Changing the line spacing in a RichTextBox control

I cooked up a small code sample that demonstrates how to easily change the line spacing of text in a RichTextBox control. This example was written by me as a solution to a question in Q&A[^]. To use the below code, you have to make a WinForm project and add six buttons, a RichTextBox control and normal TextBox to it.

Inside the structure PARAFORMAT, there are two relevant fields that play closely together: bLineSpacingRule defines how dyLineSpacing is supposed to be interpreted.

0

Single spacing. The dyLineSpacing member is ignored.

1

One-and-a-half spacing. The dyLineSpacing member is ignored.

2

Double spacing. The dyLineSpacing member is ignored.

3

The dyLineSpacing member specifies the spacing from one line to the next, in twips. However, if dyLineSpacing specifies a value that is less than single spacing, the control displays single-spaced text.

4

The dyLineSpacing member specifies the spacing from one line to the next, in twips. The control uses the exact spacing specified, even if dyLineSpacing specifies a value that is less than single spacing.

5

The value of dyLineSpacing / 20 is the spacing, in lines, from one line to the next. Thus, setting dyLineSpacing to 20 produces single-spaced text, 40 is double spaced, 60 is triple spaced, and so on.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Windows.Forms;
namespace RichTextBoxLineSpacing
{
    public partial class Form1 : Form
    {
        [DllImport("user32", CharSet = CharSet.Auto)]
        private static extern IntPtr SendMessage(HandleRef hWnd, int msg, int wParam, ref PARAFORMAT lParam);
        const int PFM_SPACEBEFORE = 0x00000040;
        const int PFM_SPACEAFTER  = 0x00000080;
        const int PFM_LINESPACING = 0x00000100;
        const int SCF_SELECTION = 1;
        const int EM_SETPARAFORMAT = 1095;
        public Form1()
        {
            InitializeComponent();
        }
        private void setLineFormat(byte rule, int space)
        {
            PARAFORMAT fmt = new PARAFORMAT();
            fmt.cbSize = Marshal.SizeOf(fmt);
            fmt.dwMask = PFM_LINESPACING;
            fmt.dyLineSpacing = space;
            fmt.bLineSpacingRule = rule;
            richTextBox1.SelectAll();
            SendMessage( new HandleRef( richTextBox1, richTextBox1.Handle ),
                         EM_SETPARAFORMAT,
                         SCF_SELECTION,
                         ref fmt
                       );
        }
        private void button1_Click(object sender, EventArgs e)
        {
            setLineFormat(0, space);
        }
        private void button2_Click(object sender, EventArgs e)
        {
            setLineFormat(1, space);
        }
        private void button3_Click(object sender, EventArgs e)
        {
            setLineFormat(2, space);
        }
        private void button4_Click(object sender, EventArgs e)
        {
            setLineFormat(3, space);
        }
        private void button5_Click(object sender, EventArgs e)
        {
            setLineFormat(4, space);
        }
        private void button6_Click(object sender, EventArgs e)
        {
            setLineFormat(5, space);
        }
        int space = 0;
        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            String val = textBox1.Text;
            bool success = int.TryParse(val, out space);
            if (success)
            {
                textBox1.BackColor = Color.White;
            }
            else
            {
                textBox1.BackColor = Color.Red;
            }
        }
    }
    [StructLayout( LayoutKind.Sequential )]
    public struct PARAFORMAT
    {
        public int cbSize;
        public uint dwMask;
        public short wNumbering;
        public short wReserved;
        public int dxStartIndent;
        public int dxRightIndent;
        public int dxOffset;
        public short wAlignment;
        public short cTabCount;
        [MarshalAs( UnmanagedType.ByValArray, SizeConst = 32 )]
        public int[] rgxTabs;
        // PARAFORMAT2 from here onwards
        public int dySpaceBefore;
        public int dySpaceAfter;
        public int dyLineSpacing;
        public short sStyle;
        public byte bLineSpacingRule;
        public byte bOutlineLevel;
        public short wShadingWeight;
        public short wShadingStyle;
        public short wNumberingStart;
        public short wNumberingStyle;
        public short wNumberingTab;
        public short wBorderSpace;
        public short wBorderWidth;
        public short wBorders;
    }
}