Detect when a specific window in another process opens or closes

.net, c#, winforms

Solution

You can use either of these options:

- Using SetWinEventHook method

- Handling UI Automation Events (Preferred) (Suggested by Hans in comments)

Solution 1 - Using SetWinEventHook method

Using `SetWinEventHook` you can listen to some events from other processes and register a `WinEventProc` callback method to receive the event when the event raised.

Here `EVENT_SYSTEM_FOREGROUND` can help us.

We limit the event receiver to receive this event from a specific process and then we check if the text of the window which causes the event is equals to `Page Setup` then we can say the `Page Setup` window in the target process is open, otherwise we can tell the `Page Setup` dialog is not open.

To keep things simple in below example I supposed a `notepad` instance is open when your application starts, but you can also use `Win32_ProcessStartTrace` to detect when a `notepad` application runs.

To be more specific and say when the dialog is closed, you can listen to `EVENT_OBJECT_DESTROY` and detect if the message is for the window which we are interested in.

public const uint EVENT_SYSTEM_FOREGROUND = 0x0003;
public const uint EVENT_OBJECT_DESTROY = 0x8001;
public const uint WINEVENT_OUTOFCONTEXT = 0;
public delegate void WinEventDelegate(IntPtr hWinEventHook, uint eventType, IntPtr hwnd,
    int idObject, int idChild, uint dwEventThread, uint dwmsEventTime);
[DllImport("user32.dll")]
public static extern IntPtr SetWinEventHook(uint eventMin, uint eventMax, IntPtr
    hmodWinEventProc, WinEventDelegate lpfnWinEventProc, uint idProcess,
    uint idThread, uint dwFlags);
[DllImport("user32.dll")]
public static extern bool UnhookWinEvent(IntPtr hWinEventHook);
[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);

IntPtr hook = IntPtr.Zero;
protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);
    var p = System.Diagnostics.Process.GetProcessesByName("notepad").FirstOrDefault();
    if (p != null)
        hook = SetWinEventHook(EVENT_SYSTEM_FOREGROUND, EVENT_SYSTEM_FOREGROUND,
        IntPtr.Zero, new WinEventDelegate(WinEventProc), 
        (uint)p.Id, 0, WINEVENT_OUTOFCONTEXT);
}
protected override void OnFormClosing(FormClosingEventArgs e)
{
    UnhookWinEvent(hook);
    base.OnFormClosing(e);
}
void WinEventProc(IntPtr hWinEventHook, uint eventType,
    IntPtr hwnd, int idObject, int idChild, uint dwEventThread, uint dwmsEventTime)
{
    string s = "Page Setup";
    StringBuilder sb = new StringBuilder(s.Length + 1);
    GetWindowText(hwnd, sb, sb.Capacity);
    if (sb.ToString() == s)
        this.Text = "Page Setup is Open";
    else
        this.Text = "Page Setup is not open";
}

Solution 2 - Handling UI Automation Events

As suggested in comments by Hans, you can use UI Automation APIs to subscribe for `WindowOpenedEvent` and `WindowClosedEvent`.

In below example I supposed there is an open instance of `notepad` and detected opening and closing of its `Page Setup` dialog:

protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);
    var notepad = System.Diagnostics.Process.GetProcessesByName("notepad")
                        .FirstOrDefault();
    if (notepad != null)
    {
        var notepadMainWindow = notepad.MainWindowHandle;
        var notepadElement = AutomationElement.FromHandle(notepadMainWindow);
        Automation.AddAutomationEventHandler(
            WindowPattern.WindowOpenedEvent, notepadElement,
            TreeScope.Subtree, (s1, e1) =>
            {
                var element = s1 as AutomationElement;
                if (element.Current.Name == "Page Setup")
                {
                    //Page setup opened.
                    this.Invoke(new Action(() =>
                    {
                        this.Text = "Page Setup Opened";
                    }));
                    Automation.AddAutomationEventHandler(
                        WindowPattern.WindowClosedEvent, element,
                        TreeScope.Subtree, (s2, e2) =>
                        {
                            //Page setup closed.
                            this.Invoke(new Action(() =>
                            {
                                this.Text = "Closed";
                            }));
                        });
                }
            });
    }
}

protected override void OnFormClosing(FormClosingEventArgs e)
{
    Automation.RemoveAllEventHandlers();
    base.OnFormClosing(e);
}

Don't forget to add reference to `UIAutomationClient` and `UIAutomationTypes` assemblies and add `using System.Windows.Automation;`.

Problem

Original source