The Dispose method(s) of the System::ComponentModel::Container class is virtual, meaning that they are not implemented, but should be implemented in all derived classes. Thats why you can not call its Dispose method.

I cant really say why you're getting the same error on the GUI_New::main class. Does it have a public (or atleast protected) Dispose method that accepts a boolean argument?

NULL is a constant defined as 0, which is why you're getting the error about trying to compare an int with a Stream^. Use the keyword nullptr, which is the NULL equivalent for the managed extensions, instead of NULL.

SaveFileDialog->FileName is a property. Two parentheses after a member name denotes a call, and properties can not be called. Just remove the parentheses