DataRow转换时对DBNULL的处理

    public static T SafeRead<T>(this IDataReader reader, string fieldName, T defaultValue)
    ...{
        try
        ...{
            object obj = reader[fieldName];
            if (obj == null || obj == System.DBNull.Value)
                return defaultValue;
 
            return (T)Convert.ChangeType(obj, defaultValue.GetType());
        }
        catch
        ...{
            return defaultValue;
        }
    }