What is an abstract class?

// Pointless abstract class
abstract class A
{

}
// All derived classes will have ID property and DoSomething method.
abstract class B
{
    private int id;

    public int ID
    {
        get { return id; }
        set { id = value; }
    }

    public void DoSomething()
    {
        Console.WriteLine("Doing it!");
    }
}
// All derived classes will have ID property and DoSomething method but they will have to implement them themselves
abstract class C
{
    public abstract int ID { get; set; }
    public abstract void DoSomething();
}
// All derived classes will have DoSomething method but they can chose to implement it themselves or use this implementation
abstract class D
{
    public virtual void DoSomething()
    {
        Console.WriteLine("Doing it!");
    }
}
// All derived classes can access InitialValue but it will not be available to non derived classes
abstract class E
{
    protected const int InitialValue = 5;
}
// All derived classes must call this class's constructor from their constructor passing an id value - see FUseage
abstract class F
{
    private int id;

    protected F(int id)
    {
        this.id = id;
    }

    public int ID
    {
        get { return id; }
        set { id = value; }
    }
}
class FUseage : F
{
    public FUseage()
        : base(0)
    { }
}