将varchar值转为int时失败

{
    public partial class useradd : Form
    {
        public string newlimit;
        public useradd(string newlimit)
        {
        this.newlimit = newlimit;
            InitializeComponent();
        }        private static string connString = "Data Source=localhost;Initial Catalog=testsysdb;Integrated Security=True;Pooling=False";        public static string ConnString
        {
            get
            {
                return connString;
            }
        }
        private void button2_Click(object sender, EventArgs e)
        {
            this.Close();//关闭该窗体
        }        private void button1_Click(object sender, EventArgs e)
        {
            string name=textBox1.Text;
            string passwd=textBox2.Text;
            string passwd2 = textBox3.Text;
            string age = textBox4.Text;
            string sex = ((this.radio_female.Checked == true) ? "男" : "女");
            string add = textBox5.Text;
            string email = textBox6.Text;
            string telephone = textBox7.Text;
            int flag = 0;            if (passwd!=passwd2)
            {
                MessageBox.Show("密码确认有误！", "提示");
                return;
            }
            if (name.Length == 0 || passwd.Length == 0 || age.Length == 0 || age.Length == 0 || sex.Length == 0 || add.Length == 0 | email.Length == 0)
            {
                MessageBox.Show("请添加完整的用户信息！", "添加用户提示");
                return;
 
            }            SqlConnection conn = new SqlConnection(useradd.ConnString);
            string sqlstr = "insert into userinfo values('" + name + "','" + passwd + "','" + newlimit + "','" + age + "','" + telephone + "','" + sex + "','" + add + "','" + email + "')";
           
            SqlCommand command = new SqlCommand(sqlstr, conn);
            conn.Open();            try
            {
                flag=command.ExecuteNonQuery();
                   
                if (flag>0)
                {
                    MessageBox.Show("用户添加成功！", "添加用户提示");
                    this.Close();
                                }
                else
                {
                    MessageBox.Show("添加用户失败！", "添加用户提示");
                }
                
            }
            catch (Exception err)
            {
                
                MessageBox.Show("系统错误："+err.Message, "添加用户提示");
            }
            conn.Close();
          
        }        private void useradd_Load(object sender, EventArgs e)
        {        }        private void button2_Click_1(object sender, EventArgs e)
        {
            this.Close();