C# 使用映射器将 DataReader 映射到对象

Question

I've been struggling with getting ExpressMapper to map from a DataReader to a object (basically, Sqlite Rows -> Objects) . I've had success with AutoMapper, but found some other issues that got me looking for another map tool.

In AutoMapper I used the following code, but in ExpressMapper all string properties are NULL and the long prop is always 0, this indicates that the mapping is failing. Tried to look some examples for ExpressMapper but did not found any. Can you help? Also, are there any other mappers besides AutoMapper (and possibly ExpressMapper) that can help me achieve this scenario?

Thanks!

 public class ImportedFiles
{
    public Int64 Id { get; set; }
    public String FileName { get; set; }
    public String Hash { get; set; }

    public ImportedFiles()
    {

    }

}

private static void ReadImportedFiles()
    {
        var lst = ReadData<ImportedFiles>("SELECT * FROM ImportedFiles").ToList();
    }

 public static IEnumerable<T> ReadData<T>(string queryString)
        {
            using (var connection = new SQLiteConnection(ConnectionBuilder.ConnectionString))
            {
                connection.Open();
                using (var cmd = connection.CreateCommand())
                {
                    cmd.CommandText = queryString;
                    using (var reader = cmd.ExecuteReader())
                        if (reader.HasRows)
                            while (reader.Read())
                                yield return Mapper.Map<IDataRecord, T>(reader);
                }
            }
        }

Answer 1

are there any other mappers besides AutoMapper (and possibly ExpressMapper) that can help me achieve this scenario?

Yes. Generally, they are called Object-Relational Mappers (shortened to O/RM or just ORM). There are many; a couple of the most popular for the .NET framework are Entity Framework and NHibernate .

I think you are using the wrong class of tool (object-to-object mappers) for the task at hand ( object-relational mapping ).

Object-to-object mappers are designed for mapping between closely aligned, strongly-typed, data-containing classes, usually to save hand-writing mapping code. They are not designed for mapping to or from more generic data containers like DataRow , or for dealing with other "plumbing" concerns that come into play when working with a relational database.

Answer 2

ExpressMapper is capable to map to a DataRow. This should solves your issue.

Just refactor your code to work with a DataRow. And pre-register the mapping.

So, if you write:

yield return Mapper.Map<DataRow, T>(row);

And do the mappings (once per application) like I suggested:

        Mapper.Register<DataRow, ImportedFiles>()
            .Member(dest => dest.Id,
                    src => DBNull.Value == src["Id"] ? 0 : (double)src["Id"])
            .Member(dest => dest.FileName,
                    src => DBNull.Value == src["FileName"] ? String.Empty : (string)src["FileName"])
            .Member(dest => dest.Hash,
                    src => DBNull.Value == src["Hash"] ? String.Empty : (string)src["Hash"])
            ;

You're able to use it.

---

[Edited]

In case you need to map more than one source (ie: more than one DataTable) then you could write a DataRowAdapter which will differentiate your mapping and let know ExpressMapper (or whatever ORM you want to use - this is appliable to all ORM) which data to expect in your DataRow.

Below a working example with your code.

yield return Mapper.Map<DataRowAdapter<T>, T>(new DataRowAdapter<T>(row));

DataRowAdapter class:

public class DataRowAdapter<T>
{
    DataRow _dataRow;

    public DataRowAdapter(DataRow dataRow)
    {
        _dataRow = dataRow;
    }

    public DataRow Current
    {
        get
        {
            return _dataRow;
        }
    }
}

And your new-brand mapping:

     Mapper.Register<DataRowAdapter<ImportedFiles>, ImportedFiles>()
        .Member(dest => dest.Id,
                src => DBNull.Value == src.Current["Id"] ? 0 : (double)src.Current["Id"])
        .Member(dest => dest.FileName,
                src => DBNull.Value == src.Current["FileName"] ? String.Empty : (string)src.Current["FileName"])
        .Member(dest => dest.Hash,
                src => DBNull.Value == src.Current["Hash"] ? String.Empty : (string)src.Current["Hash"])
        ;

Done.

Answer 3

Example using SQLiteData reader to map the selected items to Object class List

public static List<Object.Class> getObject()
    {
        List<Object.Class> list = new List<Object.Class>();
        var datareader = Database.getData();
        while (datareader.Read())
        {


            list.Add(item: new Object.Class(
               id: datareader.GetInt32(0),
            date: DateTime.Parse(datareader.GetString(6))
                )
        );


        }
        datareader.Close();

        return list;
    }