Attach same code (fields, members) to multiple child classes of different external types without duplication

Question

I have a parent class A which is coming from a library. I inherit from it in class B:A. I have the same for C:D, E:F and some more. There is plenty of code (state as fields and methods) I need to attach to each instance of B, C, E and so on. It is the same code which I do not want to duplicate.

Ideally this could be solved by having multiple parent classes, which are not supported by C#.

Instead, I thought I could use attributes to step in:

[MyAttributeWithState(foo, bar)]
public class A : B {
  // Code specific to A
}

This would allow me to attach the common code very easily without delegation or fields. But class Attributes are assigned onto types, not instances. There is only one state for the whole type. I need it per Instance.

What are my alternatives?

Answer 1

@jmcihinney had a good idea with default implementations of interface methods:

public interface IAttachedBehavior {
  protected AttachedState State { get; }

  public void DoSomething() {
    State.DoSomething();
  }
  
  public Thing[] GetThings() {
    return State.GetThings();
  }
}

public class A : B, INetworkObject {
  // Nothing to do
}