How to get focused window handler-CodePudding

I have a WPF custom virtual keyboard application. I need to host this WPF application as standalone and user should be able to use this to enter values for any window application.

How to get the current focused application window? ex: notepad, notepad or any other window applications which takes in input.

I have tried below code to get the current active window, but it returns the WPF virtual keyboard application itself since it is also in active state

var window = System.Windows.Application.Current.Windows.OfType<Window>().SingleOrDefault(w => w.IsActive);

The below code works fine however, this requires windowname as param to set the foregroundwindow

public partial class MainWindow : Window{
    
[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
    
static extern bool SetForegroundWindow(IntPtr hWnd);    
    
[DllImport("user32.dll", EntryPoint = "FindWindow", SetLastError = true)]
static extern IntPtr FindWindowByCaption(IntPtr ZeroOnly, string lpWindowName);

 private void button1_Click(object sender, RoutedEventArgs e){
    SetForegroundWindow(FindWindowByCaption(IntPtr.Zero, "Untitled - Notepad"));
    }
}

CodePudding user response：

To get the current active window handle, you could use GetForegroundWindow from the user32.dll.

In C# it should look something like

[DllImport("user32.dll")]
static extern IntPtr GetForegroundWindow();

Or in case your application window will be the active window, you could use GetWindow

[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr GetWindow(IntPtr hWnd, GetWindow_Cmd uCmd);

enum GetWindow_Cmd : uint {
    GW_HWNDFIRST = 0,
    GW_HWNDLAST = 1,
    GW_HWNDNEXT = 2,
    GW_HWNDPREV = 3,
    GW_OWNER = 4,
    GW_CHILD = 5,
    GW_ENABLEDPOPUP = 6
}

So you could pass your own window handle as first argument and GW_HWNDNEXT as second. As a result you get the handle for the window that is below your current window.

EDIT

As IInspectable mentioned, the above won't work within an WPF application.

But you could try using the following using EnumWindows:

// Get own hWnd
WindowInteropHelper windowHwnd = new(this);
IntPtr ownHwnd = windowHwnd.Handle;

// Find target hWnd
IntPtr targetHWnd = IntPtr.Zero;
NativeMethods.EnumWindows((hWnd, lParam) => 
{
  if (hWnd == ownHwnd) { return true; } // Ignore own window
  if (!NativeMethods.IsWindowVisible(hWnd)) { return true; } // Ignore hidden windows
  if (GetClassName(hWnd).Equals("Shell_TrayWnd", StringComparison.OrdinalIgnoreCase)) { return true; } // Ignore taskbar window
  if (string.IsNullOrEmpty(GetWindowTitle(hWnd))) { return true; } // Ignore windows without a title
  targetHWnd = hWnd; // Found target hWnd
  return false; // Stop iterating
}, IntPtr.Zero);

// Used functions

private string GetWindowTitle(IntPtr hWnd)
{
  int textLength = NativeMethods.GetWindowTextLength(hWnd);
  StringBuilder sb = new(textLength   1);
  NativeMethods.GetWindowText(hWnd, sb, sb.Capacity);
  return sb.ToString();
}

private string GetClassName(IntPtr hWnd)
{
  StringBuilder sb = new(256);
  NativeMethods.GetClassName(hWnd, sb, sb.Capacity);
  return sb.ToString().Trim();
}

internal static class NativeMethods
{
    [DllImport("user32.dll", SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    internal static extern bool IsWindowVisible(IntPtr hWnd);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    internal static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);

    [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
    internal static extern int GetWindowTextLength(IntPtr hWnd);

    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    internal static extern bool EnumWindows(EnumWindowsProc lpEnumFunc, IntPtr lParam);
    internal delegate bool EnumWindowsProc(IntPtr hWnd, IntPtr lParam);

    [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
    internal static extern int GetClassName(IntPtr hWnd, StringBuilder lpClassName, int nMaxCount);
}

Edit 2

To ensure the WPF hWnd is created, you could use WindowInteropHelper.EnsureHandle() method instead of the Handle property. This is important if the WPF window has not been displayed yet.