The reverse cast, contravariance, is not supported either in C#.NET

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Support for Parameter Covariance and Contravariance
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C#'s inheritance implementation doesn't support either covariance or contravariance Covariance allows overriding a member by supplying a method in the derived class to be a more specialized type For example, covariance would allow the method on a derived class, void Draw(Widget part), to override the base class, perhaps an abstract member, void Draw(Part part), where Widget derives from Part The problem with that covariance is that the base class allowed for any Part to be drawn, but the derived type allows for only the drawing of type Widget Therefore, does the derived type handle the drawing of Part's Presumably it should, but not via the implementation of void Draw(Widget part), because it handles only Widget Effectively, therefore, void Draw(Widget part) does not override void Draw(Part part), but instead, it overloads it, providing a second method with the same name Covariance is especially questionable when overriding abstract members An abstract member requires that the derived class implements a specific signature, and if the derived class specializes the signature, narrowing what is actually supported, then the full functionality required by the abstract method will be lacking Contravariance occurs when the derived type's members have broader signatures than the base type's signature For example, the base class could have a method, void Add(Widget widget) , and the derived class would override the method with void Add(Part part) This would allow addition of a Part to the class via the derived method, but since the base class always expects a Widget, it may call into the added item as though it was a Widget and fail Keeping with the strong type semantics of C#, neither covariance nor contravariance is supported
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Advanced Topic: Defining Custom Conversions
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Casting between types is not limited to types within a single inheritance chain It is possible to cast entirely unrelated types as well The key is the provision of a conversion operator between the two types C# allows types to include either explicit or implicit cast operators Any time the operation could possibly fail, such as in a cast from long to int , developers should choose to define an explicit cast operator This warns developers performing the cast to do so only when they are certain the cast will succeed, or else to be prepared to catch the exception if it doesn't They should also use explicit casts over an implicit cast when the conversion is lossy Converting from a float to an int , for example, truncates the decimal, which a return cast (from int back to float) would not recover Listing 65 shows implicit and explicit cast operators for Address to string and vice versa
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Listing 65 Defining Cast Operators
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class Address { // public static implicit operator string( Address address) { // } public static explicit operator Address( string addressText) { // } }
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In this case, you have an implicit cast from Address to string because all Address objects can be converted successfully to a string However, you have an explicit cast from string to Address because strings will not necessarily be valid addresses
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private Access Modifier
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All public members of a base class are available to the derived class However, private members are not For example, in Listing 66, the private field, _Name, is not available on Contact
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Listing 66 Private Members Are Not Inherited
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public class PdaItem { private string _Name; // } ___________________________________________________________________________ ___________________________________________________________________________ public class Contact : PdaItem { // } ___________________________________________________________________________ ___________________________________________________________________________ public class Program { public static void Main() { Contact contact = new Contact(); // ERROR:'PdaItem _Name' is inaccessible // due to its protection level // contact_Name = "Inigo Montoya"; } }
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As part of keeping with the principal of encapsulation, derived classes cannot access members declared as private This forces the base class developer to make an explicit choice as to whether a derived class gains access to a member In this case, the base class is defining an API in which _Name can be changed only via the Name property That way, if validation is added, the derived class will gain the validation benefit automatically because it was unable to access _Name directly from the start
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