E: EOleException do
        WriteLn(Format('*** Failure: %s', [E.GetBaseException.Message]));
    end;

    // Display results
    WriteLn('Read ', Value);

    //
    Sleep(1000);
  end;
end;

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

_EasyUAClient Interface
_EasyUAClient 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.