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Table 5-2. Struct Declaration Modifier Availability
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Struct Declaration Context Member of Namespace Accessibility public protected private internal protected internal
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Member of Class Member of Struct
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N/A N/A (default) Member of Namespace N/A
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(default)
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N/A (default)
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Member of Class Member of Struct N/A
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5. Data Types
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Inheritance new abstract sealed virtual override Other readonly volatile static extern
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N/A N/A (implicit) N/A N/A N/A N/A N/A N/A
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N/A (implicit) N/A N/A N/A N/A N/A N/A
N/A N/A (implicit) N/A N/A N/A N/A N/A N/A
Empty constructors and structs
An empty constructor is one that takes no parameters. Although it's valid to define an empty constructor for a class, it's not valid to define one for a struct. The compiler implicitly defines an empty constructor for a struct, the body of which sets all of the struct members to their default values. This means that it's impossible to stop the instantiation of a struct using a private empty constructor. If the accessibility of the struct makes it visible, it can always be instantiated.
Instantiation
Unlike classes, struct instances are allocated memory as soon they are declared; the new keyword is not required. However, if the new keyword is not used, all field members of the struct must be explicitly assigned values prior to use; otherwise, a compiler error occurs. If the new keyword is used, the field members of the struct will be initialized to their default values. Struct variables can never be assigned the value null.
Members
Structs can contain the following member types: constant, field, method, property, event, indexer, operator, instance constructor, static constructor, and nested type declarations. Structs cannot contain destructors, as stack-based object structs are not subject to the garbage collection process.
More Information
For comprehensive coverage of these member types, see the "Members" section later in this chapter (beginning on page 88).
Assigning and passing structs
If a struct instance is passed as a function member parameter, returned from a function member, or assigned to a variable, a copy of the complete struct will be created. The copy is
5. Data Types
independent of the original struct; any changes made to the content of the new struct will not be reflected in the original. Should the need arise to pass a struct reference, use the ref or out modifier on the parameter. The need to use ref may indicate that a class would be a more appropriate alternative. See the "Variables" section later in this chapter for full details of the ref and out parameter modifiers.
Issues with structs
Thought should be given to how a data structure will be used before deciding whether to implement a class or a struct. The decision can affect the performance of an application. There are no definitive rules; only guidelines can be offered:
As a general rule, small and simple favors structs. The larger and more complex a data structure is, the more likely it should be implemented as a class. When working in a resource-constrained environment where memory needs to be freed quickly, structs will provide a benefit over the nondeterministic garbage collection of classes. Because structs are allocated on the stack, their memory is freed as soon as they go out of scope. If speed is of paramount importance, efficiency will be gained from the stack-based nature of structs. Frequently passing a struct as a parameter or assigning structs to variables can be expensive; bear in mind that a complete copy of the contents of that struct is created each time and is more costly than copying an object reference. Frequent method calls are better served by classes; the overhead of implicit boxing can forfeit benefits gained from the fact that structs are stack-based. When a data structure will be used inside a containing structure, the approach used by the container to manage its contents can dramatically affect overall performance. In a collection, a struct will be repeatedly boxed as the Equals or GetHashCode method is called. This will cause a significant performance overhead. In an array, a struct can provide higher efficiency than a class, as there is no need to look up references.
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