ReadItem(IEasyDAClient,ServerDescriptor,DAItemDescriptor,DAReadParameters) Method

Reads a named item from an OPC server. Value, quality and timestamp are returned. Reads a 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()>
<NotNullAttribute()>
Public Overloads Shared Function ReadItem( _
   ByVal client As IEasyDAClient, _
   ByVal serverDescriptor As ServerDescriptor, _
   ByVal itemDescriptor As DAItemDescriptor, _
   ByVal readParameters As DAReadParameters _
) As DAVtq

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

[Extension()]
[NotNull()]
public static DAVtq ReadItem( 
   IEasyDAClient client,
   ServerDescriptor serverDescriptor,
   DAItemDescriptor itemDescriptor,
   DAReadParameters readParameters
)

[Extension()]
[NotNull()]
public:
static DAVtq^ ReadItem( 
   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 a DAVtq object. The object contains the value, timestamp and quality of the 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.

The ReadItem method only waits for the first update from the server (or until the timeout elapses); it does not wait until the quality becomes "uncertain" or "good".

Example

// This example shows how to read a single item, and display its value, timestamp and quality.

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

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

            DAVtq vtq;
            try
            {
                vtq = client.ReadItem("", "OPCLabs.KitServer.2", "Simulation.Random");
            }
            catch (OpcException opcException)
            {
                Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message);
                return;
            }

            Console.WriteLine("Vtq: {0}", vtq);
        }
    }
}

# This example shows how to read a single item, and display its value, timestamp and quality.

#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

try {
    $vtq = [IEasyDAClientExtension]::ReadItem($client, "", "OPCLabs.KitServer.2", "Simulation.Random")
}
catch [OpcException] {
    Write-Host "*** Failure: $($PSItem.Exception.GetBaseException().Message)"
    return
}

Write-Host "Vtq: $($vtq)"

# This example shows how to read a single item, and display its value, timestamp and quality.

# The QuickOPC package is needed. Install it using "pip install opclabs_quickopc".
import opclabs_quickopc

# Import .NET namespaces.
from OpcLabs.BaseLib.Navigation import *
from OpcLabs.EasyOpc import *
from OpcLabs.EasyOpc.DataAccess import *
from OpcLabs.EasyOpc.OperationModel import *


# Instantiate the client object.
client = EasyDAClient()

# Perform the operation.
try:
    vtq = IEasyDAClientExtension.ReadItem(client, '', 'OPCLabs.KitServer.2', 'Simulation.Random')
except OpcException as opcException:
    print('*** Failure: ' + opcException.GetBaseException().Message)
    exit()

# Display results.
print('Vtq: ', vtq, sep='')

' This example shows how to read a single item, and display its value, timestamp and quality.

Imports OpcLabs.EasyOpc.DataAccess
Imports OpcLabs.EasyOpc.OperationModel

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

            Dim vtq As DAVtq
            Try
                vtq = client.ReadItem("", "OPCLabs.KitServer.2", "Simulation.Random")
            Catch opcException As OpcException
                Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message)
                Exit Sub
            End Try

            Console.WriteLine("Vtq: {0}", vtq)
        End Sub
    End Class
End Namespace

CSharp
Python
VB.NET

// Shows how different data types can be read, including rare types and arrays of values.

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

namespace DocExamples.DataAccess._EasyDAClient
{
    partial class ReadItem
    {
        // Instantiate the client object.
        static readonly EasyDAClient Client = new EasyDAClient();

        static void ReadAndDisplay(string itemId)
        {
            Console.WriteLine();
            Console.WriteLine("Reading \"{0}\"...", itemId);

            DAVtq vtq;
            try
            {
                vtq = Client.ReadItem("OPCLabs.KitServer.2", itemId);
            }
            catch (OpcException opcException)
            {
                Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message);
                return;
            }
            Console.WriteLine("Vtq: {0}", vtq);
        }

        public static void DataTypes()
        {
            ReadAndDisplay("Simulation.Register_EMPTY");
            ReadAndDisplay("Simulation.Register_NULL");
            ReadAndDisplay("Simulation.Register_DISPATCH");

