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How to do synchronous scrolling of two RichTextBox (C# Form) [duplicate]

In my application's form, I have two RichTextBox objects. They will both always have the same number of lines of text. I would like to "synchronize" the vertical scrolling between these two, so that when the user changes the vertical scroll position on one, the other scrolls the same amount. How might I go about doing this?

Thanks Jay for your answer; after some more searching I also found the method described here. I'll outline it below for anyone else interested.
First, declare the following enums:
public enum ScrollBarType : uint {
SbHorz = 0,
SbVert = 1,
SbCtl = 2,
SbBoth = 3
}
public enum Message : uint {
WM_VSCROLL = 0x0115
}
public enum ScrollBarCommands : uint {
SB_THUMBPOSITION = 4
}
Next, add external references to GetScrollPos and SendMessage.
[DllImport( "User32.dll" )]
public extern static int GetScrollPos( IntPtr hWnd, int nBar );
[DllImport( "User32.dll" )]
public extern static int SendMessage( IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam );
Finally, add an event handler for the VScroll event of the appropriate RichTextBox:
private void myRichTextBox1_VScroll( object sender, EventArgs e )
{
int nPos = GetScrollPos( richTextBox1.Handle, (int)ScrollBarType.SbVert );
nPos <<= 16;
uint wParam = (uint)ScrollBarCommands.SB_THUMBPOSITION | (uint)nPos;
SendMessage( richTextBox2.Handle, (int)Message.WM_VSCROLL, new IntPtr( wParam ), new IntPtr( 0 ) );
}
In this case, richTextBox2's vertical scroll position will be synchronized with richTextBox1.

I did this for a small project a while ago, and here's the simplist solution I found.
Create a new control by subclassing RichTextBox:
public class SynchronizedScrollRichTextBox : System.Windows.Forms.RichTextBox
{
public event vScrollEventHandler vScroll;
public delegate void vScrollEventHandler(System.Windows.Forms.Message message);
public const int WM_VSCROLL = 0x115;
protected override void WndProc(ref System.Windows.Forms.Message msg) {
if (msg.Msg == WM_VSCROLL) {
if (vScroll != null) {
vScroll(msg);
}
}
base.WndProc(ref msg);
}
public void PubWndProc(ref System.Windows.Forms.Message msg) {
base.WndProc(ref msg);
}
}
Add the new control to your form and for each control explicitly notify the other instances of the control that its vScroll position has changed. Somthing like this:
private void scrollSyncTxtBox1_vScroll(Message msg) {
msg.HWnd = scrollSyncTxtBox2.Handle;
scrollSyncTxtBox2.PubWndProc(ref msg);
}
I think this code has problems if all the 'linked' controls don't have the same number of displayable lines.

[Visual Studio C# 2010 Express, v10.0.30319 on a Windows 7 64bit installation]
I've used Donut's solution posted above, but found a problem when scrolling to the end of RichTextBoxes that contain many lines.
If the result of GetScrollPos() is >0x7FFF then when nPos is shifted, the top bit is set. The creation of the IntPtr with the resulting wParam variable will then fail with an OverflowException. You can easily test this with the following (the second line will fail):
IntPtr ip = new IntPtr(0x7FFF0000);
IntPtr ip2 = new IntPtr(0x80000000);
A version of SendMessage() that uses UIntPtr would appear to be a solution, but I couldn't get that to work. So, I've use the following:
[DllImport("User32.dll")]
public extern static int SendMessage(IntPtr hWnd, uint msg, UInt32 wParam, UInt32 lParam);
This should be good up to 0xffff, but would fail after that. I've not yet experienced a >0xffff result from GetScrollPos(), and assume that User32.dll is unlikely to have a 64bit version of SendCommand(), but any solutions to that problem would be greatly appreciated.

