In C# WinForms, you can reaginate the contents of a RichTextBox control by adjusting the selection and scroll positions. Here’s an example of how you can achieve this:

private void ReaginateRichTextBox(RichTextBox richTextBox)
{
    // Save the current selection and scroll positions
    int selectionStart = richTextBox.SelectionStart;
    int selectionLength = richTextBox.SelectionLength;
    int scrollPosition = richTextBox.GetPositionFromCharIndex(0).Y;

    // Suspend layout to prevent flickering during reagination
    richTextBox.SuspendLayout();

    // Set the selection to the beginning of the text and scroll to the top
    richTextBox.Select(0, 0);
    richTextBox.ScrollToCaret();

    // Restore the previous selection and scroll positions
    richTextBox.Select(selectionStart, selectionLength);
    richTextBox.ScrollToCaret();

    // Resume layout and invalidate the control to force a redraw
    richTextBox.ResumeLayout();
    richTextBox.Invalidate();
}

To use this method, pass your RichTextBox control as an argument. For example:

ReaginateRichTextBox(richTextBox1);

This code saves the current selection and scroll positions, then suspends the layout of the RichTextBox control to prevent flickering. It sets the selection to the beginning of the text and scrolls to the top. After that, it restores the previous selection and scroll positions, resumes the layout, and invalidates the control to force a redraw.

This reaginates the contents of the RichTextBox, giving the appearance of starting from the top.