C# - IDataReader to Object mapping using generics

Question

How can I map a DataReader object into a class object by using generics?

For example I need to do the following:

public class Mapper<T>
    {
        public static List<T> MapObject(IDataReader dr)
        {
            List<T> objects = new List<T>();

            while (dr.Read())
            {
                //Mapping goes here...
            }

            return objects;
        }
    }

And later I need to call this class-method like the following:

IDataReder dataReader = DBUtil.Fetchdata("SELECT * FROM Book");

List<Book> bookList = Mapper<Book>.MapObject(dataReder);

foreach (Book b in bookList)
{
     Console.WriteLine(b.ID + ", " + b.BookName);
}

Note that, the Mapper - class should be able to map object of any type represented by T.

One suggestion - read into an IEnumerable<T> with a yield return. — Daniel A. White
– Daniel A. White, Commented Jul 9, 2009 at 18:02
//mapping goes here, exactly what I've showed you in my answer, you can map any object to the data reader (more exactly: injecting values from an IDataReader into an object ANY TYPE) — Omu
– Omu, Commented Jun 15, 2010 at 11:40
Why wouldn't you use a dedicated ORM then? A micro-ORM like Dapper seem to be a good fit here. — nawfal
– nawfal, Commented Jul 29, 2015 at 20:27
@BROY Honestly, with comments, answers etc always future visitors are considered too. And it's not like ORMs didnt exist in 2009 :) — nawfal
– nawfal, Commented Jul 29, 2015 at 20:35

Omu · Accepted Answer · 2010-06-15 11:42:47Z

3

I use ValueInjecter for this

I'm doing like this:

 while (dr.Read())
  {
      var o = new User();
      o.InjectFrom<DataReaderInjection>(dr);
      yield return o;
  }

you gonna need this ValueInjection for this to work:

public class DataReaderInjection : KnownSourceValueInjection<IDataReader>
    {
        protected override void Inject(IDataReader source, object target, PropertyDescriptorCollection targetProps)
        {
            for (var i = 0; i < source.FieldCount; i++)
            {
                var activeTarget = targetProps.GetByName(source.GetName(i), true);
                if (activeTarget == null) continue;

                var value = source.GetValue(i);
                if (value == DBNull.Value) continue;

                activeTarget.SetValue(target, value);
            }
        }
    }

edited Jun 15, 2010 at 11:42

answered Jun 15, 2010 at 9:00

Omu

71.6k95 gold badges286 silver badges413 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Pavel Hodek Over a year ago

Is somewhere any library of common and useful injections?

Omu Over a year ago

@PavelHodek there isn't a library but there are many in valueinjecter's main demo solution and also in the codeplex pages

nawfal Over a year ago

Few things. 1) You're using reflection to set value, this is bad for performance. Go expression route. 2) Where is DataReaderInjection and KnownSourceValueInjection in your library? I think you changed names since this answer? 3) Is it using reflection behind the scenes every time in the while reader.Read loop to get property names? I hope no, but cant convince myself without seeing KnownSourceValueInjection class.

nawfal Over a year ago

4) This is minor but still - you're not assigning to property when it is null in database (DBNull case), but what if some value is assigned to that property when new-ing in the constructor, like "var o = new User()"? In that case the user you return will be different from what is actually present in the db.

Jhonny D. Cano -Leftware- · Accepted Answer · 2009-07-09 18:04:35Z

2

Well, i don't know if it fits here, but you could be using the yield keyword

public static IEnumerable<T> MapObject(IDataReader dr, Func<IDataReader, T> convertFunction)
        {
            while (dr.Read())
            {
                yield return convertFunction(dr);
            }
        }

answered Jul 9, 2009 at 18:04

Jhonny D. Cano -Leftware-

18.1k15 gold badges84 silver badges103 bronze badges

2 Comments

Adrian Godong Over a year ago

+1 Interesting use of DI. It would be nice to have the T type provide the implementation of convertFunction as well. :D

Pavel Hodek Over a year ago

This is nice DI and it can be combined with Omu's solution.

Joel Martinez · Accepted Answer · 2009-07-09 18:06:25Z

1

You could use this LateBinder class I wrote: http://codecube.net/2008/12/new-latebinder/.