const int WM_USER = 0x400;
const int EM_GETSCROLLPOS = WM_USER + 221;
const int EM_SETSCROLLPOS = WM_USER + 222;
[System.Runtime.InteropServices.DllImport("user32.dll")]
static extern int SendMessage(IntPtr hWnd, int msg, int wParam, ref Point lParam);
private void RichTextBox1_VScroll(object sender, EventArgs e)
{
Point pt;
SendMessage(RichTextBox1.Handle, EM_GETSCROLLPOS, 0, ref pt);
SendMessage(RichTextBox2.Handle, EM_SETSCROLLPOS, 0, ref pt);
}
private void RichTextBox2_VScroll(object sender, EventArgs e)
{
Point pt;
SendMessage(RichTextBox1.Handle, EM_GETSCROLLPOS, 0, ref pt);
SendMessage(RichTextBox2.Handle, EM_SETSCROLLPOS, 0, ref pt);
}

A variation of Jay's subclass approach can be found in Joseph Kingry's answer here: Synchronizing Multiline Textbox Positions in C#.
Joseph's approach also subclasses but doesn't require a _VScroll event handler. I used that approach to do a 3-way bind between 3 boxes and added WM_HSCROLL.

#Sudhakar MuthuKrishnan's answer needs some fixes, but works. Thanks!
First GetScrollPos which rised event and then set scroll position for others.
private void RichTextBox1_VScroll(object sender, EventArgs e)
{
Point pt = new Point();
SendMessage(RichTextBox1.Handle, EM_GETSCROLLPOS, 0, ref pt);
SendMessage(RichTextBox2.Handle, EM_SETSCROLLPOS, 0, ref pt);
}
private void RichTextBox2_VScroll(object sender, EventArgs e)
{
Point pt = new Point();
SendMessage(RichTextBox2.Handle, EM_GETSCROLLPOS, 0, ref pt);
SendMessage(RichTextBox1.Handle, EM_SETSCROLLPOS, 0, ref pt);
}

How do I embed tabtip.exe inside windows

I'm trying to embed the osk in a wpf window or a user control and I've found the code below and it's working for notepad but for tabtip.exe, it's saying that it doesn't have a graphical interface??
WaitForInputIdle failed. This could be because the process does not have a graphical interface.
I tried letting it sleep for awhile instead of calling waitForInputIdle method but it throws another exception:
Process has exited, so the requested information is not available.
But in my task manager, I can still see TabTip.exe running.
namespace WpfApplication1
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
private System.Windows.Forms.Panel _panel;
private Process _process;
public MainWindow()
{
InitializeComponent();
_panel = new System.Windows.Forms.Panel();
windowsFormsHost1.Child = _panel;
}
[DllImport("user32.dll")]
private static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);
[DllImport("user32.dll", SetLastError = true)]
private static extern int GetWindowLong(IntPtr hWnd, int nIndex);
[DllImport("user32")]
private static extern IntPtr SetParent(IntPtr hWnd, IntPtr hWndParent);
[DllImport("user32")]
private static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, int uFlags);
private const int SWP_NOZORDER = 0x0004;
private const int SWP_NOACTIVATE = 0x0010;
private const int GWL_STYLE = -16;
private const int WS_CAPTION = 0x00C00000;
private const int WS_THICKFRAME = 0x00040000;
protected override void OnClosing(System.ComponentModel.CancelEventArgs e)
{
base.OnClosing(e);
if (_process != null)
{
_process.Refresh();
_process.Close();
}
}
private void ResizeEmbeddedApp()
{
if (_process == null)
return;
SetWindowPos(_process.MainWindowHandle, IntPtr.Zero, 0, 0, (int)_panel.ClientSize.Width, (int)_panel.ClientSize.Height, SWP_NOZORDER | SWP_NOACTIVATE);
}
protected override Size MeasureOverride(Size availableSize)
{
Size size = base.MeasureOverride(availableSize);
ResizeEmbeddedApp();
return size;
}
private void button1_Click_1(object sender, RoutedEventArgs e)
{
button1.Visibility = Visibility.Hidden;
ProcessStartInfo psi = new ProcessStartInfo("C:\\Program Files\\Common Files\\microsoft shared\\ink\\TabTip.exe");
_process = Process.Start(psi);
Thread.Sleep(500);
//_process.WaitForInputIdle();
SetParent(_process.MainWindowHandle, _panel.Handle);
// remove control box
int style = GetWindowLong(_process.MainWindowHandle, GWL_STYLE);
style = style & ~WS_CAPTION & ~WS_THICKFRAME;
SetWindowLong(_process.MainWindowHandle, GWL_STYLE, style);
// resize embedded application & refresh
ResizeEmbeddedApp();
}
}
}
Edit: Inspired by rene's comment, I've tried to obtain the window ptr as below and used spy++ to verify that the address that FindWindow gives is pointing to the correct window, but it's still not moving:
IntPtr KeyboardWnd = FindWindow("IPTip_Main_Window", null);
int style = GetWindowLong(KeyboardWnd, GWL_STYLE);
style = style & ~WS_CAPTION & ~WS_THICKFRAME;
SetWindowLong(KeyboardWnd, GWL_STYLE, style);
SetWindowPos(KeyboardWnd, IntPtr.Zero, 0, 0, (int)_panel.ClientSize.Width, (int)_panel.ClientSize.Height, SWP_NOZORDER | SWP_NOACTIVATE);
Edit 2: My first thought was that tab tip couldn't be resized, but then I noticed a behavior when I try to move the window across two different screen, it'll resize to fit the screen size, so I'm sure there must be a way to resize, so I started spy++(x64) to check :
Edit 3: after tinkering abit with user32 api and no progress, I've tried to use a memory scanner to scan for the x and y position of tabtip and change it, however, it's not refreshing until a repaint is triggered, I'm wondering the feasibility going down that path.

