Is this functional C#?

gistfile1.cs

class Program
{
  static void Main(string[] args)
  {
      var item = InventoryItemPure.Register(new InventoryItemName("Domain Driven Design"));
      item = InventoryItemPure.CheckIn(item, new InventoryItemCount(4));
      item = InventoryItemPure.CheckOut(item, new InventoryItemCount(3));
      item = InventoryItemPure.Deactivate(item);

      Console.ReadLine();
  }
}

public class InventoryItemPure
{
    private readonly Guid _id;
    private readonly bool _activated;
    private InventoryItemName _name;
    private InventoryItemCount _count;

    public InventoryItemPure(Guid id, bool activated, InventoryItemName name, InventoryItemCount count)
    {
        Guard.ForNull(name, "name");
        Guard.ForNull(count, "count");

        _id = id;
        _activated = activated;
        _name = name;
        _count = count;
    }

    public Guid Id  { get { return _id; }}

    public bool Activated { get { return _activated; }}

    public InventoryItemName Name { get { return _name; }}

    public InventoryItemCount Count { get { return _count; }}

    public static InventoryItemPure CheckIn(InventoryItemPure item, InventoryItemCount count)
    {
        return new InventoryItemPure(item.Id, item.Activated, item.Name, count.UpBy(count));
    }

    public static InventoryItemPure CheckOut(InventoryItemPure item, InventoryItemCount count)
    {
        return new InventoryItemPure(item.Id, item.Activated, item.Name, count.DownBy(count));
    }

    public static InventoryItemPure Deactivate(InventoryItemPure item)
    {
        return new InventoryItemPure(item.Id, false, item.Name, item.Count);
    }

    public static InventoryItemPure Register(InventoryItemName name)
    {
        Guard.ForNull(name, "name");

        return new InventoryItemPure(Guid.NewGuid(), true, name, InventoryItemCount.Empty());
    }
}

public class InventoryItemCount
{
    public readonly int _value;

    public InventoryItemCount(int value)
    {
        if (value < 0) throw new ArgumentOutOfRangeException("value", "Item count shouldn't be negative.");

        _value = value;
    }

    public int Value
    {
        get { return _value; }
    }

    public InventoryItemCount DownBy(InventoryItemCount count)
    {
        return new InventoryItemCount(_value - count.Value);
    }

    public InventoryItemCount UpBy(InventoryItemCount count)
    {
        return new InventoryItemCount(_value + count.Value);
    }

    public static InventoryItemCount Empty()
    {
        return new InventoryItemCount(0);
    }

    public override string ToString()
    {
        return _value.ToString();
    }
}

public class InventoryItemName
{
    private readonly string _value;

    public InventoryItemName(string value)
    {
        Guard.ForEmpty(value, "value");

        _value = value;
    }

    public string Value { get { return _value; }}

    public override string ToString()
    {
        return Value;
    }
}

public class Guard
{
    public static void ForNull(object o, string description)
    {
        if (o == null)
            throw new ArgumentNullException(description);
    }

    public static void ForEmpty(string value, string description)
    {
        if (string.IsNullOrEmpty(value))
            throw new ArgumentNullException(description);
    }
}