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Choosing Between Classes and Structures

Classes are reference types and structures are value types. Reference types are allocated on the heap, and memory management is handled by the garbage collector. Value types are allocated on the stack or inline and are deallocated when they go out of scope. In general, value types are cheaper to allocate and deallocate. However, if they are used in scenarios that require a significant amount of boxing and unboxing, they perform poorly as compared to reference types. For more information, see Boxing and Unboxing (C# Programming Guide).

For additional information about value types and reference types, see the Common Type System.

Consider defining a structure instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects.

Do not define a structure unless the type has all of the following characteristics:

  • It logically represents a single value, similar to primitive types (integer, double, and so on).

  • It has an instance size smaller than 16 bytes.

  • It is immutable.

  • It will not have to be boxed frequently.

If one or more of these conditions are not met, create a reference type instead of a structure. Failure to adhere to this guideline can negatively impact performance.

Portions Copyright 2005 Microsoft Corporation. All rights reserved.

Portions Copyright Addison-Wesley Corporation. All rights reserved.

For more information on design guidelines, see the "Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries" book by Krzysztof Cwalina and Brad Abrams, published by Addison-Wesley, 2005.

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