Can you try to run your handle code in STA thread? I had a similar issue with native window, which I had resolved using STA thread.
var thread = new Thread(() => {
// Your code here
});
thread.TrySetApartmentState(ApartmentState.STA);
thread.Start();

I had a similar problem, and the reason I had it was that I started a program that needed to be run by an administrator with a non-administrative program, and it would pop up with WaitForInputIdle failed. This could be because the process does not have a graphical interface, so I assume you try starting your program with an administrator

Perform a mouse Click event on another Application using C#

what i need to do is that I need to control another application installed on the same machine using my custom application. For example if i needed to use the standard windows calculator I would simply send the input events to the calculator. I have used some code snippets to make this possible and I have now triggered both mouse and keyboard events. but the problem is that i can be sure that the keyboard event will hit the target application because it has the process handle. but i cannot be sure about the mouse. and also if the target application goes into background, i cannot initiate mouse clicks on it. I need help to find a way to make sure that the mouse click is done on the application only.
I need to send mouse co-ordinates and click as well. for example "sendMouseClick("Notepad", 100, 400); which will send a click to Notepad, even though it stays minimized.
IMPORTANT NOTE
A similar question is answered previously but that is in reference to first finding the state of the other application and then sending the inputs either keyboard or mouse, what i need to do is to send an application a set of instructions that must work whether the application is in foreground or not.
For "The other Guys":: if you dont want to help or cant help, thats okay but please do know that i havent stolen the question or anything. I simply want to achieve this task in C#.
The code I have to simulate keyboard key Press is:
using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Drawing;
using System.Windows.Forms;
namespace SimulateKeyPress
{
partial class Form1 : Form
{
private Button button1 = new Button();
private Button button2 = new Button();
private Button button3 = new Button();
[STAThread]
public static void Main()
{
Application.EnableVisualStyles();
Application.Run(new Form1());
}
public Form1()
{
button1.Location = new Point(10, 10);
button1.TabIndex = 1;
button1.Text = "Click to automate Calculator";
button1.AutoSize = true;
button1.Click += new EventHandler(button1_Click);
button2.Location = new Point(150, 140);
button2.TabIndex = 0;
button2.Text = "Click to Exit Calculator";
button2.AutoSize = true;
button2.Location = new Point(80, 80);
button2.TabIndex = 2;
button2.Text = "Click to Run Calculator";
button2.AutoSize = true;
button2.Click += new EventHandler(button2_Click);
this.DoubleClick += new EventHandler(Form1_DoubleClick);
this.Controls.Add(button1);
this.Controls.Add(button2);
// this.Controls.Add(button3);
}
// Get a handle to an application window.
[DllImport("USER32.DLL", CharSet = CharSet.Unicode)]
public static extern IntPtr FindWindow(string lpClassName,
string lpWindowName);
// Activate an application window.
[DllImport("USER32.DLL")]
public static extern bool SetForegroundWindow(IntPtr hWnd);
// Send a series of key presses to the Calculator application.
private void button1_Click(object sender, EventArgs e)
{
// Get a handle to the Calculator application. The window class
// and window name were obtained using the Spy++ tool.
IntPtr calculatorHandle = FindWindow("CalcFrame", "Calculator");
//Process firstProc = new Process();
//firstProc.StartInfo.FileName = "calc.exe";
//firstProc.EnableRaisingEvents = true;
//firstProc.Start();
// Verify that Calculator is a running process.
if (calculatorHandle == IntPtr.Zero)
{
MessageBox.Show("Calculator is not running.");
return;
}
// Make Calculator the foreground application and send it
// a set of calculations.
SetForegroundWindow(calculatorHandle);
SendKeys.SendWait("1024");
SendKeys.SendWait("*");
SendKeys.SendWait("32");
SendKeys.SendWait("=");
}
private void button2_Click(object sender, EventArgs e)
{
System.Diagnostics.Process.Start("calc.exe");
}
private void button3_Click(object sender,EventArgs e)
{
Process [] proc =Process.GetProcessesByName("Calculator");
proc[0].Kill();
}
// Send a key to the button when the user double-clicks anywhere
// on the form.
private void Form1_DoubleClick(object sender, EventArgs e)
{
// Send the enter key to the button, which raises the click
// event for the button. This works because the tab stop of
// the button is 0.
SendKeys.Send("{ENTER}");
}
}
}
the previous help on stack overflow, msdn and other sites provides the code to simulate a mouse click in the same application. But i need to send mouse hits to another application.

