understanding adapter class pattern C#

I have the below code for class adapter pattern.

 class clsCollection : CollectionBase
    {
        public virtual void Add(string str)
        {
            List.Add(str);
        }   
    }
    class clsStack : Stack
    {
        public void push(string str)
        {
            this.push(str);
        } 
    }

    class clsCollectionAdapter : clsStack
    {
        public  void Add(string str)
        {
            this.push(str);
        }
    }

how this is connected to the class clscollection here? I couldnt understand how clscollectionadapter maps with the clscollection ?

Can someone helps me to clarify this?