OPC Studio User's Guide and Reference
ConstantValue<TValue>(UADataVariable,TValue) Method
Example 



OpcLabs.ServerOpcUA Assembly > OpcLabs.EasyOpc.UA.NodeSpace Namespace > UADataVariableExtension Class > ConstantValue Method : ConstantValue<TValue>(UADataVariable,TValue) Method
The type of the value of the data variable.
The data variable that will be modified and returned.

The value of this parameter cannot be null (Nothing in Visual Basic).

The constant value that will be provided by the data variable.

The value of this parameter can be null (Nothing in Visual Basic).

Makes constant data variable using generic-typed value.
Syntax
'Declaration
 
<ExtensionAttribute()>
<NotNullAttribute()>
Public Overloads Shared Function ConstantValue(Of TValue)( _
   ByVal dataVariable As UADataVariable, _
   ByVal value As TValue _
) As UADataVariable
'Usage
 
Dim dataVariable As UADataVariable
Dim value As TValue
Dim value As UADataVariable
 
value = UADataVariableExtension.ConstantValue(Of TValue)(dataVariable, value)
[Extension()]
[NotNull()]
public static UADataVariable ConstantValue<TValue>( 
   UADataVariable dataVariable,
   TValue value
)
[Extension()]
[NotNull()]
public:
static UADataVariable^ ConstantValuegeneric<typename TValue>
( 
   UADataVariable^ dataVariable,
   TValue^ value
) 

Parameters

dataVariable
The data variable that will be modified and returned.

The value of this parameter cannot be null (Nothing in Visual Basic).

value
The constant value that will be provided by the data variable.

The value of this parameter can be null (Nothing in Visual Basic).

Type Parameters

TValue
The type of the value of the data variable.

Return Value

Returns the dataVariable, internally modified as defined by the method.

This method never returns null (Nothing in Visual Basic).

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.

Remarks

This method overload infers the OPC data type and value rank of the data variable from the TValue type.

The data variable is made read-only, and its minimum sampling interval is set to System.Threading.Timeout.Infinite, effectively disabling the sampling.

This is an extension method (info: C#, VB.NET). In languages that have support for extensions methods (such as C# and VB.NET), you can use the extension method as if it were a regular method on the object that is its first parameter. In other languages (such as with Python.NET), you will call the extension as a static method, and pass it the object on which it acts as its first parameter.

Example
// This example shows how to create a data variable with pre-set constant value.
// You can use any OPC UA client, including our Connectivity Explorer and OpcCmd utility, to connect to the server. 
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
// OPC client, server and subscriber examples in C# on GitHub: https://github.com/OPCLabs/Examples-OPCStudio-CSharp .
// Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
// a commercial license in order to use Online Forums, and we reply to every post.

using System;
using OpcLabs.EasyOpc.UA;
using OpcLabs.EasyOpc.UA.NodeSpace;

namespace UAServerDocExamples._UADataVariable
{
    class ConstantValue
    {
        public static void Main1()
        {
            // Instantiate the server object.
            // By default, the server will run on endpoint URL "opc.tcp://localhost:48040/".
            var server = new EasyUAServer();

            // Create a constant, read-only data variable.
            // The type of the data variable (Int32, in this case) is inferred from the value.
            server.Add(new UADataVariable("ReadThisVariable").ConstantValue(42));

            // Start the server.
            Console.WriteLine("The server is starting...");
            server.Start();

            Console.WriteLine("The server is started.");
            Console.WriteLine();

            // Let the user decide when to stop.
            Console.WriteLine("Press Enter to stop the server...");
            Console.ReadLine();

            // Stop the server.
            Console.WriteLine("The server is stopping...");
            server.Stop();

            Console.WriteLine("The server is stopped.");
        }
    }
}
' This example shows how to create a data variable with pre-set constant value.
' You can use any OPC UA client, including our Connectivity Explorer and OpcCmd utility, to connect to the server. 
'
' Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
' OPC client and subscriber examples in VB.NET on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-VBNET .
' Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
' a commercial license in order to use Online Forums, and we reply to every post.

Imports System
Imports OpcLabs.EasyOpc.UA
Imports OpcLabs.EasyOpc.UA.NodeSpace

Namespace _UADataVariable
    Partial Friend Class ConstantValue
        Shared Sub Main1()
            ' Instantiate the server object.
            ' By default, the server will run on endpoint URL "opc.tcp://localhost:48040/".
            Dim server = New EasyUAServer()

            ' Create a constant, read-only data variable.
            ' The type of the data variable (Int32, in this case) is inferred from the value.
            server.Add(New UADataVariable("ReadThisVariable").ConstantValue(42))

            ' Start the server.
            Console.WriteLine("The server is starting...")
            server.Start()

            Console.WriteLine("The server is started.")
            Console.WriteLine()

            ' Let the user decide when to stop.
            Console.WriteLine("Press Enter to stop the server...")
            Console.ReadLine()

            ' Stop the server.
            Console.WriteLine("The server is stopping...")
            server.Stop()

            Console.WriteLine("The server is stopped.")
        End Sub
    End Class
End Namespace
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