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There are a couple ways to fix this problem. We will do so by observing that all mouse movement in each window is processed by the pnlPhoto_MouseMove event handler. This method in turn calls the UpdatePixelData method, as does all other updates to the dialog. As a result, we can associate an existing PixelDlg form with a new window by assigning the dialog at the start of our update method. The following steps make this change in our application.

ENSURE AN EXISTING PIXELDLG FORM IS ASSIGNED TO NEW CHILD INSTANCES Action 7 Locate the UpdatePixelData method. Assign the _dlgPixel field at the beginning of the method.

protected void UpdatePixelData(int xPos, int yPos) { if (IsMdiChild) _dlgPixel = PixelDlg.GlobalDialog; . . .

This now guarantees that a child form will pick up the global PixelDlg form as needed. Of course, this change also causes the dialog to be created even when it is not used. Such a change might not be appropriate in a large application with multiple utility forms such as our pixel dialog. For our purposes, it is okay. Compile and run the program to verify that our new code works. Also realize that these changes are consistent with our non-MDI application. When a single MainForm instance is present, it will now use the global PixelDlg instance to create the dialog, and all code will work as we originally intended in chapter 8. You can test this by modifying the MyPhotos project settings to use the MainForm.Main method as the entry point. The PixelDlg form is now integrated into our MDI application. The next task is to ensure that we do not open multiple windows for the same album file. 16.4.3 OPENING AN ALBUM TWICE In our current code for the Open menu, the user selects an album and a new child window is created to contain this album. This is fine as long as the selected album has not been previously opened by the user. In the case where a MainForm window already exists for the selected album, it would be more appropriate to simply display the existing window at the top of the z-order. This can be done by searching through the list of child windows for one that displays the selected album. The MdiChildren property in the Form class retrieves the collection of child forms assigned to an MDI container form. This property can be treated like any other array to search for a matching form. This property is useful whenever a specific form is desired, as we do here. It can also be used to see if any child forms are present in an MDI application and to obtain the number of MDI child forms, although checking the ActiveMdiChild property is typically a more efficient mechanism for the former task. 551

When implementing this change, we should keep in mind the fact that forms other than our MainForm class might be contained by this array. The following steps detail a solution for this change with this fact in mind.

HANDLE AN ATTEMPT TO OPEN A DISPLAYED ALBUM Action 1 In the MainForm.cs code window, add a new AlbumFile property to retrieve the file name of the displayed album. Locate the

public string AlbumFile { get { return _album.FileName; } }

private void menuOpen_Click (object sender, System.EventArgs e) {

. . . if (dlg.ShowDialog() == DialogResult.OK) { try { // See if album is already open foreach (Form f in MdiChildren) { if (f is MainForm) { MainForm mf = (MainForm) f; if (mf.AlbumFile == dlg.FileName) { if (mf.WindowState == FormWindowState.Minimized) { mf.WindowState = FormWindowState.Normal; } mf.BringToFront(); return; } } } // Open new child window for album MainForm form = new MainForm(dlg.FileName); . . . }

If a MainForm instance is found, see if it displays the selected album. If a match is found, bring the existing album to the front of the application window.