Shift + mouse wheel horizontal scrolling

Question

The use of shift + scroll wheel is fairly common for horizontal scrolling.

Both of those are fairly easy to capture. I can use the MouseWheel event with a flag set

Answer 1

In your designer file, you'll need to manually add a MouseWheel event delegate.

this.richTextBox.MouseWheel += new System.Windows.Forms.MouseEventHandler(this.RichTextBox_MouseWheel);

Then, in your code behind, you can add the following.

    private const int WM_SCROLL = 276; // Horizontal scroll 
    private const int SB_LINELEFT = 0; // Scrolls one cell left 
    private const int SB_LINERIGHT = 1; // Scrolls one line right

    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    private static extern int SendMessage(IntPtr hWnd, int wMsg, IntPtr wParam, IntPtr lParam); 

    private void RichTextBox_MouseWheel(object sender, MouseEventArgs e)
    {
        if (ModifierKeys == Keys.Shift)
        {
            var direction = e.Delta > 0 ? SB_LINELEFT : SB_LINERIGHT;

            SendMessage(this.richTextBox.Handle, WM_SCROLL, (IntPtr)direction, IntPtr.Zero);
        }
    }

For more information on the const values, see the following SO: How do I programmatically scroll a winforms datagridview control?

UPDATE

Use Alvin's solution if possible. It's way better.