在C＃中移动Form2时移动Form1

Question

I have two forms. Form2 is being opened from Form1 , like this:

Form2.ShowDialog();

StartPosition of Form2 is configured to centerParent .

I need to fix position Form2 in Form1's center, so that when I move Form2 , Form1 also changes its location. I have tried many solutions without success.

Answer 1

You would have to include the parent reference when calling the ShowDialog function, but you would also have to record the initial position difference before using the LocationChanged event.

Form2 f2 = new Form2();
f2.StartPosition = FormStartPosition.CenterParent;
f2.ShowDialog(this);

Then in the dialog form, you can wire it up like this:

Point parentOffset = Point.Empty;
bool wasShown = false;

public Form2() {
  InitializeComponent();
}

protected override void OnShown(EventArgs e) {
  parentOffset = new Point(this.Left - this.Owner.Left,
                           this.Top - this.Owner.Top);
  wasShown = true;
  base.OnShown(e);
}

protected override void OnLocationChanged(EventArgs e) {
  if (wasShown) {
    this.Owner.Location = new Point(this.Left - parentOffset.X, 
                                    this.Top - parentOffset.Y);
  }
  base.OnLocationChanged(e);
}

This code isn't doing any error checking, demonstration code only.

Answer 2

Please do beware that this is in general a very undesirable UI feature. Dialogs are annoying because they disable the rest of the windows in the app. Which prevents the user from activating a window to have a look at its content. All that the user can do is move a dialog out of the way. You are intentionally preventing this from working.

Anyhoo, easy enough to implement with the LocationChanged event. Paste this code in the dialog form class:

    private Point oldLocation = new Point(int.MaxValue, 0);

    protected override void OnLocationChanged(EventArgs e) {
        if (oldLocation.X != int.MaxValue && this.Owner != null) {
            this.Owner.Location = new Point(
                this.Owner.Left + this.Left - oldLocation.X,
                this.Owner.Top + this.Top - oldLocation.Y);
        }
        oldLocation = this.Location;
        base.OnLocationChanged(e);
    }