I wrote another post with usage: http://codecube.net/2008/12/using-the-latebinder/

answered Jul 9, 2009 at 18:06

Joel Martinez

47.9k26 gold badges136 silver badges186 bronze badges

Comments

Nick Berardi · Accepted Answer · 2009-07-09 18:06:40Z

1

This is going to be very hard to do for the reason that you are basically trying to map two unknowns together. In your generic object the type is unknown, and in your datareader the table is unknown.

So what I would suggest is you create some kind of column attribute to attach to the properties of you entity. And then look through those property attributes and try to look up the data from those attributes in the datareader.

Your biggest problem is going to be, what happens if one of the properties isn't found in the reader, or vice-versa, one of the columns in the reader isn't found in the entity.

Good luck, but if you want to do something like this, you probably want a ORM or at the very least some kind of Active Record implementation.

answered Jul 9, 2009 at 18:06

Nick Berardi

55k15 gold badges118 silver badges137 bronze badges

1 Comment

Omu Over a year ago

look at my answer, it's not that hard :), and there's no attributes required

Reed Copsey · Accepted Answer · 2009-07-09 18:07:03Z

1

The easiest way I can think of offhand would be to supply a Func<T,T> delegate for converting each column and constructing your book.

Alternatively, if you followed some conventions, you could potentially handle this via reflection. For example, if each column mapped to a property in the resulting object using the same name, and you restricted T in your Mapper to providing a constructable T, you could use reflection to set the value of each property to the value in the corresponding column.

answered Jul 9, 2009 at 18:07

Reed Copsey

567k80 gold badges1.2k silver badges1.4k bronze badges

Comments

ramnik · Accepted Answer · 2009-07-09 18:14:30Z

1

I don't think you'll be able to get around defining the relationship between fields in some form. Take a look at this article and pay particular attention to how the mapping is defined, it may work for you.

http://www.c-sharpcorner.com/UploadFile/rmcochran/elegant_dal05212006130957PM/elegant_dal.aspx

answered Jul 9, 2009 at 18:14

ramnik

5452 silver badges12 bronze badges

Comments

nWorx · Accepted Answer · 2009-07-09 18:29:03Z

1

what about following

abstract class DataMapper
{
    abstract public object Map(IDataReader);
}

class BookMapper : DataMapper
{
   override public object Map(IDataReader reader)
   {
       ///some mapping stuff
       return book;
   }
}

public class Mapper<T>
{
    public static List<T> MapObject(IDataReader dr)
    {
        List<T> objects = new List<T>();
        DataMapper myMapper = getMapperFor(T);
        while (dr.Read())
        {
            objects.Add((T)myMapper(dr));
        }

        return objects;
    }

    private DataMapper getMapperFor(T myType)
    {
       //switch case or if or whatever
       ...
       if(T is Book) return bookMapper;

    }
}

Don't know if it is syntactically correct, but I hope u get the idea.

answered Jul 9, 2009 at 18:29

nWorx

2,1551 gold badge17 silver badges37 bronze badges

1 Comment

nawfal Over a year ago

You could avoid the if else conditions and rely on polymorphism provided the Book class itself implements some Func<IDataRecord, Book>.

Giorgi · Accepted Answer · 2010-08-18 06:42:09Z

1

What about using Fluent Ado.net ?

answered Aug 18, 2010 at 6:42

Giorgi

31k13 gold badges92 silver badges128 bronze badges

Comments

zackevans · Accepted Answer · 2010-12-08 08:54:06Z

1

Have a look at http://CapriSoft.CodePlex.com

answered Dec 8, 2010 at 8:54

zackevans

111 bronze badge

Comments

Robin Orheden · Accepted Answer · 2010-12-08 09:06:48Z

1

I would recommend that you'd use AutoMapper for this.

answered Dec 8, 2010 at 9:06

Robin Orheden

2,76423 silver badges24 bronze badges

Collectives™ on Stack Overflow

C# - IDataReader to Object mapping using generics

10 Answers 10

4 Comments

2 Comments

Comments

1 Comment

Comments

Comments

1 Comment

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

10 Answers 10

4 Comments

2 Comments

Comments

1 Comment

Comments

Comments

1 Comment

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related