 <p>Your code doesn't make sense because you never use dogList. Is UpdateListBox a method in your form in the dog class? It should be a method in your form. Then you can write something like:</p> <pre><code>public void UpdateListbox() { dogListBox.Items.Clear(); foreach (Dog dog in dog.dogList) { dogListBox.Items.Add(dog); } } </code></pre> <p>The form should be updating the listbox not the dog.</p> <p>I also want to mention that even if the above works, its logically incorrect because a dog should not have a list of dogs. But thats another issue.</p>