C# interface with implementation without abstract class?

I am writing a library and I want to have an interface

public interface ISkeleton
 {
     IEnumerable<IBone> Bones { get; }
     void Attach(IBone bone);
     void Detach(IBone bone);
 }
 

The Attach() and Detach() implementation actually should be the same for every ISkeleton. Thus, it could essentially be:

public abstract class Skeleton
 {
     public IEnumerable<IBone> Bones { get { return _mBones; } }
 
     public List<IBone> _mBones = new List<IBone>();
 
     public void Attach(IBone bone)
     {
          bone.Transformation.ToLocal(this);
          _mBones.add();
     }
 
     public void Detach(IBone bone)
     {
         bone.Transformation.ToWorld(this);
          _mBones.Remove(bone);
     }
 }
 

But C# doesn't allow multiple inheritance. So among various issues, users have to remember to inherit from Skeleton every time they want to implement Skeleton.

I could use extension methods

public static class Skeleton
 {   
     public static void Attach(this ISkeleton skeleton, IBone bone)
     {
          bone.Transformation.ToLocal(skeleton);
          skeleton.Bones.add(bone);
     }
 
     public static void Detach(this ISkeleton skeleton, IBone bone)
     {
         bone.Transformation.ToWorld(this);
          skeleton.Bones.Remove(bone);
     }
 }
 

But then I need to have

public interface ISkeleton
 {   
     ICollection<IBone> Bones { get; }
 }
 

Which I do not want, because it is not covariant and users can bypass the Attach() and Detach() methods.

QUESTION: Must I really use an abstract Skeleton class or are there any or tricks and methods?

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