[ACCEPTED]-How can I make some items in a ListBox bold?-listbox

You need to change listbox's DrawMode to 4 DrawMode.OwnerDrawFixed. Check out these 3 articles on msdn:
DrawMode Enumeration
ListBox.DrawItem Event
Graphics.DrawString Method

Also look at this question 2 on msdn forums:
Question on ListBox items

A simple example (both items 1 - Black-Arial-10-Bold):

 public partial class Form1 : Form
 {
    public Form1()
    {
        InitializeComponent();

        ListBox1.Items.AddRange(new Object[] { "First Item", "Second Item"});
        ListBox1.DrawMode = DrawMode.OwnerDrawFixed;
    }

    private void ListBox1_DrawItem(object sender, DrawItemEventArgs e)
    {
        e.DrawBackground();
        e.Graphics.DrawString(ListBox1.Items[e.Index].ToString(), new Font("Arial", 10, FontStyle.Bold), Brushes.Black, e.Bounds);
        e.DrawFocusRectangle();
    }
 }

To add to Mindaugas Mozūras's solution, I 6 had a problem where my e.Bounds wasn't large enough 5 and text was getting cut off. To solve this 4 problem (thanks to a post here), you override 3 the OnMeasureItem event and changes your DrawMode to 2 DrawMode.OwnerDrawVariable.

In designer:

listBox.DrawMode = DrawMode.OwnerDrawVariable;

In handler:

void listBox_MeasureItem(object sender, MeasureItemEventArgs e)
{
     e.ItemHeight = 18;
}

Solved my issues 1 of having the height cut off text.

A more generic example that uses sender, and 6 actually respects foreground color (if the 5 item is selected, for example, or the user 4 uses some another color set, where black 3 foreground color is not really readable) and 2 current ListBox font:

    private void listBoxDrawItem (object sender, DrawItemEventArgs e)
    {
        Font f = e.Font;
        if (e.Index == 1) //TODO: Your condition to make text bold
            f = new Font(e.Font, FontStyle.Bold);
        e.DrawBackground();
        e.Graphics.DrawString(((ListBox)(sender)).Items[e.Index].ToString(), f, new SolidBrush(e.ForeColor), e.Bounds);
        e.DrawFocusRectangle();
    }

You need to have DrawMode 1 set to OwnerDrawFixed (for example, in designer).

Following is the code demonstrating the 1 same.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            foreach (FontFamily fam in FontFamily.Families)
            {
                listBox1.Items.Add(fam.Name);
            }
            listBox1.DrawMode = DrawMode.OwnerDrawFixed; // 属性里设置

        }

        private void listBox1_DrawItem(object sender, DrawItemEventArgs e)
        {
            e.DrawBackground();
            e.Graphics.DrawString(listBox1.Items[e.Index].ToString(), new Font(listBox1.Items[e.Index].ToString(), listBox1.Font.Size), Brushes.Black, e.Bounds);
            //e.DrawFocusRectangle();
        }
    }
}

Make the selected item bold

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        ListBox1.Items.AddRange(new Object[] { "me", "myself", "bob"});

        // set the draw mode to fixed
        ListBox1.DrawMode = DrawMode.OwnerDrawFixed;
    }

    private void ListBox1_DrawItem(object sender, DrawItemEventArgs e)
    {
        // draw the background
        e.DrawBackground();

        // get the font
        Font font = new Font(e.Font, (e.State & DrawItemState.Selected) == DrawItemState.Selected ? FontStyle.Bold : FontStyle.Regular);

        // draw the text
        e.Graphics.DrawString(ListBox1.Items[e.Index].ToString(), font, new SolidBrush(ListBox1.ForeColor), e.Bounds);

        e.DrawFocusRectangle();
    }
}

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