System.Data.SqlTypes.SqlNullValueException: 数据为空。不能对空值调用此方法或

有可能读出的数据为NULL，可以这样改：

方法一：
while (reader.Read())
{
     for (int i = 0; i < 7; i++)
     {
         if (reader.IsDBNull(i))
         {
             continue;
         }
         readstring[i] = reader.GetString(i);
     }
}
方法二：

string sql = "select ISNULL([Name],''),ISNULL([Address],''),ISNULL([WebAddress],''),ISNULL([Telephone],''),ISNULL([Description],''),ISNULL([LeftTopPic],''),ISNULL([LeftBottomPic],'') from [t_Customer] where [CustomerID] = " + ctid; 

方法三：
while (reader.Read())
{
     for (int i = 0; i < 7; i++)
     {
         readstring[i] = reader[i].ToString();
     }
}

用 is DBNull 

如：
user.Phone = reader["phone"] is DBNull ? "" : (string)reader["phone"];