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6 Type and Member Basics
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public override String ToString() { return String.Format("({0}, {1})", m_x, m_y); } public static void Main() { Point p = new Point(3, 4); // The C# compiler emits the callvirt instruction here but the // JIT compiler will optimize this call and produce code that // calls ToString nonvirtually because p's type is Point, // which is a sealed class Console.WriteLine(p.ToString()); } } n
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Security and predictability A class must protect its own state and not allow itself to ever become corrupted . When a class is unsealed, a derived class can access and manipulate the base class s state if any data fields or methods that internally manipulate fields are accessible and not private . In addition, a virtual method can be overridden by a derived class, and the derived class can decide whether to call the base class s implementation . By making a method, property, or event virtual, the base class is giving up some control over its behavior and its state . Unless carefully thought out, this can cause the object to behave unpredictably, and it opens up potential security holes .
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The problem with a sealed class is that it can be a big inconvenience to users of the type . Occasionally, developers want to create a class derived from an existing type in order to attach some additional fields or state information for their application s own use . In fact, they may even want to define some helper or convenience methods on the derived type to manipulate these additional fields . Since sealed classes restrict this ability, I made a proposal to the CLR team that they introduce a new class modifier called closed. A closed class can be used as a base class, but its behavior is closed and not subject to interference by a derived class . Basically, a closed base class would prohibit a derived class from accessing any of the base class s non-public members . This would allow the base class to change with the knowledge that it will not impact a derived class . Ideally, compilers would change the default access modifier for types to closed because this would be the safest choice without being too restrictive . It is too early to know if this idea will make its way into the CLR and programming languages . However, I am very hopeful it will someday . By the way, you could almost accomplish today what closed is designed to do; it s just that it is very inconvenient . Basically, when you implement your class, make sure you seal all the virtual methods you inherit (including the methods defined by System.Object) . Also, don t define any methods that may become a versioning burden in the future such as protected or virtual methods . Here is an example:
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Part II Designing Types
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public class SimulatedClosedClass : Object { public sealed override Boolean Equals(Object obj) { return base.Equals(obj); } public sealed override Int32 GetHashCode() { return base.GetHashCode(); } public sealed override String ToString() { return base.ToString(); } // Unfortunately, C# won't let you seal the Finalize method // Define additional public or private members here... // Do not define any protected or virtual members }
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Unfortunately, the compilers and the CLR do not support closed types today . Here are the guidelines I follow when I define my own classes:
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When defining a class, I always explicitly make it sealed unless I truly intend for the class to be a base class that allows specialization by derived classes . As stated earlier, this is the opposite of what C# and many other compilers default to today . I also default to making the class internal unless I want the class to be publicly exposed outside of my assembly . Fortunately, if you do not explicitly indicate a type s visibility, the C# compiler defaults to internal . If I really feel that it is important to define a class that others can derive but I do not want to allow specialization, I will simulate creating a closed class by using the above technique of sealing the virtual methods that my class inherits . Inside the class, I always define my data fields as private and I never waver on this . Fortunately, C# does default to making fields private. I d actually prefer it if C# mandated that all fields be private and that you could not make fields protected, internal, public, and so on . Exposing state is the easiest way to get into problems, have your object behave unpredictably, and open potential security holes . This is true even if you just declare some fields as internal. Even within a single assembly, it is too hard to track all code that references a field, especially if several developers are writing code that gets compiled into the same assembly . Inside the class, I always define my methods, properties, and events as private and nonvirtual . Fortunately, C# defaults to this as well . Certainly, I ll make a method, property, or event public to expose some functionality from the type . I try to avoid making any of these members protected or internal, as this would be exposing my type to some potential vulnerability . However, I would sooner make a member protected or internal than I would make a member virtual because a virtual member gives up a lot of control and really relies on the proper behavior of the derived class . There is an old OOP adage that goes like this: when things get too complicated, make more types . When an implementation of some algorithm starts to get complicated, I define helper types that encapsulate discrete pieces of functionality . If I m defining
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