You have to set the WS_EX_NOACTIVATE
extended style on your window. The easiest way is by using P/Invoke.
private const int WS_EX_NOACTIVATE = 0x08000000;
private const int GWL_EXSTYLE = -20;
[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
private static extern int GetWindowLong(IntPtr hwnd, int index);
[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
private static extern int SetWindowLong(IntPtr hwnd, int index, int newStyle);
...
var window = new Window();
window.SourceInitialized += (s, e) => {
var interopHelper = new WindowInteropHelper(window);
int exStyle = GetWindowLong(interopHelper.Handle, GWL_EXSTYLE);
SetWindowLong(interopHelper.Handle, GWL_EXSTYLE, exStyle | WS_EX_NOACTIVATE);
};
window.Show();
You can also use WPF interop class HwndSource
to create your window. Its constructor accepts the extended window style.