Why the New Modifier in C#.NET

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Table 61 Why the New Modifier
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Activity Programmer A defines class Person that includes properties FirstName and LastName
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Code public class Person { public string FirstName { get {return _FirstName; } set { _FirstName =value; } } private string _FirstName; public string LastName { get {return _LastName; } set { _LastName =value; } } private string _LastName; }
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Programmer B derives from Person and public class Contact : Person defines Contact with the additional property, { Name In addition, she defines the Program public string Name class whose Main() method instantiates { Contact, assigns Name, and then prints out the get name { return FirstName + " " + LastName; } set { string names =valueSplit(' '); // Error handling not shown FirstName = names[0]; LastName = names[1]; } } }
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Later, Programmer A adds the Name property public class Person but instead of implementing the getter as FirstName + " " + LastName, she implements { public string Name it as LastName + ", " + FirstName { Furthermore, she doesn't define the property get as virtual, and she uses the property in a { DisplayName() method return LastName + ", " + FirstName; } set { string names =valueSplit(', '); // Error handling not shown LastName = names[0]; FirstName = names[1];
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Activity
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Code }
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FirstName = names[1];
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} public static void Display(Person person) { // Display <LastName>, <FirstName> ConsoleWriteLine( personName ); } }
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Because PersonName is not virtual, Programmer A will expect Display() to use the Person implementation, even if a Person-derived data type, Contact, is passed in However, Programmer B would expect ContactName to be used in all cases where the variable data type is a Contact (Programmer B would have no code where PersonName was used, since no PersonName property existed initially) To allow the addition of PersonName without breaking either programmer's expected behavior, you cannot assume virtual was intended Furthermore, since C# requires an override member to explicitly use the override modifier, some other semantic must be assumed, instead of allowing the addition of a member in the base class to cause the derived class to no longer compile The semantic is the new modifier, and it hides a redeclared member of the derived class from the base class Instead of calling the most derived member, a member of the base class calls the most derived member in the inheritance chain prior to the member with the new modifier If the inheritance chain contains only two classes, then a member in the base class will behave as though no method was declared on the derived class (if the derived implementation redeclared the base class member) Although the compiler will report the warning shown in either Output 62 or Output 63, if neither override nor new is specified, then new will be assumed, thereby maintaining the desired version safety Consider Listing 614, for example Its output appears in Output 64
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Listing 614 override versus new Modifier
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public class Program { public class BaseClass { public void DisplayName() { ConsoleWriteLine("BaseClass"); } } public class DerivedClass: BaseClass { // Compiler WARNING new modifier assumed public virtual void DisplayName() { ConsoleWriteLine("DerivedClass"); } } public class SubDerivedClass : DerivedClass { public override void DisplayName() { ConsoleWriteLine("SubDerivedClass");
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} } public class SuperSubDerivedClass : SubDerivedClass { public new void DisplayName() { ConsoleWriteLine("SuperSubDerivedClass"); } } public static void Main() { SuperSubDerivedClass superSubDerivedClass = new SuperSubDerivedClass(); SubDerivedClass subDerivedClass = superSubDerivedClass; DerivedClass derivedClass = superSubDerivedClass; BaseClass baseClass = superSubDerivedClass; superSubDerivedClassDisplayName(); subDerivedClassDisplayName(); derivedClassDisplayName(); baseClassDisplayName(); } }
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Output 64
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SuperSubDerivedClass SubDerivedClass SubDerivedClass BaseClass
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These results occur for the following reasons
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SuperSubDerivedClass: SuperSubDerivedClassDisplayName() displays SuperSubDerivedClass
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because there is no derived class and hence, no overload
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SubDerivedClass: SubDerivedClassDisplayName() is the most derived member to override a base class's virtual member SuperSubDerivedClassDisplayName() is hidden because of its new modifier SubDerivedClass: DerivedClassDisplayName() is virtual and SubDerivedClassDisplayName() is the most derived member to override it As before, SuperSubDerivedClassDisplayName() is hidden because of the new modifier BaseClass: BaseClassDisplayName() does not redeclare any base class member and it is not
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virtual; therefore, it is called directly When it comes to the CIL, the new modifier has no effect on what code the compiler generates Its only effect is to remove the compiler warning that would appear otherwise
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