按照C#入门经典的 windows编程中的一个例子做出错,求解答!

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 WindowsFormsApplication3
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            buttonOK.Enabled = false;
        }        private void buttonOK_Click(object sender, EventArgs e)
        {
            string output;
            output = "Name: " + this.textBoxName.Text + "\r\n";
            output += "Address: " + this.textBoxAddress.Text + "\r\n";
            output += "Occupation: " + this.textBoxOccupation.Text + "\r\n";
            output += "Age: " + this.textBoxAge.Text;
            this.textBoxOutput.Text = output;
        }        private void buttonHelp_Click(object sender, EventArgs e)
        {
            string output;
            output = "Name: " + this.textBoxName.Text + "\r\n";
            output += "Address: " + this.textBoxAddress.Text + "\r\n";
            output += "Occupation: " + this.textBoxOccupation.Text + "\r\n";
            output += "Age: " + this.textBoxAge.Text;
            this.textBoxOutput.Text = output;
        }        private void textBoxEmpty_Validating(object sender, System.ComponentModel.CancelEventArgs e)
        {
            TextBox tb = (TextBox)sender;
            if (tb.Text.Length == 0)
                tb.BackColor = Color.Red;
            else
                tb.BackColor = System.Drawing.SystemColors.Window;
            ValidateOK();    //这里出错
        }        private void textBoxOccupation_Validating(object sender, CancelEventArgs e)
        {        }
    }错误在红色那里  ValidateOK();  Error显示:  
The name 'ValidateOK' does not exist in the current context
求解答,为什么错. 书的例子是这样写的.