为什么getHashCode()很重要?

我想知道 object.GetHashCode() 用于。我读到它被集合用来唯一地标识键。但我想测试一下,结果不是我所期望的。

struct Animal
{
    public string Name { get; set; }
    public int Age { get; set; }

    public Animal(string name, int age) : this()
    {
        Name = name;
        Age = age;
    }

    public override int GetHashCode()
    {
        return Age.GetHashCode();
    }
}

object doggy = new Animal("Dog", 25);
object cat = new Animal("Cat", 25);

Hashtable table = new Hashtable();
table.Add(doggy, "Dog");
table.Add(cat, "Cat");

Console.WriteLine("{0}", table[cat]);
Console.WriteLine("{0}", table[doggy]);

我本以为“cat”会覆盖“dog”或某种错误,告诉我“key already exists”,但输出是

“猫” “狗”