# IOpcNodeModellingOrder Interface

<span class="api-meta">since v2.50</span>

<dl class="api-definition">
<dt>Namespace</dt>
<dd><a href="../namespaces/opc.uafx">Opc.UaFx</a></dd>
<dt>Assembly</dt>
<dd>Opc.UaFx.Advanced.dll</dd>
<dt>Package</dt>
<dd>Opc.UaFx.Advanced v2.59.0</dd>
<dt>Members</dt>
<dd><a href="./members">IOpcNodeModellingOrder Members</a></dd>
<dt>Also in</dt>
<dd>Opc.UaFx.Client</dd>
</dl>

Provides the modelling rule of an instance, which defines whether the instance exists in each instance of the type declaring it.

## Syntax

<div class="api-signature" data-lang="csharp">

```csharp
public interface IOpcNodeModellingOrder
```

</div>

<div class="api-signature" data-lang="vbnet">

```vbnet
Public Interface IOpcNodeModellingOrder
```

</div>

## Derived types

- [OpcNodeFieldInfo](https://www.traeger.de/en/docs/development/opcua/api/v2.59/opc.uafx.opcnodefieldinfo/)
- [OpcNodeMemberInfo](https://www.traeger.de/en/docs/development/opcua/api/v2.59/opc.uafx.opcnodememberinfo/)
- [OpcNodeMethodInfo](https://www.traeger.de/en/docs/development/opcua/api/v2.59/opc.uafx.opcnodemethodinfo/)
- [OpcNodeObjectInfo](https://www.traeger.de/en/docs/development/opcua/api/v2.59/opc.uafx.opcnodeobjectinfo/)
- [OpcNodePropertyInfo](https://www.traeger.de/en/docs/development/opcua/api/v2.59/opc.uafx.opcnodepropertyinfo/)

## Properties

| Name | Description |
|---|---|
| [IsOptional](https://www.traeger.de/en/docs/development/opcua/api/v2.59/opc.uafx.iopcnodemodellingorder/members#isoptional) | Gets a value indicating whether the instance might not necessarily exist. |
| [Rule](https://www.traeger.de/en/docs/development/opcua/api/v2.59/opc.uafx.iopcnodemodellingorder/members#rule) | Gets the modelling rule of the instance. |

## Applies to

| Package | Versions |
|---|---|
| Opc.UaFx.Advanced | v2.50–v2.59 |