How does a factory know which type of object to create?

Question

I believe the factory method design pattern is appropriate for what I\'m trying to do, but I\'m not sure how much responsibility (knowledge of subclasses it creates) to give it.

Answer 1

If this is for windows I would try to guess content type and then use factory. In fact I did this some time ago.

Here is a class to guess content type of a file:

using System;
using System.IO;
using System.Runtime.InteropServices;

namespace Nexum.Abor.Common
{
    /// 

    /// This will work only on windows
    /// 
    public class MimeTypeFinder
    {
        [DllImport(@"urlmon.dll", CharSet = CharSet.Auto)]
        private extern static UInt32 FindMimeFromData(
            UInt32 pBC,
            [MarshalAs(UnmanagedType.LPStr)] String pwzUrl,
            [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer,
            UInt32 cbSize,
            [MarshalAs(UnmanagedType.LPStr)]String pwzMimeProposed,
            UInt32 dwMimeFlags,
            out UInt32 ppwzMimeOut,
            UInt32 dwReserverd
        );

        public string getMimeFromFile(string filename)
        {
            if (!File.Exists(filename))
                throw new FileNotFoundException(filename + " not found");

            var buffer = new byte[256];
            using (var fs = new FileStream(filename, FileMode.Open))
            {
                if (fs.Length >= 256)
                    fs.Read(buffer, 0, 256);
                else
                    fs.Read(buffer, 0, (int)fs.Length);
            }
            try
            {
                UInt32 mimetype;
                FindMimeFromData(0, null, buffer, 256, null, 0, out mimetype, 0);
                var mimeTypePtr = new IntPtr(mimetype);
                var mime = Marshal.PtrToStringUni(mimeTypePtr);
                Marshal.FreeCoTaskMem(mimeTypePtr);
                return mime;
            }
            catch (Exception)
            {
                return "unknown/unknown";
            }
        }
    }
}