ConfigManager.cs

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

namespace ConfigManager
{
    public class ConfigManager : IDisposable
    {
        private static readonly XmlSerializer serializer = new XmlSerializer(typeof(Entry[]), new XmlRootAttribute() { ElementName = "Entries" });

        private readonly string filepath;
        private Dictionary<string, object> items = new Dictionary<string, object>();

        public ConfigManager(string filepath)
        {
            this.filepath = filepath;
            if (!File.Exists(filename))
                this.Save();
            else
                this.Load();
        }

        public T GetWithDefault<T>(string key, T def)
        {
            object value;
            if (!this.items.TryGetValue(key, out value))
            {
                value = def;
                this.items.Add(key, value);
            }
            return (T)value;
        }
        
        public object this[string key]
        {
            get
            {
                object value;
                this.items.TryGetValue(key, out value);
                return value;
            }
            set
            {
                this.items[key] = value;
            }
        }

        public void Load()
        {
            using (var stream = File.OpenRead(this.filepath))
            {
                this.items = ((Entry[])serializer.Deserialize(stream)).ToDictionary(x => x.Key, x => x.Value);
            }
        }

        public void Save()
        {
            using (var stream = File.OpenWrite(this.filepath))
            {
                serializer.Serialize(stream, this.items.Select(x => new Entry() { Key = x.Key, Value = x.Value }).ToArray());
            }
        }

        public void Dispose()
        {
            this.Save();
        }
        
        public class Entry
        {
            public string Key { get; set; }
            public object Value { get; set; }
        }
    }
}

Program.cs

using System;
using System.IO;

namespace ConfigManager
{
    class Program
    {
        static void Main(string[] args)
        {
            // Start from a clean slate
            File.Delete("test.xml");

            using (var configManager = new ConfigManager("test.xml"))
            {
                // Getting a value which doesn't exist, with a default
                int val = (int)configManager.GetWithDefault("test", 5);
                Console.WriteLine(val);
            }

            using (var configManager = new ConfigManager("test.xml"))
            {
                // Getting a value which does exist, with a default. Demonstrates that the defaulit value was set in the last example
                // Also showing that GetWithDefault usage does not require a cast, since it infers the type from the default argument
                int val = configManager.GetWithDefault("test", 6);
                Console.WriteLine(val);

                // Using [] to set a value
                configManager["key"] = "value";
            }

            using (var configManager = new ConfigManager("test.xml"))
            {
                // Using [] to get a value
                string val = (string)configManager["key"];
                Console.WriteLine(val);
            }

            Console.ReadLine();
        }
    }
}