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Implicit reference conversion allows use of a reference type where an instance of a different reference type is expected.
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Any reference type is implicitly convertible to System.Object. Any reference type is implicitly convertible to any class it derives from. Any reference type is implicitly convertible to any interface it implements. Any array type is implicitly convertible to System.Array. Arrays of the same dimension with underlying types that support implicit conversion are implicitly convertible. Any delegate type is implicitly convertible to System.Delegate. The null value is implicitly convertible to any reference type.
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Implicit boxing conversion allows the conversion of any value type to System.Object or any interface that the value type implements. The process of boxing is discussed in the "Boxing" and "Unboxing" sections earlier in this chapter.
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Explicit conversion requires the use of the cast expression; this is the same as Java, where the type to be cast is preceded by a set of brackets containing the target type. For example:
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float f = 23897.5473F; byte b = (byte)f;
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// cast float to byte (losing data)
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Using a cast where an implicit conversion exists incurs no penalty and can improve the readability of code, clarifying the programmer's intentions. We discuss the different types of explicit conversion in the following sections.
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Explicit numeric conversion
Explicit numeric conversion allows conversion from any numeric type to another. Depending on the types and values converted, information loss or exceptions can occur. See the section
5. Data Types
"The checked and unchecked Keywords" in 4 for details of how to handle numeric overflow.
Explicit enumeration conversion
Explicit enumeration conversion supports the following conversions:
From all simple numeric types to any enum type From any enum type to any simple numeric type From any enum type to any other enum type
Explicit reference conversion
Explicit reference conversion permits the conversion of one reference type to another. If an explicit reference conversion fails at runtime, the CLR will throw a System.InvalidCastException.
Explicit unboxing conversion
Unboxing conversions allow a previously boxed value type to be unboxed. The process of unboxing is discussed in the "Boxing" and "Unboxing" sections earlier in this chapter.
User-Defined Conversion
C# allows the programmer to define custom mechanisms for the implicit and explicit conversion of user-defined types. We'll discuss the syntax for defining custom conversions in the "Operators" section later in this chapter.
Members
Members are the programming elements and constructs that are contained in namespaces, classes, structs, and interfaces. Members are divided into three categories: functional, data, and type. Functional members are those that contain executable code, data members are constant or variable values, and type members are nested type declarations.
Member Types and Declaration Context
Table 5-11 contrasts the member types available in Java and C#. A Java developer will be familiar with many of the C# member types, but C# also adds some new ones. For the C# member types, we identify their valid declaration contexts.
Table 5-11. A Cross-Language Comparison of Member Types
Java Member Functional Members Constructor Instance initializer Static initializer
C# Member Instance constructor N/A Static constructor
C# Member Context Namespace N/A N/A N/A
Class Struct Interface N/A N/A N/A
N/A N/A
5. Data Types
Finalizer Method N/A N/A N/A N/A Data Members Constant Field Java Member Type Members Class Interface N/A N/A N/A
Destructor Method Property Event Indexer Operator Constant Field C# Member Class Interface Delegate Struct Enum
N/A N/A N/A N/A N/A N/A N/A N/A Namespace
N/A N/A N/A Class Struct Interface N/A N/A N/A N/A N/A
Versioning and Inheritance of Members
In Java, all methods not declared static or final are implicitly declared virtual. A derived class that implements a method with the same signature and return type as an inherited virtual method implicitly overrides that method. When a method is invoked on an object, the most overridden version of the method available to the runtime type of the object is used irrespective of the reference type used to refer to the object. This behavior is the basis of polymorphism. This approach can cause problems in derived classes when releasing new versions of a base class:
If a new version of a base class introduces a method with the same signature and return type as a method already declared in a derived class, any attempts to invoke that method will result in the invocation of the overridden method in the derived class. This will almost certainly give results different from those intended by the base class developers. If the method is marked as final to avoid this, the derived class will fail to compile. If a new version of the base class introduces a method with the same signature but different return type from a method already declared in a derived class, the derived class will fail to compile.
Although not common, these problems are more probable when deriving from third-party classes, where it's not possible to coordinate versioning. To overcome these problems, C# offers two alternative approaches to member inheritance: overriding and hiding.
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