S14 How to create System Instances (prior to SDK 3.5 SP7)

The Automation Platform architecture contains several system instances that are mandatory, like Object Manager, Language Model Manager, and so on. All these instances have in common that they are automatically initialized as singletons during startup. It is often useful for plug-in developers to create something similar, i.e. a class that is automatically instantiated during startup. In many cases, this instance will have to attach to one or more events of the other system instances. This article shows how to do that correctly.

How to declare a system instance

Creating a system instance is very easy. The only thing that has to be done is to attach the _3S.CoDeSys.Core.Components.SystemInterfaceAttribute to the class.

namespace SampleNamespace
{
 [TypeGuid("my-type-guid")]
 [SystemInterface("SampleNamespace.IMyInterfaceForMySystemInstance")]
 public class MySystemInstance : IMyInterfaceForMySystemInstance
 {
 }
}

The default constructor of this class will automatically be called when the Automation Platform application is starting. Each system interface needs a name. You are free to choose whatever name you like (but it must be unique). However, it is very convenient to choose the namespace-qualified name of the interface that is implemented, for two reasons:

  1. This name is very likely to be unique.
  2. When you are accessing this system instance from a different plug-in, you do not need to duplicate the string literally, but you could use that code snippet:
var obj = ComponentManager.Singleton.InstanceFactory.GetSystemInstance(typeof(IMyInterfaceForMySystemInstance).FullName);

Here, the compiler provides the correct string, so the risk of a misspelled name is not so high.

How to attach to the events of other system instances

Often, we need system instances to attach to some events of other system instances. For example, our sample instance needs to be notified whenever the primary project changes in the IDE, so it needs to attach to the PrimaryProjectSwitched event.

The basic problem is that the order in which system instances are created during startup is not deterministic at all. That means, if the default constructor of our implementation is reached, it is not guaranteed yet that another system instance like the Engine is already present. The following pattern works around this problem reliably.

public MySystemInstance()
{
 // The AllSystemInstancesCreated event is triggered after all system instances were created.
 ComponentManager.Singleton.AllSystemInstancesCreated+= OnAllSystemInstancesCreated;
}

private void OnAllSystemInstancesCreated(object sender, EventArgs e)
{
 // All system instances have been created, so we can now access then Engine.
 AttachToEngine(); 
 // Cleanup our event subscription.
 ComponentManager.Singleton.AllSystemInstancesCreated-= OnAllSystemInstancesCreated;
}

private void AttachToEngine()
{
 // ...
}