Maybe this could help you
Code
The task
Getting the mouse's current position
Sending the mouse event
Windows forms
...
using System.Runtime.InteropServices;
namespace yournamespace
{
public partial class yourclassname
{
[DllImport("user32.dll", CharSet = CharSet.Auto, CallingConvention = CallingConvention.StdCall)]
public static extern void mouse_event(uint dwFlags, uint dx, uint dy, uint cButtons, uint dwExtraInfo);
private const int MOUSEEVENTF_LEFTDOWN = 0x02;
private const int MOUSEEVENTF_LEFTUP = 0x04;
private const int MOUSEEVENTF_RIGHTDOWN = 0x08;
private const int MOUSEEVENTF_RIGHTUP = 0x10;
int X = Cursor.Position.X;
int Y = Cursor.Position.Y;
mouse_event(MOUSEEVENTF_LEFTDOWN | MOUSEEVENTF_LEFTUP, X, Y, 0, 0);
}
}
WPF
Things are a bit harder in WPF
double mousePointX;
double mousePointY;
[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool GetCursorPos(out POINT lpPoint);
[StructLayout(LayoutKind.Sequential)]
public struct POINT
{
public int X;
public int Y;
public POINT(int x, int y)
{
this.X = x;
this.Y = y;
}
}
private void WritePoint(object sender, RoutedEventArgs e)
{
POINT p;
if (GetCursorPos(out p))
{
System.Console.WriteLine(Convert.ToString(p.X) + ";" + Convert.ToString(p.Y));
}
}
[DllImport("User32.dll")]
static extern IntPtr GetDC(IntPtr hwnd);
[DllImport("gdi32.dll")]
static extern int GetDeviceCaps(IntPtr hdc, int nIndex);
[DllImport("user32.dll")]
static extern bool ReleaseDC(IntPtr hWnd, IntPtr hDC);
private Point ConvertPixelsToUnits(int x, int y)
{
// get the system DPI
IntPtr dDC = GetDC(IntPtr.Zero); // Get desktop DC
int dpi = GetDeviceCaps(dDC, 88);
bool rv = ReleaseDC(IntPtr.Zero, dDC);
// WPF's physical unit size is calculated by taking the
// "Device-Independant Unit Size" (always 1/96)
// and scaling it by the system DPI
double physicalUnitSize = (1d / 96d) * (double)dpi;
Point wpfUnits = new Point(physicalUnitSize * (double)x,
physicalUnitSize * (double)y);
return wpfUnits;
}
private void WriteMouseCoordinatesInWPFUnits()
{
POINT p;
if (GetCursorPos(out p))
{
Point wpfPoint = ConvertPixelsToUnits(p.X, p.Y);
System.Console.WriteLine(Convert.ToString(wpfPoint.X) + ";" + Convert.ToString(wpfPoint.Y));
mousePointY = wpfPoint.Y;
mousePointX = wpfPoint.X
}
}
Now the most important part of the code
[DllImport("user32.dll", CharSet = CharSet.Auto, CallingConvention = CallingConvention.StdCall)]
public static extern void mouse_event(uint dwFlags, uint dx, uint dy, uint cButtons, uint dwExtraInfo);
private const int MOUSEEVENTF_LEFTDOWN = 0x02;
private const int MOUSEEVENTF_LEFTUP = 0x04;
private const int MOUSEEVENTF_RIGHTDOWN = 0x08;
private const int MOUSEEVENTF_RIGHTUP = 0x10;
...
mouse_event(MOUSEEVENTF_LEFTDOWN | MOUSEEVENTF_LEFTUP, Convert.ToUInt32(mousePointX), Convert.ToUInt32(mousePointY), 0, 0);
...
Warning
The code is tested
The code is not a "copy & paste code

WPF Window fixed width mouse cursor

I am building a WPF 4.5 Application that has controls that enable the User to "Lock" and "Unlock" the Application's Height.
In order to lock the Height, I am following this StackOverflow answer regarding setting the MinHeight and MaxHeight to the same value.
In order to unlock the Height, I set MinHeight=0 and MaxHeight=double.PositiveInfinity
This all appears to be working fine.
The problem I'm encountering that I haven't been able to solve is that when the height is "Locked", when I mouseover the right edge of the Application Window, the cursor turns into the horizontal resize cursor.
Is there a way I can disable that so that the cursor stays as the regular pointer in WPF?
I am on WPF 4.5.
I saw this post that has answers showing how to do it in Win32: WPF: Make window unresizeable, but keep the frame?.
This post is over 3 years old, and I was just wondering (hoping) maybe WPF has evolved since then.
Thank you very much in advance!
Philip

On your startup Window (MainWindow.xaml), try making a binding for the Window's ResizeMode property and then modifying it to 'NoResize' when you don't want it to be resizable. To make it resizable, change it to 'CanResize'.
Hope that helps!

