GroupedValueList.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Ical.Net.Collections.Interfaces;
using Ical.Net.Collections.Proxies;

namespace Ical.Net.Collections
{
    public class GroupedValueList<TGroup, TInterface, TItem, TValueType> :
        GroupedList<TGroup, TInterface>
        where TInterface : class, IGroupedObject<TGroup>, IValueObject<TValueType>
        where TItem : new()        
    {
        public void Set(TGroup group, TValueType value) => Set(group, new[] { value });

        public void Set(TGroup group, IEnumerable<TValueType> values)
        {
            if (ContainsKey(group))
            {
                AllOf(group)?.FirstOrDefault()?.SetValue(values);
                return;
            }

            // No matching item was found, add a new item to the list
            var obj = Activator.CreateInstance(typeof(TItem)) as TInterface;
            obj.Group = group;
            Add(obj);
            obj.SetValue(values);
        }

        public TType Get<TType>(TGroup group)
        {
            var firstItem = AllOf(group).FirstOrDefault();
            if (firstItem?.Values != null)
            {
                return firstItem
                    .Values
                    .OfType<TType>()
                    .FirstOrDefault();
            }
            return default;
        }

        public IList<TType> GetMany<TType>(TGroup group)
            => new GroupedValueListProxy<TGroup, TInterface, TItem, TValueType, TType>(this, group);
    }
}