Create object instance of correct subtype from raw bytes

Question

I have a binary file format I cannot change, and in C# I need to deserialize class instances from it again.

The file contains several byte blocks which represent object instances of a base class. However, one byte in the block specifies which subclass the object once was.

For example, a byte block looks like the following:

• General settings which the base class has (let's say Collectible)
• A byte determining what kind of additional details follow (either Coin or ItemBox)
• According to the byte, either details about the Coin (e.g. the coins value) or the ItemBox (e.g. which item it has and how fast it regenerates)

I currently coded a constructor for the Collectible class accepting a BinaryReader to read from. However, after I read the general settings and know which sub type follows, I can't downcast the Collectible class to Coin or ItemBox depending on the type when I'm already in the constructor.

public Collectible(BinaryReader reader)
{
    // General collectible stuff, position, synced anim...
    X = reader.ReadInt32();
    Y = reader.ReadInt32();
    MusicStartCode = reader.ReadInt32();
    MusicBpm = reader.ReadInt32();
    // ...

    // Tricky part: sub-type specific information follows
    CollectibleType type = (CollectibleType)reader.ReadByte();
    switch (type)
    {
        case CollectibleType.Coin:
            // What to do now?
            break;
        case CollectibleType.ItemBox:
            // I ran out of ideas
            break;
    }
}

How do I solve such a problem elegantly and deserialize the binary format back into object instances? I thought of some kind of seeking towards the byte determining the sub type and then create the specific sub class instances. But where would I do that, is there a typical design pattern for this problem?

On a sidenote I think that this format was written by a C++ program originally which just dumped the instances bytes into the file and had no problem casting a byte array pointer back to a class when loading it. But I can't do that in C# or can I (I want to avoid unsafe code)?

Answer 1

How about something along the lines of this:

public class CollectionFactory
{
    private readonly BinaryReader _reader;
    public CollectionFactory(BinaryReader reader)
    {
        if (reader == null)
        {
            throw ArgumentNullException(reader);
        }

        _reader = reader;
    }

    public CollectionType CreateInstance()
    {
        using (_reader)
        {
            // General collectible stuff, position, synced anim...
            var x = _reader.ReadInt32();
            var y = _reader.ReadInt32();
            var musicStartCode = reader.ReadInt32();
            var musicBpm = reader.ReadInt32();
            // ...

            // Tricky part: sub-type specific information follows
            CollectibleType collectableType = (CollectibleType)reader.ReadByte();

            if (collectableType is Coin)
            {
                return new Coin 
                {
                    X = x,      
                    Y = y,
                    MusicStartCode = musicStartCode
                    // etc..
                };
            }
            if (collectableType is ItemBox)
            {
                return new ItemBox
                {
                    X = x,      
                    Y = y,
                    MusicStartCode = musicStartCode
                    // etc..
                };
            }

            return null;
        }
    }
}

And then consume it:

var collectionFactory = new CollectionFactory(reader);
var collType = collectionFactory.CreateInstance();

// cast to specific type if needed here.
var itemBox = collType as ItemBox;
if (itembox != null)
{
   // Do stuff..
}