You need to set MinWidth = MaxWidth = Width = your desired width as mentioned in this StackOverflow answer regarding setting the MinHeight and MaxHeight to the same value.
In addition you need to hook the winproc for your window and process the WM_NCHITTEST message.
#region Vertical Resize Only
// ReSharper disable InconsistentNaming
private const int WM_NCHITTEST = 0x0084;
private const int HTBORDER = 18;
private const int HTBOTTOM = 15;
private const int HTBOTTOMLEFT = 16;
private const int HTBOTTOMRIGHT = 17;
private const int HTLEFT = 10;
private const int HTRIGHT = 11;
private const int HTTOP = 12;
private const int HTTOPLEFT = 13;
private const int HTTOPRIGHT = 14;
[DllImport("user32.dll", CharSet = CharSet.Auto)]
public static extern IntPtr DefWindowProc(
IntPtr hWnd,
int msg,
IntPtr wParam,
IntPtr lParam);
// ReSharper restore InconsistentNaming
#endregion Vertical Resize Only
public CanConfigurationDialog()
{
InitializeComponent();
Loaded += MainWindowLoaded;
}
#region Vertical Resize Only
private void MainWindowLoaded(object sender, RoutedEventArgs e)
{
try
{
// Obtain the window handle for WPF application
var mainWindowPtr = new WindowInteropHelper(this).Handle;
var mainWindowSrc = HwndSource.FromHwnd(mainWindowPtr);
mainWindowSrc?.AddHook(WndProc);
}
catch (Exception)
{
;
}
}
private static IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
{
// Override the window hit test
// and if the cursor is over a resize border,
// return a standard border result instead.
if (msg == WM_NCHITTEST)
{
handled = true;
var htLocation = DefWindowProc(hwnd, msg, wParam, lParam).ToInt32();
switch (htLocation)
{
case HTTOP:
case HTTOPLEFT:
case HTTOPRIGHT:
htLocation = HTTOP;
break;
case HTBOTTOM:
case HTBOTTOMLEFT:
case HTBOTTOMRIGHT:
htLocation = HTBOTTOM;
break;
case HTLEFT:
case HTRIGHT:
htLocation = HTBORDER;
break;
}
return new IntPtr(htLocation);
}
return IntPtr.Zero;
}
#endregion Vertical Resize Only
This will prevent the horizontal resize cursor from being displayed!
Q.E.D.

How to prevent TextBox auto scrolls when append text?

I have a multi-line TextBox with a vertical scrollbar that logs data from real-time processing. Currently, whenever a new line is added by textBox.AppendText(), the TextBox scrolls to the bottom so you can see the last entry, this great. But I have a checkbox to indicate whether TextBox is allowed to auto-scroll. Is there any way to do this?
Note:
I want to use the TextBox because the added text has multi-lines and alignment by whitespace, so it's not simple to use with a ListBox or a ListView.
I tried to add a new line by textBox.Text += text, but the TextBox constantly scrolls to the top.
If we have a solution to do that, then one more question is how to prevent the TextBox auto scrolls when the user uses the scrollbar to view somewhere else in the TextBox while the TextBox appends text?
private void OnTextLog(string text)
{
if (chkAutoScroll.Checked)
{
// This always auto scrolls to the bottom.
txtLog.AppendText(Environment.NewLine);
txtLog.AppendText(text);
// This always auto scrolls to the top.
