virtual properties

public virtual ICollection<B> Prop { get; set; }

Translates almost directly to:

private ICollection<B> m_Prop;

public virtual ICollection<B> get_Prop()
{
    return m_Prop;
}

public virtual void set_Prop(ICollection<B> value)
{
    m_Prop = value;
}

Thus, the virtual keyword allows you to override the property in sub-classes just as you would the above get/set methods:

public override ICollection<B> Prop
{
    get { return null; }
    set { }
}