WinForms: topmost form loses focus on hiding dialog called from it - forms

Question

I have a WinForms project with a main topmost form from which a non-modal dialog is displayed. I need to hide (not close) the dialog if it loses the input focus - no matter what was the reason (the user clicked the main form, switched to another app, etc). The following bare part of the project source code shows what is going on:
public partial class MainForm : Form
{
Form _dialog = new Form();
public MainForm()
{
InitializeComponent();
this.TopMost = true;
this.Text = "Main Form";
_dialog.Text = "Dialog";
_dialog.Owner = this;
_dialog.TopMost = true;
_dialog.Deactivate += Dialog_Deactivate;
_dialog.FormClosing += Dialog_FormClosing;
}
private void Dialog_Deactivate(object sender, EventArgs e)
{
_dialog.Hide();
}
private void Dialog_FormClosing(object sender, FormClosingEventArgs e)
{
_dialog.Hide();
e.Cancel = true;
}
private void ButtonShowDialog_Click(object sender, EventArgs e)
{
_dialog.Show();
}
}
The main problem I am trying to solve is the following. If the user opened the dialog and clicks the main form like I depicted on the following screenshot
, the dialog becomes hidden as expected, but the main form loses the focus and another app that was previously active becomes active in the background - the Windows Explorer on the next screenshot:
Is it a known issue in Windows or WinForms? How to cause the main form not to lose the focus in this construction?

Answer 1

The issue seems to be that when the Main Form is clicked, it triggers a WM_WINDOWPOSCHANGING event. Since the child dialog is open, the hwndInsertAfter handle is the child dialog. But then in the Dialog_Deactivate the child dialog is hidden, causing the Main Form to fall behind all the other windows because the hwndInsertAfter handle is no longer a visible window.
Possible solutions are 1) set the child dialog owner to null before calling Hide() or 2) use a different event, like LostFocus, i.e:
public class MainForm3 : Form {
Form _dialog = null;
public MainForm3() {
this.Text = "Main Formmmmmmmm";
Button btn = new Button { Text = "Show" };
btn.Click += ButtonShowDialog_Click;
this.Controls.Add(btn);
}
bool b = false;
protected override void WndProc(ref Message m) {
base.WndProc(ref m);
if (_dialog != null && _dialog.Visible)
b = true;
if (b)
Debug.WriteLine(m);
int WM_WINDOWPOSCHANGING = 0x46;
if (b && m.Msg == WM_WINDOWPOSCHANGING) {
var wp = Marshal.PtrToStructure<WINDOWPOS>(m.LParam);
Debug.WriteLine("hwnd: " + wp.hwnd + " " + GetWindowText(wp.hwnd));
Debug.WriteLine("hwndInsertAfter: " + wp.hwndInsertAfter + " " + GetWindowText(wp.hwndInsertAfter));
Debug.WriteLine("flags: " + wp.flags);
}
}
private void Dialog_Deactivate(object sender, EventArgs e) {
_dialog.Owner = null; // solution 1
_dialog.Hide();
}
private void _dialog_LostFocus(object sender, EventArgs e) { // solution 2
_dialog.Hide();
}
private void Dialog_FormClosing(object sender, FormClosingEventArgs e) {
if (_dialog.Visible) {
_dialog.Hide();
e.Cancel = true;
}
}
private void ButtonShowDialog_Click(object sender, EventArgs e) {
if (_dialog == null) {
_dialog = new Form();
_dialog.Text = "Dialoggggggg";
//_dialog.Deactivate += Dialog_Deactivate;
_dialog.LostFocus += _dialog_LostFocus; // solution 2, use LostFocus instead
_dialog.FormClosing += Dialog_FormClosing;
}
_dialog.Owner = this;
_dialog.Show();
}
[StructLayout(LayoutKind.Sequential)]
public struct WINDOWPOS {
public IntPtr hwnd, hwndInsertAfter;
public int x, y, cx, cy;
public SWP flags;
}
[Flags]
public enum SWP : uint {
SWP_ASYNCWINDOWPOS = 0x4000,
SWP_DEFERERASE = 0x2000,
SWP_DRAWFRAME = 0x0020,
SWP_FRAMECHANGED = 0x0020,
SWP_HIDEWINDOW = 0x0080,
SWP_NOACTIVATE = 0x0010,
SWP_NOCOPYBITS = 0x0100,
SWP_NOMOVE = 0x0002,
SWP_NOOWNERZORDER = 0x0200,
SWP_NOREDRAW = 0x0008,
SWP_NOREPOSITION = 0x0200,
SWP_NOSENDCHANGING = 0x0400,
SWP_NOSIZE = 0x0001,
SWP_NOZORDER = 0x0004,
SWP_SHOWWINDOW = 0x0040
}
[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);
private static String GetWindowText(IntPtr hWnd) {
StringBuilder sb = new StringBuilder(256);
GetWindowText(hWnd, sb, sb.Capacity);
return sb.ToString();
}
}

Answer 2

Try calling Activate() on the main form after the dialog is hidden, i.e:
private void Dialog_Deactivate(object sender, EventArgs e)
{
_dialog.Hide();
this.Activate();
}