ReadItemValue(IEasyDAClient,DAItemArguments,DAReadParameters) Method

Reads a single named item from an OPC-DA server. Only the item value is returned (quality and timestamp are not returned). Reads a value of a named item using an item arguments objects, and read parameters.

Syntax

Visual Basic
C#
C++/CLI

'Declaration
 
<ExtensionAttribute()>
<CanBeNullAttribute()>
Public Overloads Shared Function ReadItemValue( _
   ByVal client As IEasyDAClient, _
   ByVal itemArguments As DAItemArguments, _
   ByVal readParameters As DAReadParameters _
) As Object

'Usage
 
Dim client As IEasyDAClient
Dim itemArguments As DAItemArguments
Dim readParameters As DAReadParameters
Dim value As Object
 
value = IEasyDAClientExtension.ReadItemValue(client, itemArguments, readParameters)

[Extension()]
[CanBeNull()]
public static object ReadItemValue( 
   IEasyDAClient client,
   DAItemArguments itemArguments,
   DAReadParameters readParameters
)

[Extension()]
[CanBeNull()]
public:
static Object^ ReadItemValue( 
   IEasyDAClient^ client,
   DAItemArguments^ itemArguments,
   DAReadParameters^ readParameters
)

Parameters

client

The client object that will perform the operation.

This is typically the 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).

readParameters

The read parameters (such as data source or value age).

Because the DAReadParameters has an implicit conversion from System.Int32 and DADataSource, in languages that support implicit conversion operators (such as C# or VB.NET), you can simply use an integer (representing the value age in milliseconds), or an element of the DADataSource enumeration in place of this parameter, and the corresponding OPC DA read parameters object will be constructed automatically. When the implicit conversion operators are not supported (such as with Python.NET), you can use the DAReadParameters Constructor(Int32) or DAReadParameters Constructor(DADataSource) 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

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.

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.

Remarks

Use this method if you are only interested in the actual value of the OPC item. Use ReadItem method if you also need the quality or timestamp. The ReadItemValue method makes it very easy to obtain the actual data value with just one function call.

The ReadItemValue method requires that the value quality is "good".

The server(s) can be local or can be remotely accessed via DCOM.

Optionally, a specific data type can be requested, or an access path can be specified (OPC DA 1.0 only).

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

// This example shows how to read and display value of a single item.
// One of the shortest examples possible.
//
// 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.OperationModel;

namespace DocExamples.DataAccess._EasyDAClient
{
    partial class ReadItemValue
    {
        public static void Main1()
        {
            // Instantiate the client object.
            var client = new EasyDAClient();

            Console.WriteLine("Reading item value...");
            object value;
            try
            {
                value = client.ReadItemValue("", "OPCLabs.KitServer.2", "Demo.Ramp");
            }
            catch (OpcException opcException)
            {
                Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message);
                return;
            }

            Console.WriteLine(value);
        }
    }
}

# This example shows how to read and display value of a single item.
# One of the shortest examples possible.
#
# 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.DataAccess
using namespace OpcLabs.EasyOpc.OperationModel

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

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

Write-Host "Reading item value..."
try {
    $value = [IEasyDAClientExtension]::ReadItemValue($client, "", "OPCLabs.KitServer.2", "Demo.Ramp")
}
catch [OpcException] {
    Write-Host "*** Failure: $($PSItem.Exception.GetBaseException().Message)"
    return
}

Write-Host $value

' This example shows how to read and display value of a single item.
' One of the shortest examples possible.
'
' 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.OperationModel

Namespace DataAccess._EasyDAClient
    Partial Friend Class ReadItemValue
        Public Shared Sub Main1()
            Dim client As New EasyDAClient()

            Console.WriteLine("Reading item...")
            Try
                Console.WriteLine(client.ReadItemValue("", "OPCLabs.KitServer.2", "Demo.Ramp"))
            Catch opcException As OpcException
                Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message)
                Exit Sub
            End Try
        End Sub
    End Class
End Namespace

Python

# This example shows how to read value of a single item, 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.DataAccess import *
from OpcLabs.EasyOpc.OperationModel import *


# Instantiate the client object
client = EasyDAClient()

print('Reading item value...')
try:
    value = IEasyDAClientExtension.ReadItemValue(client, '', 'OPCLabs.KitServer.2', 'Demo.Single')
except OpcException as opcException:
    print('*** Failure: ' + opcException.GetBaseException().Message, sep='')
    exit()

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

CSharp
VB.NET
Python

// This example shows how to read and display value of a single item.
// One of the shortest examples possible.
//
// 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.OperationModel;

namespace DocExamples.DataAccess.Xml
{
    partial class ReadItemValue
    {
        public static void Main1Xml()
        {
            // Instantiate the client object.
            var client = new EasyDAClient();

            Console.WriteLine("Reading item value...");
            object value;
            try
            {
                value = client.ReadItemValue("http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx", "Dynamic/Analog Types/Double");
            }
            catch (OpcException opcException)
            {
                Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message);
                return;
            }

            Console.WriteLine(value);
        }
    }
}

' This example shows how to read and display value of a single item.
' One of the shortest examples possible.
'
' 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.OperationModel

Namespace DataAccess.Xml
    Partial Friend Class ReadItemValue
        Public Shared Sub Main1Xml()
            Dim client As New EasyDAClient()

            Console.WriteLine("Reading item value...")
            Dim value As Object
            Try
                value = client.ReadItemValue("http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx", "Dynamic/Analog Types/Double")
            Catch opcException As OpcException
                Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message)
                Exit Sub
            End Try

            Console.WriteLine(value)
        End Sub
    End Class
End Namespace

# This example shows how to read and display value of a single item.
# One of the shortest examples possible.
#
# 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 import *
from OpcLabs.EasyOpc.DataAccess import *
from OpcLabs.EasyOpc.OperationModel import *

# Instantiate the client object.
client = EasyDAClient()

print('Reading item value...')
try:
    value = IEasyDAClientExtension.ReadItemValue(client, ServerDescriptor('http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx'), DAItemDescriptor('Dynamic/Analog Types/Double'))
except OpcException as opcException:
    print('*** Failure: ' + opcException.GetBaseException().Message, sep='')
    exit()

# Display results
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

Reference

IEasyDAClientExtension Class
IEasyDAClientExtension Members
Overload List

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