c#未将对象引用设置到对象的实例

Question

运行对数据库进行存取的时候，注册用户出现以下问题，单独运行第一段用户不存在时，可以实现，想要注册新用户就出现问题，属实是不知道报错部分该怎么改了

``` MySqlConnection conn = null;            
try            
{                
      conn = new MySqlConnection(sign);                
      conn.Open();               
      string checkNameSql = "select * from userinfo where name='{0}'";              
      checkNameSql = string.Format(checkNameSql, textBox1.Text);                
      MySqlCommand cmdCheckName = new MySqlCommand(checkNameSql, conn);                
      int isRepeatName = (int)cmdCheckName.ExecuteScalar();//用于查询数据时，返回查询结果//会报错               
      if (isRepeatName != 0)                
      {                    
            MessageBox.Show("用户名已存在！");                    
            return;                
       }                
       string signnew = "insert into userinfo(name,password) values('{0}','{1}')";               
       signnew = string.Format(signnew, textBox1.Text, textBox2.Text);            
       MySqlCommand cmd = new MySqlCommand(signnew, conn);                
       int returnvalue = cmd.ExecuteNonQuery();//用于插入、更新、和删除数组              
       if (returnvalue != -1)             
       {                   
              MessageBox.Show("注册成功！");                
        }        
｝

Answer 1

 signnew = string.Format(signnew, textBox1.Text, textBox2.Text);            
       MySqlCommand cmd = new MySqlCommand(signnew, conn);                
       int returnvalue = cmd.ExecuteNonQuery();//用于插入、更新、和删除数组  

这一块插入数据库值报错了