I’ve been faced with a problem of automatically scrolling a
richtextbox. For example: scrolling one pixel every 40 milliseconds.
Using C# under Visual Studio 2005, I’ve dropped a new
richtextbox onto my form and expected to see a method similar to
scroll(int pixels) under the
richtextbox’s available methods.
Using intellisense, we get the following relevant options:
ScrollCaret() will scroll our
textbox to a given text, surely that doesn't satisfy our needs.
Ok, so what else have we got? Surprisingly that is all.
So how do we achieve a smooth auto scrolling effect?
We're going to use the Win32 API
SendMessage method to send a scroll message to our
textbox asking it to scroll itself in pixels.
In case you are not familiar with messages, I strongly recommend reading the following explanation: message_loop.
So How Does a Scroll Message Look Like?
Using Microsoft Spy ++ we are going to view all scroll messages sent to our
First, launch your
ScrollTextBox application which should be a form with a rich/
textbox with some text in it. This will give you a scroller to play with.
Then run Spy++ and click the log message icon (ctrl + m).
Drag and drop the scope onto your
textbox, next click the Messages tab and select only the Scrollbar option under the Message groups, Click OK.
A new dialog should appear: “Massages (Window 000206B6) though you will probably get a different window number.
Now scroll your
textbox using the up/down arrows, or dragging the scroller and watch Spy logs all scroll messages received by the
A Typical Scroll Message
<0007>000206B6 S WM_VSCROLL nScrollCode:SB_THUMBTRACK nPos:16 hwndScrollBar:null)
<0007> is just the logger line number
WM_VSCROLL is the type of the message, in our case a Vertical scroll message
SB_THUMBTRACK is the first parameter of the message.
The first message parameter tells us what type of scroll is performed.
In the example I have given, I'm dragging the scroller.
A full list of first parameter values is given below:
nPos specifies the position to scroll to, It is part of the first parameter and is available only for
hwndScrollBar is the second parameter of the message and is always
Here is a list of low-order words of
wParam values to a
What our simple application will do is send a
WM_VSCROLL message to the
textbox using the
SB_THUMBTRACK parameter. It will set the high-order word of
wParam to the desired position we want the scroller to be at.
In order to send messages, we'll need to import the
sendmessage method from the Win32 API:
[DllImport("User32.dll", CharSet = CharSet.Auto, EntryPoint = "SendMessage")]
protected static extern IntPtr SendMessage
(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);
Now we can send messages to any handle we get our hands on.
int SB_LINEUP = 0;
ptrWparam = new IntPtr(SB_LINEUP);
ptrLparam = new IntPtr(0);
SendMessage(txtArea.Handle, WM_VSCROLL, ptrWparam, ptrLparam);
The above code will scroll
txtArea one line up. To scroll a
textbox by n pixels value, we'll need to do a little bit more work.
What we need is a way to determine where our scroller is at the moment. Then we could increase its position by a pixel value and all that's left is to send a new
SB_THUMBTRACK message with the new position back to the
textbox. Using the Win32 API
GetScrollInfo method, we can get all kinds of information on a scroller bar:
GetScrollInfo(IntPtr hwnd, int fnBar, ref SCROLLINFO lpsi);
Hwnd is an handle to our
fnBar is the size of the structure
lpsi is a structure that will hold information about the scroller.
public uint cbSize;
public uint fMask;
public int nMin;
public int nMax;
public uint nPage;
public int nPos;
public int nTrackPos;
For more information about the structure, visit MSDN.
What we'll need is the
nPos variable which will hold the current scroller position. So our basic scroll method will do something like this:
void scroll(IntPtr handle, int pixles)
SCROLLINFO si = new SCROLLINFO();
si.cbSize = (uint)Marshal.SizeOf(si);
si.fMask = (uint)ScrollInfoMask.SIF_ALL;
GetScrollInfo(handle, (int)ScrollBarDirection.SB_VERT, ref si);
si.nPos += pixles;
SetScrollInfo(handle, (int)ScrollBarDirection.SB_VERT, ref si, true);
IntPtr ptrWparam = new IntPtr(SB_THUMBTRACK + 0x10000 * si.nPos);
IntPtr ptrLparam = new IntPtr(0);
SendMessage(handle, WM_VSCROLL, ptrWparam, ptrLparam);
That's it now to get the filling that the
textbox is scrolling itself smoothly. Use a timer to call the above method say every 40 milliseconds and increase the scroller position by 1 pixel each time.
Surprisingly this simple "feature" is not built into the
I've spent some time looking for a quick solution for my problem, but couldn't find one.
So I've decided to post my solution. I hope it will come in handy, In case you have any questions or comments, please feel free to post or you could email me.
- 1st February, 2008: Initial post