Question

I have a simple plugin system in a piece of code I have written. Basically, it takes a Type, which inherits from a base class, and calls “CreateInstance” on it. I then invoke the methods in this instance. The super class lives in an SDK DLL and contains no special logic/code, just skeleton methods which are overridden when a plugin is made.

I want to be able to make an object available to the plugin developers, called “SystemLog”, which will let them write log output. My question is, how can I populate this object when I create the instance, unless I make it public with get/set accessors? I would like it to be read-only to the plugin developer, as I don’t want them overwriting it in some way.

Answer 1

I see two questions here:

1. How to pass SystemLog to the plugin wants to use it?
2. How to protect SystemLog from plugins?

Let me start from the 2-nd one. You can declare something like

public interface ISystemLogger 
{ 
  void log(String message);
}

And implement simple kind of wrapper on top of the SystemLog (instance of this wrapper you could pass to the plugins). So plugins know only about ISystemLoggger interface and not about any kind of it’s implementation

private sealed class SystemLoggerPluginWrapper : ISystemLogger
{
  private readonly SystemLog inner;

  public SystemLoggerPluginWrapper(SystemLog inner) {
    this.inner = inner;
  }

  public void log(string message)
  {
    inner.log(message);
  }
}

1. If you do not use any kind of dependency injection and create instance manually you could call pluginType.GetConstructors() iterate though them and if there is one that has exactly single parameter of type ISystemLogger instead if calling CreateInstance(Type) you call CreateInstance(Type, Object[]) and pass an instance of wrapper above.