ReadItem<TValue>(IEasyDAClient,ServerDescriptor,DANodeDescriptor,DAReadParameters) Method

Reads a typed named item from an OPC server. A typed value, quality and timestamp are returned. Reads a typed 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(Of TValue)( _
   ByVal client As IEasyDAClient, _
   ByVal serverDescriptor As ServerDescriptor, _
   ByVal nodeDescriptor As DANodeDescriptor, _
   ByVal readParameters As DAReadParameters _
) As DAVtq(Of TValue)

'Usage
 
Dim client As IEasyDAClient
Dim serverDescriptor As ServerDescriptor
Dim nodeDescriptor As DANodeDescriptor
Dim readParameters As DAReadParameters
Dim value As DAVtq(Of TValue)
 
value = IEasyDAClientExtension2.ReadItem(Of TValue)(client, serverDescriptor, nodeDescriptor, readParameters)

[Extension()]
[NotNull()]
public static DAVtq<TValue> ReadItem<TValue>( 
   IEasyDAClient client,
   ServerDescriptor serverDescriptor,
   DANodeDescriptor nodeDescriptor,
   DAReadParameters readParameters
)

[Extension()]
[NotNull()]
public:
static DAVtq<TValue^>^ ReadItemgeneric<typename TValue>
( 
   IEasyDAClient^ client,
   ServerDescriptor^ serverDescriptor,
   DANodeDescriptor^ nodeDescriptor,
   DAReadParameters^ readParameters
)

Parameters

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

Type Parameters

TValue: The type of item value.

Return Value

If successful, the function returns a OpcLabs.EasyOpc.DataAccess.Generic.DAVtq<TValue> 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".

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

IEasyDAClientExtension2 Class
IEasyDAClientExtension2 Members
Overload List

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