ReadMultipleItems Method (IEasyDAClient)

Reads multiple named items from an OPC server or OPC servers, using array of argument objects as an input. Values, qualities and timestamps are returned.

Syntax

Visual Basic
C#
C++/CLI

'Declaration
 
<ElementsNotNullAttribute()>
<NotNullAttribute()>
Function ReadMultipleItems( _
   ByVal argumentsArray() As DAReadItemArguments _
) As DAVtqResult()

'Usage
 
Dim instance As IEasyDAClient
Dim argumentsArray() As DAReadItemArguments
Dim value() As DAVtqResult
 
value = instance.ReadMultipleItems(argumentsArray)

[ElementsNotNull()]
[NotNull()]
DAVtqResult[] ReadMultipleItems( 
   DAReadItemArguments[] argumentsArray
)

[ElementsNotNull()]
[NotNull()]
array<DAVtqResult^>^ ReadMultipleItems( 
   array<DAReadItemArguments^>^ argumentsArray
)

Parameters

argumentsArray: Array of arguments, one element per each OPC item involved in the operation.

Return Value

The function returns an array of OpcLabs.EasyOpc.DataAccess.OperationModel.DAVtqResult objects. The indices of elements in the output array are the same as those in the input array, .

Exceptions

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.

Remarks

The servers 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 ReadMultipleItems method only waits for the first update from the server (or until the timeout elapses) for each item; it does not wait until the quality becomes "uncertain" or "good". The function performs all individual operations in parallel, but only returns after all individual operations are completed (or their timeouts elapse).

This method does not throw an exception in case of OPC operation failures. Instead, the eventual exception related to each item is returned in Exception property of each returned OpcLabs.EasyOpc.DataAccess.OperationModel.DAVtqResult element.

The size of the input array will become the size of the output array. The element positions (indices) in the output array are the same as in the input array.

This is a multiple-operation method. In a properly written program, it does not throw any exceptions. You should therefore not put try/catch statements or similar constructs around calls to this method. The only exceptions thrown by this method are for usage errors, i.e. when your code violates the usage contract of the method, such as passing in invalid arguments or calling the method when the state of the object does not allow it. Any operation-related errors (i.e. errors that depend on external conditions that your code cannot reliably check) are indicated in the result objects returned by the method. For more information, see Multiple-operation Methods and Do not catch any exceptions with asynchronous or multiple-operation methods.

Example

CSharp
PowerShell
PHP
VB.NET

// This example shows how to read 4 items at once, and display their values, timestamps and qualities.

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

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

            DAVtqResult[] vtqResults = client.ReadMultipleItems("OPCLabs.KitServer.2",
                new DAItemDescriptor[]
                    {
                        "Simulation.Random", "Trends.Ramp (1 min)", "Trends.Sine (1 min)", "Simulation.Register_I4"
                    });

            for (int i = 0; i < vtqResults.Length; i++)
            {
                Debug.Assert(vtqResults[i] != null);

                if (vtqResults[i].Succeeded)
                    Console.WriteLine("vtqResults[{0}].Vtq: {1}", i, vtqResults[i].Vtq);
                else
                    Console.WriteLine("vtqResults[{0}] *** Failure: {1}", i, vtqResults[i].ErrorMessageBrief);
            }
        }
    }
}

# This example shows how to read 4 items at once, and display their values, timestamps and qualities.

#requires -Version 5.1
using namespace OpcLabs.EasyOpc.DataAccess

# 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

$vtqResults = [IEasyDAClientExtension]::ReadMultipleItems($client, "OPCLabs.KitServer.2", @(
    (New-Object DAItemDescriptor("Simulation.Random")),
    (New-Object DAItemDescriptor("Trends.Ramp (1 min)")),
    (New-Object DAItemDescriptor("Trends.Sine (1 min)")),
    (New-Object DAItemDescriptor("Simulation.Register_I4"))
    ))

for ($i = 0; $i -lt $vtqResults.Length; $i++) {
    $vtqResult = $vtqResults[$i]
    if ($vtqResult.Succeeded) {
        Write-Host "vtqResults[$($i)].Vtq: $($vtqResult.Vtq)"
    }
    else {
        Write-Host "vtqResults[$($i)] *** Failure: $($vtqResult.ErrorMessageBrief)"
    }
}

// This example shows how to read 4 items at once, and display their values, timestamps and qualities.

$ReadItemArguments1 = new COM("OpcLabs.EasyOpc.DataAccess.OperationModel.DAReadItemArguments");
$ReadItemArguments1->ServerDescriptor->ServerClass = "OPCLabs.KitServer.2";
$ReadItemArguments1->ItemDescriptor->ItemID = "Simulation.Random";

$ReadItemArguments2 = new COM("OpcLabs.EasyOpc.DataAccess.OperationModel.DAReadItemArguments");
$ReadItemArguments2->ServerDescriptor->ServerClass = "OPCLabs.KitServer.2";
$ReadItemArguments2->ItemDescriptor->ItemID = "Trends.Ramp (1 min)";

$ReadItemArguments3 = new COM("OpcLabs.EasyOpc.DataAccess.OperationModel.DAReadItemArguments");
$ReadItemArguments3->ServerDescriptor->ServerClass = "OPCLabs.KitServer.2";
$ReadItemArguments3->ItemDescriptor->ItemID = "Trends.Sine (1 min)";