//txtLog.Text += Environment.NewLine + text;
}
else
{
// I want to append the text without scrolls right here.
}
}
Update 1: As saggio suggests, I also think the solution to this problem is to determine the position of the first character in the current text that is displayed in the TextBox before appending text and restoring it after that. But how to do this? I tried to record the current cursor position like this, but it did not help:
int selpoint = txtLog.SelectionStart;
txtLog.AppendText(Environment.NewLine);
txtLog.AppendText(text);
txtLog.SelectionStart = selpoint;
Update 2 (the issue was resolved): I found a solution that can solve my issue here on Stack Overflow. I have optimized their code to suit my case as follows:
// Constants for extern calls to various scrollbar functions
private const int SB_VERT = 0x1;
private const int WM_VSCROLL = 0x115;
private const int SB_THUMBPOSITION = 0x4;
[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern int GetScrollPos(IntPtr hWnd, int nBar);
[DllImport("user32.dll")]
private static extern int SetScrollPos(IntPtr hWnd, int nBar, int nPos, bool bRedraw);
[DllImport("user32.dll")]
private static extern bool PostMessageA(IntPtr hWnd, int nBar, int wParam, int lParam);
[DllImport("user32.dll")]
private static extern bool GetScrollRange(IntPtr hWnd, int nBar, out int lpMinPos, out int lpMaxPos);
private void AppendTextToTextBox(TextBox textbox, string text, bool autoscroll)
{
int savedVpos = GetScrollPos(textbox.Handle, SB_VERT);
textbox.AppendText(text + Environment.NewLine);
if (autoscroll)
{
int VSmin, VSmax;
GetScrollRange(textbox.Handle, SB_VERT, out VSmin, out VSmax);
int sbOffset = (int)((textbox.ClientSize.Height - SystemInformation.HorizontalScrollBarHeight) / (textbox.Font.Height));
savedVpos = VSmax - sbOffset;
}
SetScrollPos(textbox.Handle, SB_VERT, savedVpos, true);
PostMessageA(textbox.Handle, WM_VSCROLL, SB_THUMBPOSITION + 0x10000 * savedVpos, 0);
}
private void OnTextLog(string text)
{
AppendTextToTextBox(txtLog.Text, Environment.NewLine + text, chkAutoScroll.Checked);
}
Another way:
private const int SB_VERT = 0x1;
private const int WM_VSCROLL = 0x115;
private const int SB_THUMBPOSITION = 0x4;
private const int SB_BOTTOM = 0x7;
[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern int GetScrollPos(IntPtr hWnd, int nBar);
[DllImport("user32.dll")]
private static extern int SetScrollPos(IntPtr hWnd, int nBar, int nPos, bool bRedraw);
[DllImport("user32.dll")]
private static extern bool PostMessageA(IntPtr hWnd, int nBar, int wParam, int lParam);
private void AppendTextToTextBox(TextBox textbox, string text, bool autoscroll)
{
int savedVpos = GetScrollPos(textbox.Handle, SB_VERT);
textbox.AppendText(text + Environment.NewLine);
if (autoscroll)
{
PostMessageA(textbox.Handle, WM_VSCROLL, SB_BOTTOM, 0);
}
else
{
SetScrollPos(textbox.Handle, SB_VERT, savedVpos, true);
PostMessageA(textbox.Handle, WM_VSCROLL, SB_THUMBPOSITION + 0x10000 * savedVpos, 0);
}
}
I post these solutions for those who have a similar issue. Thanks for cgyDeveloper's source code.
Does anyone have a more straightforward way?

