using System;

using System.Collections.Generic;

using System.IO;

using System.Xml.Serialization;

public class Program

{

    public static void Main()

    {

        Preferences preferences = null;

        var xmlString = Data.XML;

        using (var stream = new StringReader(xmlString))

        {

            var serializer = new XmlSerializer(typeof(Preferences));

            preferences = (Preferences)serializer.Deserialize(stream);

        }

        var node0 = new Font()

        {

            Fname = "New One",

            Role = "console",

            Size = "12"

        };

        var node1 = new Font()

        {

            Fname = "Helvetica",

            Role = "titles",

            Size = "10"

        };

        if (preferences.Font.Contains(node0))

        {

            // Not expecting to enter here

            Console.WriteLine($"'{nameof(node0)}' is already present");

        }

        if (preferences.Font.Contains(node1))

        {

            Console.WriteLine($"'{nameof(node1)}' is already present");

        }   

    }

}

​

[XmlRoot(ElementName = "font")]

public class Font : IEquatable<Font>

{

    [XmlElement(ElementName = "size")]

    public string Size { get; set; }

    [XmlElement(ElementName = "fname")]

    public string Fname { get; set; }

    [XmlAttribute(AttributeName = "role")]

    public string Role { get; set; }

​

    public bool Equals(Font font)

    {

        if (font == null) return false;

​

        return (Size == font.Size) && (Fname == font.Fname) && (Role == font.Role);

    }

}

​

[XmlRoot(ElementName = "preferences")]

public class Preferences

{

    [XmlElement(ElementName = "font")]

    public List<Font> Font { get; set; }

}

​

    public static class Data

    {

        public static string XML = @"

<preferences>

  <font role=""console"">

    <size>9</size>

    <fname>Courier</fname>

  </font>

  <font role=""default"">

    <fname>Times New Roman</fname>

    <size>14</size>

  </font>

  <font role=""titles"">

    <size>10</size>

    <fname>Helvetica</fname>

  </font>

</preferences>";

    }

​