求助一下```
//连接字符串string connectionString = @"Data Source=User;Initial Catalog=guanli;Integrated Security=True";
//建立连接
SqlConnection myCon = new SqlConnection(connectionString);
myCon.Open();
//读取所输入用户名的密码
SqlCommand myComm = myCon.CreateCommand();
string sql = "select username,password,usertype from T_User where(username = N'"+txtUsername.Text.Trim()+"')";
myComm.CommandText = sql;
SqlDataReader myReader = myComm.ExecuteReader();
能详细解说一下这段代码吗???