This seems pretty straight forward but I may be missing something. Use append text to scroll to the position if Autochecked is true and just add the text if you do not wish to scroll.
Update...I was missing something. You want to set the selection point and then scroll to the caret. See below.
if (chkAutoScroll.Checked)
{
// This always auto scrolls to the bottom.
txtLog.AppendText(Environment.NewLine);
txtLog.AppendText(text);
// This always auto scrolls to the top.
//txtLog.Text += Environment.NewLine + text;
}
else
{
int caretPos = txtLog.Text.Length;
txtLog.Text += Environment.NewLine + text;
txtLog.Select(caretPos, 0);
txtLog.ScrollToLine(txtLog.GetLineIndexFromCharacterIndex(caretPos));
}

You have to do it something like this,
textBox1.AppendText("Your text here");
// this selects the index zero as the location of your caret
textBox1.Select(0, 0);
// Scrolls to the caret :)
textBox1.ScrollToCaret();
Tested and working on VS2010 c# Winforms, i dont know about WPF but google probably has the answer for you.

The desired actions are:
To turn on autoscrolling when the scrollbar is dragged to the bottom.
To turn off autoscrolling when the scollbar is dragged anywhere else.
create the following class
public class AutoScrollTextBox : TextBox
{
protected override void OnInitialized(EventArgs e)
{
base.OnInitialized(e);
VerticalScrollBarVisibility = ScrollBarVisibility.Auto;
HorizontalScrollBarVisibility = ScrollBarVisibility.Disabled;
}
protected override void OnTextChanged(TextChangedEventArgs e)
{
bool isScrolledToEnd = VerticalOffset + ViewportHeight == ExtentHeight;
base.OnTextChanged(e);
CaretIndex = Text.Length;
if (isScrolledToEnd)
{
ScrollToEnd();
}
}
}
And replace the TextBox with AutoScrollTextBox in your XML and append to the TextToDisplay binding as stuff arrives for display
<local:AutoScrollTextBox Text="{Binding TextToDisplay }" />