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Boxing takes an instance of a value type and converts it to an object or interface type. For example:
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// Box an int variable int myInt = 100; object myIntObject = myInt; System.Console.WriteLine("myIntObject = " + myInt.ToString()); // Box a long literal object myLongObject = 4500L;
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5. Data Types
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This example uses simple data types, but the same syntax also works for boxing structs. Structs can also be boxed into instances of interfaces they implement. For example, boxing a struct named MyStruct that implements the interface ISomeInterface is achieved as follows:
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MyStruct x = new MyStruct(); ISomeInterface y = x;
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The runtime implements boxing by instantiating a container object of the appropriate type and copying the data from the value type into it. It's important to understand that the boxed instance contains a copy of the source value. Any changes made to the original value are not reflected in the boxed instance.
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The C# compiler will implicitly box value types as required for example, invoking a function member of a struct or passing a value type where an object is expected. Given the overheads associated with boxing, overuse can affect program performance. Where performance is an issue, you should write programs to avoid the unnecessary use of implicit boxing.
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Unboxing is the reverse of boxing. It takes an object representing a previously boxed value and re-creates a value type from it. For example:
// Box an int variable int myInt = 100; object myIntObject = myInt; System.Console.WriteLine("myIntObject = " + myInt.ToString()); // Unbox int myOtherInt = (int)myIntObject;
As can be seen in this example, the previously boxed object must be explicitly cast to the appropriate type. The runtime checks that the boxed instance is being unboxed as the correct type; otherwise, it throws a System.InvalidCastException. It isn't possible to create a value type representation of any reference type using unboxing; unboxing works only on objects that contain previously boxed values.
Value Types
All value types in C# are of type struct or enum. Both struct and enum are types that were not implemented in Java, an omission that is frequently debated by language theorists. A set of inbuilt value types in C#, referred to as simple types, provides the same functionality as primitive types (int, long, float, and so forth) in Java, but their implementation is very different. All .NET inbuilt value types are implemented as structs; this is essential to enabling the boxing of inbuilt value types.
5. Data Types
Structs
A struct is a data structure that is similar to a class. The most important differences between classes and structs are a consequence of structs being a value type:
When instantiated, a struct is allocated memory on the stack or inline if it's a member of a heap resident object, such as a class. Memory allocated to a struct instance contains the member data, not references to the data. Struct instances are disposed of as soon as they lose scope; they are not garbage collected.
Declaration
A struct declaration takes the following form: [attributes] [modifiers] struct identifier : interfaces { body } Apart from the fact that structs do not support inheritance (discussed in the next section), structs are declared the same way as classes. For example, a public struct named MyStruct that implements the IMyInterface interface is declared as follows:
public struct MyStruct : IMyInterface { // function and data members }
Inheritance
Structs do not support user-defined inheritance. However, all structs implicitly derive from System.ValueType, which is in turn derived from System.Object.
Modifiers
The applicability of modifiers to a struct declaration depends on the context in which the struct is declared. Structs can be declared as top-level types (that is, direct members of an enclosing namespace) or nested within the definition of another struct or class. Table 5-2 summarizes modifier availability.
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