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



OpcLabs.ServerOpcUA Assembly > OpcLabs.EasyOpc.UA.NodeSpace Namespace > UADataVariableExtension Class > WriteValueFunction Method : WriteValueFunction<TValue>(UADataVariable,Func<TValue,UAStatusCode>) 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 write value function, i.e. the function that accepts the value to be written, performs the write operation, and returns the status code that is the result of the OPC UA Write.

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

Makes data variable that consumes the attribute data using the write value function. The declared type is specified using generic type parameter.
Syntax
'Declaration
 
<ExtensionAttribute()>
<NotNullAttribute()>
Public Overloads Shared Function WriteValueFunction(Of TValue)( _
   ByVal dataVariable As UADataVariable, _
   ByVal writeValueFunction As Func(Of TValue,UAStatusCode) _
) As UADataVariable
'Usage
 
Dim dataVariable As UADataVariable
Dim writeValueFunction As Func(Of TValue,UAStatusCode)
Dim value As UADataVariable
 
value = UADataVariableExtension.WriteValueFunction(Of TValue)(dataVariable, writeValueFunction)
[Extension()]
[NotNull()]
public:
static UADataVariable^ WriteValueFunctiongeneric<typename TValue>
( 
   UADataVariable^ dataVariable,
   Func<TValue^,UAStatusCode^>^ writeValueFunction
) 

Parameters

dataVariable
The data variable that will be modified and returned.

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

writeValueFunction
The write value function, i.e. the function that accepts the value to be written, performs the write operation, and returns the status code that is the result of the OPC UA Write.

The value of this parameter cannot 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 writeValueFunction is allowed to return null, which is interpreted as a "Good" status code.

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 use a function to define what happens with the value when an OPC client writes to a data
// variable. This is an example of the push data consumption model.
// 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 WriteValueFunction
    {
        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 writable data variable and add a function that will be called when the data variable is written to.
            // The function returns a status code that indicates the outcome of the Write operation. We have chosen to only
            // allow non-negative values to be written to the variable.
            server.Add(new UADataVariable("WriteToThisVariable").WriteValueFunction<int>(
                value =>
                {
                    if (value < 0)
                    {
                        Console.WriteLine($"Value rejected: {value}");
                        return UACodeBits.BadOutOfRange;
                    }
                    Console.WriteLine($"Value written: {value}");
                    return null;    // "Good"
                }));

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

            Console.WriteLine("The server is started.");
            Console.WriteLine("Any value written to the example data variable will be displayed on the console.");
            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 use a function to define what happens with the value when an OPC client writes to a data
' variable. This is an example of the push data consumption model.
' 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 WriteValueFunction
        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 writable data variable and add a function that will be called when the data variable is written to.
            ' The function returns a status code that indicates the outcome of the Write operation. We have chosen to only
            ' allow non-negative values to be written to the variable.
            server.Add(New UADataVariable("WriteToThisVariable").WriteValueFunction(Of Integer)(
                Function(value)
                    If (value < 0) Then
                        Console.WriteLine($"Value rejected: {value}")
                        Return UACodeBits.BadOutOfRange
                    End If
                    Console.WriteLine($"Value written: {value}")
                    Return Nothing ' "Good"
                End Function
                ))


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

            Console.WriteLine("The server is started.")
            Console.WriteLine("Any value written to the example data variable will be displayed on the console.")
            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