$ReadItemArguments4 = new COM("OpcLabs.EasyOpc.DataAccess.OperationModel.DAReadItemArguments");
$ReadItemArguments4->ServerDescriptor->ServerClass = "OPCLabs.KitServer.2";
$ReadItemArguments4->ItemDescriptor->ItemID = "Simulation.Register_I4";

$arguments[0] = $ReadItemArguments1;
$arguments[1] = $ReadItemArguments2;
$arguments[2] = $ReadItemArguments3;
$arguments[3] = $ReadItemArguments4;

$Client = new COM("OpcLabs.EasyOpc.DataAccess.EasyDAClient");

$results = $Client->ReadMultipleItems($arguments);

for ($i = 0; $i < count($results); $i++)
{
    $VtqResult = $results[$i];
    if ($VtqResult->Succeeded)
        printf("results[d].Vtq.ToString()s\n", $i, $VtqResult->Vtq->ToString);
    else
        printf("results[d]: *** Failures\n", $i, $VtqResult->ErrorMessageBrief);
}

' This example shows how to read 4 items at once, and display their values, timestamps and qualities.

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

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

            Dim vtqResults() As DAVtqResult = client.ReadMultipleItems(
                "OPCLabs.KitServer.2",
                New DAItemDescriptor() {"Simulation.Random", "Trends.Ramp (1 min)", "Trends.Sine (1 min)", "Simulation.Register_I4"})

            For i = 0 To vtqResults.Length - 1
                Debug.Assert(vtqResults(i) IsNot Nothing)

                If vtqResults(i).Succeeded Then
                    Console.WriteLine("vtqResult[{0}].Vtq: {1}", i, vtqResults(i).Vtq)
                Else
                    Console.WriteLine("vtqResult[{0}] *** Failure: {1}", i, vtqResults(i).ErrorMessageBrief)
                End If
            Next i
        End Sub
    End Class
End Namespace

CSharp
PowerShell
VB.NET

// This example shows how to read 4 items from an OPC XML-DA server at once, and display their values, timestamps 
// and qualities.

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

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

            DAVtqResult[] vtqResults = client.ReadMultipleItems(
                new ServerDescriptor { UrlString = "http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx" },
                new DAItemDescriptor[]
                    {
                        "Dynamic/Analog Types/Double", 
                        "Dynamic/Analog Types/Double[]", 
                        "Dynamic/Analog Types/Int", 
                        "SomeUnknownItem"
                    });

            for (int i = 0; i < vtqResults.Length; i++)
            {
                Debug.Assert(vtqResults[i] != null);

                if (!(vtqResults[i].Exception is null))
                {
                    Console.WriteLine($"vtqResults[{i}] *** Failure: {vtqResults[i].ErrorMessageBrief}");
                    continue;
                }
                Console.WriteLine($"vtqResults[{i}].Vtq: {vtqResults[i].Vtq}");
            }
        }
    }
}

# This example shows how to read 4 items from an OPC XML-DA server at once, and display their values, timestamps 
# and qualities.

#requires -Version 5.1
using namespace OpcLabs.EasyOpc
using namespace OpcLabs.EasyOpc.DataAccess

# 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

$serverDescriptor = New-Object ServerDescriptor -Property @{
    UrlString = "http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx"
}

$vtqResults = [IEasyDAClientExtension]::ReadMultipleItems($client, 
    $serverDescriptor, 
    @(
        (New-Object DAItemDescriptor("Dynamic/Analog Types/Double")),
        (New-Object DAItemDescriptor("Dynamic/Analog Types/Double[]")),
        (New-Object DAItemDescriptor("Dynamic/Analog Types/Int")),
        (New-Object DAItemDescriptor("SomeUnknownItem"))
        ))

for ($i = 0; $i -lt $vtqResults.Length; $i++) {
    $vtqResult = $vtqResults[$i]
    if ($vtqResult.Succeeded) {
        Write-Host "vtqResults[$($i)].Vtq: $($vtqResult.Vtq)"
    }
    else {
        Write-Host "vtqResults[$($i)] *** Failure: $($vtqResult.ErrorMessageBrief)"
    }
}

' This example shows how to read 4 items from an OPC XML-DA server at once, and display their values, timestamps 
' and qualities.

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

Namespace DocExamples.DataAccess.Xml
    Partial Friend Class ReadMultipleItems
        Public Shared Sub Main1Xml()
            Dim client = New EasyDAClient()

            Dim vtqResults() As DAVtqResult = client.ReadMultipleItems(
                    New ServerDescriptor() With {.UrlString = "http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx"},
                    New DAItemDescriptor() _
                    { _
                        "Dynamic/Analog Types/Double",
                        "Dynamic/Analog Types/Double[]",
                        "Dynamic/Analog Types/Int",
                        "SomeUnknownItem"
                    })

            For i = 0 To vtqResults.Length - 1
                Debug.Assert(vtqResults(i) IsNot Nothing)

                If vtqResults(i).Exception IsNot Nothing Then
                    Console.WriteLine("vtqResult[{0}] *** Failure: {1}", i, vtqResults(i).ErrorMessageBrief)
                    Continue For
                End If
                Console.WriteLine("vtqResult[{0}].Vtq: {1}", i, vtqResults(i).Vtq)
            Next i
        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

IEasyDAClient Interface
IEasyDAClient Members