Question

was declared but never referenced是什么意思

Answer 1

was declared but never referenced
声明，但从未引用

Therefore, this static hashtable, which essentially existed for the life of the application itself, was never removing the keys that referenced each form.
因此，这个静态hashtable（在应用程序的生存期内一直存在）永远不会删除引用每个窗体的那些键。