ReadItemValue(IEasyDAClient,ServerDescriptor,DAItemDescriptor,DAReadParameters) Method

Reads a named item from an OPC server. Only the item value is returned (quality and timestamp are not returned). Reads a value of named item using descriptor objects for the OPC server and OPC-DA item, and specifying read parameters such as the data source or value age..

Syntax

Visual Basic
C#
C++/CLI

'Declaration
 
<ExtensionAttribute()>
<CanBeNullAttribute()>
Public Overloads Shared Function ReadItemValue( _
   ByVal client As IEasyDAClient, _
   ByVal serverDescriptor As ServerDescriptor, _
   ByVal itemDescriptor As DAItemDescriptor, _
   ByVal readParameters As DAReadParameters _
) As Object

'Usage
 
Dim client As IEasyDAClient
Dim serverDescriptor As ServerDescriptor
Dim itemDescriptor As DAItemDescriptor
Dim readParameters As DAReadParameters
Dim value As Object
 
value = IEasyDAClientExtension.ReadItemValue(client, serverDescriptor, itemDescriptor, readParameters)

[Extension()]
[CanBeNull()]
public static object ReadItemValue( 
   IEasyDAClient client,
   ServerDescriptor serverDescriptor,
   DAItemDescriptor itemDescriptor,
   DAReadParameters readParameters
)

[Extension()]
[CanBeNull()]
public:
static Object^ ReadItemValue( 
   IEasyDAClient^ client,
   ServerDescriptor^ serverDescriptor,
   DAItemDescriptor^ itemDescriptor,
   DAReadParameters^ readParameters
)

Parameters

client: The client object that will perform the operation.
serverDescriptor: The OPC server involved in the operation.
itemDescriptor: The OPC item involved in the operation.
readParameters: Read parameters, such as the data source or value age.

Return Value

If successful, the function returns the actual value of OPC item requested.

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

The server can be local or can be remotely accessed via DCOM. Optionally, an access path can be specified or a specific data type can be requested.

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 waits until the quality becomes "good", or the timeout elapses.

The data source (a device, OPC server's cache, or a value from OPC client component memory cache) is determined based on the readParameters argument.

Example

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

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.

#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 value of a single item, and display it.

# 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)
    exit()

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

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

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

CSharp
PowerShell
Python
VB.NET

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

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.

#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 value of a single item, and display it.

# 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)
    exit()

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

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

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

CSharp
Python

// This example shows how to read a value of a single item from the device and display its value.

using System;
using OpcLabs.EasyOpc.DataAccess;
using OpcLabs.EasyOpc.OperationModel;

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

            Console.WriteLine("Reading item value...");
            object value;
            try
            {
                // DADataSource enumeration:
                // Selects the data source for OPC reads (from device, from OPC cache, or dynamically determined).
                // The data source (memory, OPC cache or OPC device) selection is based on the desired value age and
                // current status of data received from the server.

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

            Console.WriteLine(value);
        }
    }
}

# This example shows how to read a value of a single item from the device and display its value.

# 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:
    # DADataSource enumeration:
    # Selects the data source for OPC reads (from device, from OPC cache, or dynamically determined).
    # The data source (memory, OPC cache or OPC device) selection is based on the desired value age and
    # current status of data received from the server.
    value = IEasyDAClientExtension.ReadItemValue(client,
                                                 ServerDescriptor('OPCLabs.KitServer.2'),
                                                 DAItemDescriptor('Demo.Single'),
                                                 DAReadParameters(DADataSource.Device))
except OpcException as opcException:
    print('*** Failure: ' + opcException.GetBaseException().Message)
    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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