ReadItemValue(IEasyDAClient,ServerDescriptor,DAItemDescriptor) 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.

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 _
) As Object

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

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

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

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.

Return Value

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

Exceptions

Exception	Description
OpcLabs.EasyOpc.OperationModel.OpcException	Thrown when the OPC operation fails.
System.ArgumentNullException	A `null` reference (`Nothing` in Visual Basic) is passed to a method that does not accept it as a valid argument.

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.

Example

CSharp
PowerShell
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 "../../../Assemblies/net47/OpcLabs.EasyOpcClassic.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.

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

Namespace DocExamples.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
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 "../../../Assemblies/net47/OpcLabs.EasyOpcClassic.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.

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

Namespace DocExamples.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

Requirements

Target Platforms: .NET Framework: Windows 10 (selected versions), Windows 11 (selected versions), Windows Server 2012, Windows Server 2016; .NET Core, .NET 5, .NET 6: Linux, macOS, Microsoft Windows

Reference

IEasyDAClientExtension Class
IEasyDAClientExtension Members
Overload List