How to read the highlighted/Selected Text from any window using c#.

i tried 2 approaches.

1. Send "^c" whenever user selects some thing. But in this case my clipboard is flooded with lots of unnecessary data. Sometime it copied passwords also.

so i switched my approach to 2nd method, send message method.

see this sample code

 [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 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(5000);
            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;
        }
    }

this code working fine in Notepad. But if i try to capture from another applications like Mozilla firefox, or Visual Studio IDE, it's not returning the text.

Can anybody please help me, where i am doing wrong? First of all, i have chosen the right approach?