GetFocus - Win32api help

我正试图从用户机器上打开的表单中获取所选文本。目前我已经尝试使用GetFocus，它被定义为

    '[DllImport("user32.dll")]
    static extern int GetFocus();'

在api中，它说-Retrieves the handle to the window that has the keyboard focus, if the window is attached to the calling thread's message queue.，这解释了为什么我的应用程序可以从应用程序的一部分窗口中获取选定的文本，但不能从外部窗口中获取，例如pdf。

我可以使用哪种替代的win32方法来达到这个目的？

谢谢。

编辑：这是目前的尝试

[DllImport("user32.dll"(]static extern int GetFocus((；

[DllImport("user32.dll")]
static extern bool AttachThreadInput(uint idAttach, uint idAttachTo, bool fAttach);
[DllImport("kernel32.dll")]
static extern uint GetCurrentThreadId();
[DllImport("user32.dll")]
static extern uint GetWindowThreadProcessId(int hWnd, int ProcessId);
[DllImport("user32.dll")]
static extern int GetForegroundWindow();
[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = false)]
static extern int SendMessage(int hWnd, int Msg, int wParam, StringBuilder lParam);

// second overload of SendMessage
[DllImport("user32.dll")]
private static extern int SendMessage(IntPtr hWnd, uint Msg, out int wParam, out int lParam);
const int WM_SETTEXT = 12;
const int WM_GETTEXT = 13;
private static string PerformCopy()
{
    try
    {
        //Wait 5 seconds to give us a chance to give focus to some edit window,
        //notepad for example
        System.Threading.Thread.Sleep(1000);
        StringBuilder builder = new StringBuilder(500);
        int foregroundWindowHandle = GetForegroundWindow();
        uint remoteThreadId = GetWindowThreadProcessId(foregroundWindowHandle, 0);
        uint currentThreadId = GetCurrentThreadId();
        //AttachTrheadInput is needed so we can get the handle of a focused window in another app
        AttachThreadInput(remoteThreadId, currentThreadId, true);
        //Get the handle of a focused window

        int focused = GetFocus();


        //Now detach since we got the focused handle
        AttachThreadInput(remoteThreadId, currentThreadId, false);
        //Get the text from the active window into the stringbuilder
        SendMessage(focused, WM_GETTEXT, builder.Capacity, builder);
        return builder.ToString();
    }
    catch (System.Exception oException)
    {
        throw oException;
    }
}