c# - How to scroll a RichTextBox control to a given point regardless of caret position

Question

In my WinForms app I have a RichTextBox control containing a long text. I need to scroll it programmatically to a given point (expressed as a character index), regardless of where the selection caret is located. I need a method like this:

//scroll the control so that the 3512th character is visible.
rtf.ScrollToPosition(3512);

All answers to similar questions that I have found use the ScrollToCaret() method, which is fine if you want to scroll to the caret position. But I need to scroll to a different position rather than the caret, and without changing the caret position. How do I do this?

Thank you.

Answer 1

You can use the same methodology implemented in the TextBoxBase.ScrollToCaret Method to accomplish this. This methodology is based on using the COM based Text Object Model implemented by the underlying RichEdit control.

The following defines the extension method ScrollToCharPosition. It uses abbreviated definitions of the ITextDocument and ITextRange interfaces.

public static class RTBExtensions
{

    public static void ScrollToCharPosition(this RichTextBox rtb, Int32 charPosition)
    {
        const Int32 WM_USER = 0x400;
        const Int32 EM_GETOLEINTERFACE = WM_USER + 60;
        const Int32 tomStart = 32;

        if (charPosition < 0 || charPosition > rtb.TextLength - 1)
        {
            throw new ArgumentOutOfRangeException(nameof(charPosition), $"{nameof(charPosition)} must be in the range of 0 to {rtb.TextLength - 1}.");
        }

        // retrieve the rtb's OLEINTERFACE and use the Interop Marshaller to cast it as an ITextDocument
        // The control calls the AddRef method for the object before returning, so the calling application must call the Release method when it is done with the object.
        ITextDocument doc = null;
        SendMessage(new HandleRef(rtb, rtb.Handle), EM_GETOLEINTERFACE, IntPtr.Zero, ref doc);
        ITextRange rng = null;
        if (doc != null)
        {
            try
            {
                rng = (RTBExtensions.ITextRange)doc.Range(charPosition, charPosition);
                rng.ScrollIntoView(tomStart);
            }
            finally
            {
                if (rng != null)
                {
                    Marshal.ReleaseComObject(rng);
                }
                Marshal.ReleaseComObject(doc);
            }
        }
    }

    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    private extern static IntPtr SendMessage(HandleRef hWnd, Int32 msg, IntPtr wParam, ref ITextDocument lParam);

    [ComImport, Guid("8CC497C0-A1DF-11CE-8098-00AA0047BE5D")]
    private interface ITextDocument
    {
        [MethodImpl((short)0, MethodCodeType = MethodCodeType.Runtime)]
        void _VtblGap1_17();
        [return: MarshalAs(UnmanagedType.Interface)]
        [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime), DispId(15)]
        ITextRange Range([In] int cp1, [In] int cp2);
    }

    [ComImport, Guid("8CC497C2-A1DF-11CE-8098-00AA0047BE5D")]
    private interface ITextRange
    {
        [MethodImpl((short)0, MethodCodeType = MethodCodeType.Runtime)]
        void _VtblGap1_49();
        [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime), DispId(0x242)]
        void ScrollIntoView([In] int Value);
    }
}

Example Usage: richtextbox1.ScrollToCharPosition(50)