建立regax对象 然后用正则表达式判断下就可以了！

可以说的具体点么？
我是初学者，有些东西不懂，下面是我的代码：
    private void submitButton_Click(object sender, System.EventArgs e)
        {
            if(userNameTextBox.Text.Length<3||userNameTextBox.Text.Length>12)
            {
                nameInformationLabel.ForeColor=Color.Red;
            }
            else 
            {
                nameInformationLabel.ForeColor=Color.Black;
                if(passwordTextBox.Text!=passwordAgainTextBox.Text||passwordTextBox.Text.Length<6||passwordAgainTextBox.Text.Length<6)
                {
                    passwordAgainInformationLabel.Text="两次输入密码不一致或者过短!";
                    passwordAgainInformationLabel.ForeColor=Color.Red;
                }
                else 
                {
                    passwordAgainInformationLabel.Text="";
                    if(emailTextBox.Text.Length==0)
                    {
                        emailInformationLabel.Text="不能为空!";
                        emailInformationLabel.ForeColor=Color.Red;
                    }
                    else
                    {
                        emailInformationLabel.Text="";

                        SqlConnection conn=new SqlConnection("data source=A385A52FFF9B4DF;database=ForecastDB;uid=sa;pwd=");
                        SqlCommand myCommand=new SqlCommand();
                                        
                        string strsql = "select id,password from Customer where id='" + userNameTextBox.Text + "'";
                        SqlCommand cmd = new SqlCommand(strsql, conn);
                        conn.Open();
                        SqlDataReader rd = cmd.ExecuteReader();

                        if(rd.Read())
                        {
                            conn.Close();
                            MessageBox.Show("用户名已存在！");
                        }
                        else
                        {
                            conn.Close();
                            //建立用户信息
                            myCommand.CommandText="Insert into Customer values('"+Convert.ToString(this.userNameTextBox.Text)+"','"+Convert.ToString(this.passwordTextBox.Text)+"','"+Convert.ToString(this.emailTextBox.Text)+"')";
                            myCommand.Connection =conn;
                            conn.Open();
                            myCommand.ExecuteNonQuery();
                            conn.Close();

                            MessageBox.Show("您已经注册成功！\n请登陆系统！","提示",MessageBoxButtons.OK,MessageBoxIcon.Information);
                            User_loginForm form=new User_loginForm();
                            form.Show();
                            this.Hide();
                        }
                    }
                }
            }
        }
emailTextBox是填写email的控件，请问如何改写代码才能使得程序可以判断所填写信息为合法的email格式？

谢谢！！

就是如果在emailTextBox控件里填写的信息中不包括"@"符号，就说明注册失败！！

你这又是何必呢?有自带的验证控件不用,干什么还要写一大堆!

如果你用不来验证控件的话,只能自己写前太验证代码.
可以使用字符串比较,既字符串对象的Equals的方法.
提示:
string email=this.emailTextBox.Text.Trim();
if((email.Equals("@")==-1) || (email.Equals(".")==-1))
{
    MessageBox.Show("Email格式不正确");
}
else
//其他验证代码

如果你是初学还是找本关于算法的东西好好学习学习
要不是的话，干脆就直接用里面的控件多好....只需要自己配置一下就行了

Regex chk=new Regex(@"^[\w\.-]+@[\w\.-]+\.[a-zA-Z]+$", RegexOptions.Compiled | RegexOptions.IgnoreCase);
if(!chk.IsMatch(EmailAddress)){
    throw new Exception("电子邮件地址不合法。");
}

其实你验证了也不一定是真的,还不如给注册者一点自由呢,也省自己的数据库,还能提高自己的速度,与人方便与已方便