Fun with interfaces Any object or list of objects to formatted string

IFilterItem.cs

void Main()
{
	_filters.Add("Integer", new FilterItem<int>(999));
	_filters.Add("IntegerList", new FilterListItem<int>(new [] { 1, 2, 3, 863, 2452, 24 }));
	_filters.Add("string", new FilterItem<string>("test str"));
	
	foreach (var kvp in _filters)
	{
		kvp.Value.GetQueryString().Dump();
	}
}

Dictionary<string, IFilterItem> _filters = new Dictionary<string, IFilterItem>();

public interface IFilterItem
{
	object StorageObject();
	string GetQueryString();
	
	Func<IFilterItem, string> StringConverter { get; }
}
public class FilterItem<T> : IFilterItem
{
	public FilterItem()
	{ }
	public FilterItem(T value)
	{
		_value = value;
	}
	public FilterItem(Func<IFilterItem, string> stringConverter)
	{
		if (stringConverter != null)
			StringConverter = stringConverter;
	}
	
	T _value;
	
	public virtual void SetOrAdd(T value)
	{
		_value = value;
	}
	
	public virtual object StorageObject()
	{
		return _value;
	}
	public virtual string GetQueryString()
	{
		return StringConverter(this);
	}
	
	Func<IFilterItem, string> _StringConverter = (x => x.StorageObject().ToString());
	public virtual Func<IFilterItem, string> StringConverter
	{
		get { return _StringConverter; }
		private set { _StringConverter = value; }
	}
}
public class FilterListItem<T> : FilterItem<T>, IFilterItem
{
	public FilterListItem()
		: base(x => string.Join(",", ((List<T>)x.StorageObject()).Select(i => i.ToString()).ToArray()))
	{ }
	public FilterListItem(IEnumerable<T> values)
		: this()
	{
		AddRange(values);
	}
	
	List<T> _value = new List<T>();
	
	public override void SetOrAdd(T value)
	{
		_value.Add(value);
	}
	public void AddRange(IEnumerable<T> value)
	{
		_value.AddRange(value);
	}
	
	public override object StorageObject()
	{
		return _value;
	}
}