为什么用C#编写richtextbox字体样式和字体大小随下拉框中comboBox的值变化不起作用
如图所示，我要实现的功能是当选择下拉菜单comboBox中字体类型和字体大小的时候，richtextbox选中的文本会变为相应的字体。可是我的代码根本没反应（注：comboBox1的名字为fontCB，comboBox2的名字为sizeCB,RichTextBox的名字为rtb）：

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 font in FontFamily.Families)
                this.fontCB.Items.Add(font.Name);//设置fontCB下拉菜单内容
            this.fontCB.SelectedItem = "宋体";//设置fontCB显示内容
            foreach (string name in FontSizeName)
                this.sizeCB.Items.Add(name);//设置sizeCB下拉菜单内容
            this.sizeCB.SelectedItem = "10";//设置sizeCb显示内容

        }
         public string[] FontSizeName =
            {
                "8","9","10","12","14","16","18","20","22","24","26","28","36","48","72"
            };
        //定义字号数组
         public float[] FontSize =
            {
                8,9,10,12,14,16,18,20,22,24,26,28,36,48,72
            };
         private string fontName;
         private int fontSize;
         private void fontCB_SelectedIndexChanged(object sender, EventArgs e)
         {
             try
             {
                 this.rtb.SelectionFont = new Font(this.fontCB.Text, this.rtb.SelectionFont.Size, this.rtb.SelectionFont.Style);
             }
       &nbs