How to scroll Window in other process with Win32API

Question

Im trying to create a program where I can send some process id of a process (that may be firefox, ie, notepad etc) to a method that scrolls window of the process.

I have been trying with GetScrollBarInfo and SetScrollPos which I found at pinvoke without any success. Im not sure if this is the right way or not. I started playing with GetScrollBarInfo, but it doesn't seem to work.

I tried the code found at http://www.pinvoke.net/default.aspx/user32.getscrollbarinfo

[StructLayout(LayoutKind.Sequential)]
public struct SCROLLBARINFO
{
    public int cbSize;
    public Rectangle rcScrollBar;
    public int dxyLineButton;
    public int xyThumbTop;
    public int xyThumbBottom;
    public int reserved;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 6)]
    public int[] rgstate;
}

private const uint OBJID_HSCROLL = 0xFFFFFFFA;
private const uint OBJID_VSCROLL = 0xFFFFFFFB;
private const uint OBJID_CLIENT = 0xFFFFFFFC;

private int Scroll(int ProcessID) 
{
    IntPtr handle = Process.GetProcessById(ProcessID).MainWindowHandle;
    SCROLLBARINFO psbi = new SCROLLBARINFO();
    psbi.cbSize = Marshal.SizeOf(psbi);
    int nResult = GetScrollBarInfo(handle, OBJID_CLIENT, ref psbi);
    if (nResult == 0)
    {
        int nLatError = Marshal.GetLastWin32Error();
    }
}

GetLastWin32Error() returns errorcode 122 which means "The data area passed to a system call is too small", according to http://msdn.microsoft.com/en-us/library/windows/desktop/ms681382(v=vs.85).aspx

Im not sure what I do wrong. How can I solve this?

Answer 1

You could send a WM_MOUSEWHEEL message to do what you want. For example, to scroll down once in a new notepad window using C++:

HWND hwnd = FindWindowEx(FindWindow(NULL, "Untitled - Notepad"), NULL, "Edit", NULL);
RECT r;
GetClientRect(hwnd, &r);
SendMessage(hwnd, WM_MOUSEWHEEL, MAKEWPARAM(0, WHEEL_DELTA * -1), MAKELPARAM(r.right / 2, r.bottom / 2));

To adapt that to C#, you could do something such as this:

[DllImport("user32.dll", CharSet = CharSet.Auto)]
static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, Int32 wParam, ref Point lParam);

private void ScrollWindow(IntPtr hwnd, Point p, int scrolls = -1)
{
    SendMessage(hwnd, WM_MOUSEWHEEL, (WHEEL_DELTA * scrolls) << 16, ref p);
}

Which could be used to scroll down once in a new notepad window like this:

//Imports
[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);
...
//Actual code
IntPtr hwnd = FindWindowEx(FindWindow(null, "Untitled - Notepad"), IntPtr.Zero, "Edit", null);
Point p = new Point(0, 0);
ScrollWindow(hwnd, p);

Some programs will require the lParam sent to be a point that's actually above the scrolled area, while others such as notepad will not.

Answer 2

If you're trying to scroll the window of another process, you need to, in effect, simulate clicks on the scroll bar or key presses. The cleanest way to do that is to use UI Automation, which has both .NET and native interfaces.

By asking for the scrollbar info, you're simply getting information about how the scrollbar is displayed. That's not going to give you a way to scroll the window content. You have to get the target application to scroll the content by making it think the user is operating the scrollbar.