            ReadAndDisplay("Simulation.ReadValue_I2");
            ReadAndDisplay("Simulation.ReadValue_I4");
            ReadAndDisplay("Simulation.ReadValue_R4");
            ReadAndDisplay("Simulation.ReadValue_R8");
            ReadAndDisplay("Simulation.ReadValue_CY");
            ReadAndDisplay("Simulation.ReadValue_DATE");
            ReadAndDisplay("Simulation.ReadValue_BSTR");
            ReadAndDisplay("Simulation.ReadValue_BOOL");
            ReadAndDisplay("Simulation.ReadValue_DECIMAL");
            ReadAndDisplay("Simulation.ReadValue_I1");
            ReadAndDisplay("Simulation.ReadValue_UI1");
            ReadAndDisplay("Simulation.ReadValue_UI2");
            ReadAndDisplay("Simulation.ReadValue_UI4");
            ReadAndDisplay("Simulation.ReadValue_INT");
            ReadAndDisplay("Simulation.ReadValue_UINT");

            ReadAndDisplay("Simulation.ReadValue_ArrayOfI2");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfI4");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfR4");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfR8");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfCY");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfDATE");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfBSTR");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfBOOL");
            //ReadAndDisplay("Simulation.ReadValue_ArrayOfDECIMAL");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfI1");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfUI1");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfUI2");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfUI4");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfINT");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfUINT");
        }
    }
}

# Shows how different data types can be read, including rare types and arrays of values.

# 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 *


def readAndDisplay(itemId):
    global client
    #
    print()
    print('Reading "', itemId, '"...', sep='')
    #
    try:
        vtq = IEasyDAClientExtension.ReadItem(client, ServerDescriptor('OPCLabs.KitServer.2'), DAItemDescriptor(itemId))
    except OpcException as opcException:
        print('*** Failure: ' + opcException.GetBaseException().Message)
        return
    print('Vtq: ', vtq, sep='')


# Instantiate the client object.
client = EasyDAClient()

#
readAndDisplay('Simulation.Register_EMPTY')
readAndDisplay('Simulation.Register_NULL')
readAndDisplay('Simulation.Register_DISPATCH')

readAndDisplay('Simulation.ReadValue_I2')
readAndDisplay('Simulation.ReadValue_I4')
readAndDisplay('Simulation.ReadValue_R4')
readAndDisplay('Simulation.ReadValue_R8')
readAndDisplay('Simulation.ReadValue_CY')
readAndDisplay('Simulation.ReadValue_DATE')
readAndDisplay('Simulation.ReadValue_BSTR')
readAndDisplay('Simulation.ReadValue_BOOL')
readAndDisplay('Simulation.ReadValue_DECIMAL')
readAndDisplay('Simulation.ReadValue_I1')
readAndDisplay('Simulation.ReadValue_UI1')
readAndDisplay('Simulation.ReadValue_UI2')
readAndDisplay('Simulation.ReadValue_UI4')
readAndDisplay('Simulation.ReadValue_INT')
readAndDisplay('Simulation.ReadValue_UINT')

readAndDisplay('Simulation.ReadValue_ArrayOfI2')
readAndDisplay('Simulation.ReadValue_ArrayOfI4')
readAndDisplay('Simulation.ReadValue_ArrayOfR4')
readAndDisplay('Simulation.ReadValue_ArrayOfR8')
readAndDisplay('Simulation.ReadValue_ArrayOfCY')
readAndDisplay('Simulation.ReadValue_ArrayOfDATE')
readAndDisplay('Simulation.ReadValue_ArrayOfBSTR')
readAndDisplay('Simulation.ReadValue_ArrayOfBOOL')
# readAndDisplay('Simulation.ReadValue_ArrayOfDECIMAL')
readAndDisplay('Simulation.ReadValue_ArrayOfI1')
readAndDisplay('Simulation.ReadValue_ArrayOfUI1')
readAndDisplay('Simulation.ReadValue_ArrayOfUI2')
readAndDisplay('Simulation.ReadValue_ArrayOfUI4')
readAndDisplay('Simulation.ReadValue_ArrayOfINT')
readAndDisplay('Simulation.ReadValue_ArrayOfUINT')

print()
print('Finished.')

