GUITHREADINFO gti = new GUITHREADINFO();
IntPtr hWnd = GetForegroundWindow();
uint processId;
uint activeThreadId = GetWindowThreadProcessId(hWnd, out processId);
if (GetInfo(activeThreadId, out gti))
{
    int EM_REPLACESEL = 0x00C2;

    int error = Marshal.GetLastWin32Error();
    int result = SendMessageW(gti.hwndCaret, EM_REPLACESEL, -1, passed);//3rd param doesn't change anything
    error = Marshal.GetLastWin32Error();
}

[DllImport("User32.dll", EntryPoint = "SendMessageW", SetLastError = true, CharSet = CharSet.Unicode)]
public static extern int SendMessageW(IntPtr hWnd, int uMsg, int wParam, string lParam);

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

[DllImport("user32.dll", SetLastError = true)]
static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);
//"Borrowed" code
public static bool GetInfo(uint hwnd, out GUITHREADINFO lpgui)
{
     uint lpdwProcessId;
     uint threadId = GetWindowThreadProcessId(hwnd, out lpdwProcessId);

     lpgui = new GUITHREADINFO();
     lpgui.cbSize = Marshal.SizeOf(lpgui);

    return GetGUIThreadInfo(threadId, ref lpgui);
}

[StructLayout(LayoutKind.Sequential)]
public struct RECT
{
    public int iLeft;
    public int iTop;
    public int iRight;
    public int iBottom;
}

[StructLayout(LayoutKind.Sequential)]
public struct GUITHREADINFO
{
    public int cbSize;
    public int flags;
    public IntPtr hwndActive;
    public IntPtr hwndFocus;
    public IntPtr hwndCapture;
    public IntPtr hwndMenuOwner;
    public IntPtr hwndMoveSize;
    public IntPtr hwndCaret;
    public RECT rectCaret;
}