Binary tree (count nodes) interview question

Given a LEFT and a RIGHT property that returns the underlying LEFT tree and the underlying RIGHT tree respectively, find the total count of the nodes in the tree.
class BinaryTree<T>
private BinaryTree<T> left;
private BinaryTree<T> right;
public int CountNodes<T>(this IBinaryTree<T> tree)
        // TODO: What goes here?
public int CountNodes<T>(this IBinaryTree<T> t) 
   return  1 +  t.left.CountNodes() + t.right.CountNodes();

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