QuickOPC User's Guide and Reference
ReadItemValue<TValue>(IEasyDAClient,String,String,String,Int32) Method



OpcLabs.EasyOpcClassicCore Assembly > OpcLabs.EasyOpc.DataAccess.Extensions Namespace > IEasyDAClientExtension2 Class > ReadItemValue Method : ReadItemValue<TValue>(IEasyDAClient,String,String,String,Int32) Method
The type of item value.
The client object that will perform the operation.
Name of the machine. Determines the computer on which the OPC server is located. May be an empty string, in which case the OPC server is assumed to exist on the local computer or at the computer specified for it by DCOM configuration.
Contains ProgID of the OPC server to read from.
Contains OPC item identifier.
The age of value you would like to obtain (in milliseconds).
Reads a typed named item from an OPC server. Only the item's typed value is returned (quality and timestamp are not returned). Reads a value of typed named item using individual parameters specifying its location, a specific data type, and value age.
Syntax
'Declaration
 
<ExtensionAttribute()>
<NotNullAttribute()>
Public Overloads Shared Function ReadItemValue(Of TValue)( _
   ByVal client As IEasyDAClient, _
   ByVal machineName As String, _
   ByVal serverClass As String, _
   ByVal itemId As String, _
   ByVal valueAge As Integer _
) As TValue
'Usage
 
Dim client As IEasyDAClient
Dim machineName As String
Dim serverClass As String
Dim itemId As String
Dim valueAge As Integer
Dim value As TValue
 
value = IEasyDAClientExtension2.ReadItemValue(Of TValue)(client, machineName, serverClass, itemId, valueAge)
[Extension()]
[NotNull()]
public static TValue ReadItemValue<TValue>( 
   IEasyDAClient client,
   string machineName,
   string serverClass,
   string itemId,
   int valueAge
)
[Extension()]
[NotNull()]
public:
static TValue^ ReadItemValuegeneric<typename TValue>
( 
   IEasyDAClient^ client,
   String^ machineName,
   String^ serverClass,
   String^ itemId,
   int valueAge
) 

Parameters

client
The client object that will perform the operation.
machineName
Name of the machine. Determines the computer on which the OPC server is located. May be an empty string, in which case the OPC server is assumed to exist on the local computer or at the computer specified for it by DCOM configuration.
serverClass
Contains ProgID of the OPC server to read from.
itemId
Contains OPC item identifier.
valueAge
The age of value you would like to obtain (in milliseconds).

Type Parameters

TValue
The type of item value.

Return Value

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

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.

The value of an argument is outside the allowable range of values as defined by the invoked method.

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.

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 typed 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 typed 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 dynamically determined based on the argument.

Requirements

Target Platforms: .NET Framework: Windows 10 (selected versions), Windows 11 (selected versions), Windows Server 2016, Windows Server 2022; .NET: Linux, macOS, Microsoft Windows

See Also