哪来的错误

哪来的异常？
string typeName = DropDownList1.SelectedValue.ToString();
  string sql = "select * from POWERTYPE where TYPENAME='" + typeName + "'";
  if (dataOperate.seleSQL(sql) > 0)
  {
  SqlDataReader reader = dataOperate.getRow(sql);
  reader.Read();
  TextBox3.Text = (string)reader["PRICE0"];
  }


  public static int seleSQL(string sql)
  {
  SqlConnection con = createCon();
  con.Open();
  SqlCommand com = new SqlCommand(sql, con);
  try
  {
  int i = Convert.ToInt32(com.ExecuteScalar()); //调试此处有异常
  //int i = (int)com.ExecuteScalar();
  con.Close();
  return i;
  }
  catch //(Exception e)
  {
  con.Close();
  return 0;
  }
  }

 大家帮忙看看，该怎么修改？

------解决方案--------------------
你要是判断是否有记录，直接
string sql = "select * from POWERTYPE where TYPENAME='" + typeName + "'";
SqlDataReader reader = dataOperate.getRow(sql);
if(reader.HasRows)
{
reader.Read();
TextBox3.Text = (string)reader["PRICE0"];
}

或者
if(reader.Read())
{
TextBox3.Text = (string)reader["PRICE0"];
}

没有必要执行seleSQL啊


string sql = "select * from POWERTYPE where TYPENAME='" + typeName + "'";
if(reader.Read())
{
TextBox3.Text = (string)reader["PRICE0"];
}

第一行第一列的数据是数字或者返回null 才能进行Convert.ToInt32，其他都不能执行Convert.ToInt32的