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Listing 64: Implicit Base Type Casting
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public class Program { pulic static void Main() { // Derived types can be implicitly converted to // base types Contact contact = new Contact(); PdaItem item = contact; // // Base types must be cast explicitly to derived types contact = (Contact)item; // } }
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The derived type, Contact, is a PdaItem and can be assigned directly to a PdaItem This is known as an implicit conversion because no specific operator is required and the conversion will, on principle, always succeed; it will not throw an exception The reverse, however, is not true A PdaItem is not necessarily a Contact; it could be an Appointment or some other undefined, derived type Therefore, casting from the base type to the derived type requires an explicit cast, which at runtime could fail To perform an explicit cast, identify the target type within parentheses prior to the original reference, as Listing 64 demonstrates With the explicit cast, the programmer essentially communicates to the compiler to trust her, she knows what she is doing, and the C# compiler allows the conversion as long as the target type is derived from the originating type Although the C# compiler allows an explicit conversion at compile time between potentially compatible types, the CLR will still verify the explicit cast at execution time, throwing an exception if in fact the object instance is not of the targeted type The C# compiler allows the cast operator even when the type hierarchy allows an implicit cast For example, the assignment from contact to item could use a cast operator as follows:
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item = (PdaItem)contact;
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or even when no cast is necessary:
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contact = (Contact)contact;
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6: Inheritance
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BEGINNER TOPIC Casting within the Inheritance Chain An implicit conversion to a base class does not instantiate a new instance Instead, the same instance is simply referred to as the base type and the capabilities (the accessible members) are those of the base type It is just like referring to a CD-ROM drive (CDROM) as a storage device Since not all storage devices support an eject operation, a CDROM that is viewed as a storage device cannot be ejected either, and a call to storageDeviceEject() would not compile even though the instantiated object may have been a CDROM object that supported the Eject() method Similarly, casting down from the base class to the derived class simply begins referring to the type more specifically, expanding the available operations The restriction is that the actual instantiated type must be an instance of the targeted type (or something derived from it)
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ADVANCED TOPIC Defining Custom Conversions Conversion between types is not limited to types within a single inheritance chain It is possible to convert between 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 conversion operators Anytime the operation could possibly fail, such as in a cast from long to int, developers should choose to define an explicit conversion operator This warns developers performing the conversion to do so only when they are certain the conversion will succeed, or else to be prepared to catch the exception if it doesn t They should also use explicit conversions over an implicit conversion 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 conversion operators for Address to string and vice versa
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Derivation
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Listing 65: Defining Cast Operators
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class GPSCoordinates { // public static implicit operator UTMCoordinates( GPSCoordinates coordinates) { // } }
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In this case, you have an implicit conversion from GPSCoordinates to UTMCoordinates A similar conversion could be written to reverse the process Note that an explicit conversion could also be written by replacing implicit with explicit
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private Access Modifier 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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