Multiline textbox scroll with scrollbar in WPF

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C#

WPF

I've added a textbox and scroll bar to a control in WPF. When I'm clicking on the bottom arrow of the scroll bar, the textbox scrolls to the end.

My problem is: I want to scroll the textbox with the Scrollbar line by line.

How can I do this?

What I have tried:

private void DisclaimerScrollBar_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
      {

          txbDisclaimerDesc.Focus();
          if (e.NewValue > e.OldValue)
          {
              for (int i = 0; i < txbDisclaimerDesc.LineCount - 1; i++)
              {
                  txbDisclaimerDesc.ScrollToLine(i);
              }
          }
          else
              txbDisclaimerDesc.ScrollToHome();
      }