System.InvalidCastException

当我运行这段代码时，我得到了错误:

类型为"System"的未处理异常。在@override.exe中发生InvalidCastException附加信息:无法施放"动物"类型的对象。输入"animals.dog"。

class Animal
{
    public string name = "sdsfsdf";
    public virtual void bark() {
        Console.WriteLine("woohhoo");
    }
}
class Dog : Animal
{
    public int id = 11;         
    public  override void bark()
    {
        Console.WriteLine("woof");
    }
}
class Program
{
    static void Main(string[] args)
    {
        Animal a = new Animal();    
        //bark;
        a.bark();
        Dog d = new Dog();
        d.bark();
        // take out the virtual and override keyword infront of the method bark and see the difference*/
        Animal z = new Dog();
        z.bark();
        Console.WriteLine(z.name);
        Dog f = (Dog) new Animal();
        f.bark();
        Console.ReadKey();               
    }
}

Animal a = (Animal) new Dog();