DatFile.cs

using System.Collections.Generic;
using System.Xml.Serialization;

[XmlRoot(ElementName = "datafile")]
public class DatFile
{
    [XmlRoot(ElementName = "header")]
    public class DatHeader
    {
        [XmlElement(ElementName = "name")]
        public string Name;

        [XmlElement(ElementName = "description")]
        public string Description;

        [XmlElement(ElementName = "version")]
        public string Version;

        [XmlElement(ElementName = "author")]
        public string Author;

        [XmlElement(ElementName = "homepage")]
        public string Homepage;

        [XmlElement(ElementName = "url")]
        public string Url;
    }
    
    [XmlRoot(ElementName = "game")]
    public class DatGame
    {
        [XmlRoot(ElementName = "rom")]
        public class DatRom
        {
            [XmlAttribute(AttributeName = "name")]
            public string Name;

            [XmlAttribute(AttributeName = "size")]
            public long Size;

            [XmlAttribute(AttributeName = "crc")]
            public string Crc32;

            [XmlAttribute(AttributeName = "md5")]
            public string Md5;

            [XmlAttribute(AttributeName = "sha1")]
            public string Sha1;

            public bool ShouldSerializeSize() => Size > 0;
        }
        
        [XmlAttribute(AttributeName = "name")]
        public string Name;

        [XmlElement(ElementName = "description")]
        public string Description;

        [XmlElement(ElementName = "rom")]
        public List<DatRom> Roms = new List<DatRom>();
    }

    [XmlElement(ElementName = "header")]
    public DatHeader Header = new DatHeader();

    [XmlElement(ElementName = "game")]
    public List<DatGame> Games = new List<DatGame>();
}