靜態類

引用類時的 static 關鍵字有三個效果：

1. 你無法建立靜態類的例項（這甚至會刪除預設建構函式）
2. 類中的所有屬性和方法也必須是靜態的。
3. static 類是 sealed 類，意味著它不能被繼承。

public static class Foo
{
    //Notice there is no constructor as this cannot be an instance
    public static int Counter { get; set; }
    public static int GetCount()
    {
        return Counter;
    }
}

public class Program 
{
    static void Main(string[] args)
    {
        Foo.Counter++;
        Console.WriteLine(Foo.GetCount()); //this will print 1
        
        //var foo1 = new Foo(); 
        //this line would break the code as the Foo class does not have a constructor
    }
}