Structures and Classes

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11/18/2006

Visual Basic unifies the syntax for structures and classes, with the result that both entities support most of the same features. However, there are also important differences between structures and classes.

Classes have the advantage of being reference types — passing a reference is more efficient than passing a structure variable with all its data. On the other hand, structures do not require allocation of memory on the global heap.

Because you cannot inherit from a structure, structures should be used only for objects that do not need to be extended. Use structures when the object you wish to create has a small instance size, and take into account the performance characteristics of classes versus structures.

Similarities

Structures and classes are similar in the following respects:

Both are container types, meaning that they contain other types as members.
Both have members, which can include constructors, methods, properties, fields, constants, enumerations, events, and event handlers. However, do not confuse these members with the declared elements of a structure.
Members of both can have individualized access levels. For example, one member can be declared Public and another Private.
Both can implement interfaces.
Both can have shared constructors, with or without parameters.
Both can expose a default property, provided that property takes at least one parameter.
Both can declare and raise events, and both can declare delegates.

Differences

Structures and classes differ in the following particulars:

Structures are value types; classes are reference types. A variable of a structure type contains the structure's data, rather than containing a reference to the data as a class type does.
Structures use stack allocation; classes use heap allocation.
All structure elements are Public by default; class variables and constants are Private by default, while other class members are Public by default. This behavior for class members provides compatibility with the Visual Basic 6.0 system of defaults.
A structure must have at least one nonshared variable or nonshared, noncustom event element; a class can be completely empty.
Structure elements cannot be declared as Protected; class members can.
A structure procedure can handle events only if it is a Shared (Visual Basic) Sub procedure, and only by means of the AddHandler Statement; any class procedure can handle events, using either the Handles keyword or the AddHandler statement. For more information, see Events and Event Handlers.
Structure variable declarations cannot specify initializers or initial sizes for arrays; class variable declarations can.
Structures implicitly inherit from the System.ValueType class and cannot inherit from any other type; classes can inherit from any class or classes other than System.ValueType.
Structures are not inheritable; classes are.
Structures are never terminated, so the common language runtime (CLR) never calls the Finalize method on any structure; classes are terminated by the garbage collector (GC), which calls Finalize on a class when it detects there are no active references remaining.
A structure does not require a constructor; a class does.
Structures can have nonshared constructors only if they take parameters; classes can have them with or without parameters.

Every structure has an implicit public constructor without parameters. This constructor initializes all the structure's data elements to their default values. You cannot redefine this behavior.

Instances and Variables

Because structures are value types, each structure variable is permanently bound to an individual structure instance. But classes are reference types, and an object variable can refer to various class instances at different times. This distinction affects your usage of structures and classes in the following ways:

Initialization. A structure variable implicitly includes an initialization of the elements using the structure's parameterless constructor. Therefore, Dim s As struct1 is equivalent to Dim s As struct1 = New struct1().
Assigning Variables. When you assign one structure variable to another, or pass a structure instance to a procedure argument, the current values of all the variable elements are copied to the new structure. When you assign one object variable to another, or pass an object variable to a procedure, only the reference pointer is copied.
Assigning Nothing. You can assign the value Nothing (Visual Basic) to a structure variable, but the instance continues to be associated with the variable. You can still call its methods and access its data elements, although variable elements are reinitialized by the assignment.

In contrast, if you set an object variable to Nothing, you dissociate it from any class instance, and you cannot access any members through the variable until you assign another instance to it.
Multiple Instances. An object variable can have different class instances assigned to it at different times, and several object variables can refer to the same class instance at the same time. Changes you make to the values of class members affect those members when accessed through another variable pointing to the same instance.

Structure elements, however, are isolated within their own instance. Changes to their values are not reflected in any other structure variables, even in other instances of the same Structure declaration.
Equality. Equality testing of two structures must be performed with an element-by-element test. Two object variables can be compared using the Equals method. Equals indicates whether the two variables point to the same instance.

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Structures and Classes

Similarities

Differences

Instances and Variables

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