检查类是否已实例化

Question

In my current program I have a main window and a secondary window that pops up when a button is pressed. If the secondary window is currently shown but doesn't have focus the button will instead bring it to focus.

At this time I am creating a new instance of the secondary window as the main window loads and simply checking its status with SubWindow.IsDisposed and SubWindow.CanFocus

I have found that if I do not create a new instance at the beginning SubWindow.IsDisposed throws an exception. As long as I'd previously created an instance of SubWindow the check runs fine.

My question- The current version works fine but is there a better way of doing this? It is not a huge concern, but it feels like it'd be better to be able to check for existence without having to guarantee that it has existed at least once before.

Answer 1

You could do a check on SubWindow to see if it is null. If thats the case then instantiate the SubWindow otherwise it exists.

Answer 2

VoodooChild got me on the right track. (SubWindow == null) returns false though when the window has opened once and then been closed.

Currently using

(SubWindow == null || SubWindow.IsDisposed)

which works for all cases so far.

Answer 3

You can have static counter property in your class. Increment on instantiation, decrement on disposal. That's in general... in your case you better follow VoodooChild's advice.

Answer 4

Implement the second window using a singleton pattern.

public class SecondForm : Form
{
       public static m_myInstance= new SecondForm();
       public static bool m_visible = false;

       public SecondForm ()
       {
              InitializeComponent()               
       }

       public SecondForm Instance()
       {
            return m_myInstance;
       }

      public static void Show()
      {
          ...
      }

}