密封

应用于类时，sealed 修饰符可防止其他类继承它。

class A { }
sealed class B : A { }
class C : B { } //error : Cannot derive from the sealed class

当应用于 virtual 方法（或虚拟属性）时，sealed 修饰符会阻止在派生类中覆盖此方法（属性）。

public class A 
{
    public sealed override string ToString() // Virtual method inherited from class Object
    {
        return "Do not override me!";
    }
}

public class B: A 
{
    public override string ToString() // Compile time error
    { 
        return "An attempt to override"; 
    }
}