' Shows how different data types can be read, including rare types and arrays of values.

Imports OpcLabs.EasyOpc.DataAccess
Imports OpcLabs.EasyOpc.OperationModel

Namespace DataAccess._EasyDAClient
    Partial Friend Class ReadItem
        Private Shared ReadOnly DAClient As New EasyDAClient()

        Private Shared Sub ReadAndDisplay(itemId As String)
            Console.WriteLine()
            Console.WriteLine("Reading ""{0}""...", itemId)

            Dim vtq As DAVtq
            Try
                vtq = DAClient.ReadItem("OPCLabs.KitServer.2", itemId)
            Catch opcException As OpcException
                Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message)
                Exit Sub
            End Try

            Console.WriteLine("Vtq.ToString(): {0}", vtq.ToString())
        End Sub

        Shared Sub DataTypes()
            ReadAndDisplay("Simulation.Register_EMPTY")
            ReadAndDisplay("Simulation.Register_NULL")
            ReadAndDisplay("Simulation.Register_DISPATCH")

            ReadAndDisplay("Simulation.ReadValue_I2")
            ReadAndDisplay("Simulation.ReadValue_I4")
            ReadAndDisplay("Simulation.ReadValue_R4")
            ReadAndDisplay("Simulation.ReadValue_R8")
            ReadAndDisplay("Simulation.ReadValue_CY")
            ReadAndDisplay("Simulation.ReadValue_DATE")
            ReadAndDisplay("Simulation.ReadValue_BSTR")
            ReadAndDisplay("Simulation.ReadValue_BOOL")
            ReadAndDisplay("Simulation.ReadValue_DECIMAL")
            ReadAndDisplay("Simulation.ReadValue_I1")
            ReadAndDisplay("Simulation.ReadValue_UI1")
            ReadAndDisplay("Simulation.ReadValue_UI2")
            ReadAndDisplay("Simulation.ReadValue_UI4")
            ReadAndDisplay("Simulation.ReadValue_INT")
            ReadAndDisplay("Simulation.ReadValue_UINT")

            ReadAndDisplay("Simulation.ReadValue_ArrayOfI2")
            ReadAndDisplay("Simulation.ReadValue_ArrayOfI4")
            ReadAndDisplay("Simulation.ReadValue_ArrayOfR4")
            ReadAndDisplay("Simulation.ReadValue_ArrayOfR8")
            ReadAndDisplay("Simulation.ReadValue_ArrayOfCY")
            ReadAndDisplay("Simulation.ReadValue_ArrayOfDATE")
            ReadAndDisplay("Simulation.ReadValue_ArrayOfBSTR")
            ReadAndDisplay("Simulation.ReadValue_ArrayOfBOOL")
            'ReadAndDisplay("Simulation.ReadValue_ArrayOfDECIMAL");
            ReadAndDisplay("Simulation.ReadValue_ArrayOfI1")
            ReadAndDisplay("Simulation.ReadValue_ArrayOfUI1")
            ReadAndDisplay("Simulation.ReadValue_ArrayOfUI2")
            ReadAndDisplay("Simulation.ReadValue_ArrayOfUI4")
            ReadAndDisplay("Simulation.ReadValue_ArrayOfINT")
            ReadAndDisplay("Simulation.ReadValue_ArrayOfUINT")
        End Sub
    End Class
End Namespace

CSharp
Python

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

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

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

            Console.WriteLine("Reading item...");
            DAVtq vtq;
            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.

                vtq = client.ReadItem("OPCLabs.KitServer.2", "Simulation.Random", DADataSource.Device);
            }
            catch (OpcException opcException)
            {
                Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message);
                return;
            }

            Console.WriteLine("Vtq: {0}", vtq);
        }
    }
}

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

# 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...')
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.
    vtq = IEasyDAClientExtension.ReadItem(client,
                                          ServerDescriptor('OPCLabs.KitServer.2'),
                                          DAItemDescriptor('Simulation.Random'),
                                          DAReadParameters(DADataSource.Device))
except OpcException as opcException:
    print('*** Failure: ' + opcException.GetBaseException().Message)
    exit()

# Display results.
print('Vtq: ', vtq, 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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