Using Structures in VB.NET

Introduction

Structures are complex data types that encapsulate small pieces of tightly related data items. As with Classes, Structures can contain data members as well as member methods to work with data inside a structure. Since the in-memory representation of Structure is 'value type', memory management is handled efficiently.

Structures Vs Classes

Structures	Classes
Value type	Reference type
Supports data members, code members (methods) and events	Supports data members, code members (methods) and events
Can not inherit	Supports inheritance
Preferable when you perform large number of operations on each instance	Preferable when you need to initialize one or more members upon creation
Can not control initialization using structure variable	Can have parameterized constructors
Less flexible, limited event handling support	More flexible, unlimited event handling support

Declaration

Structures are declared using Structure ... End Structure statements. Between these two statements, there must be at least one member declared. This member can be of any data type, non-shared and non-event. The following example shows how to encapsulate product info into a structure called Product.

Private Structure Product
    'Declare data members
    Public ProductID as Integer
    Public ProductName as String
    Private UnitPrice as Single

    'Declare code members
    Public Function GetPrice() as Single
        GetPrice = UnitPrice
    End Function
End Structure

In the above example, the Product structure is declared as Private. That means you can not access it from the object of the class where you defined it. As you can see, the data member UnitPrice is declared as Private, yes, you are right, you can't access it outside the structure.

Note that there is a function GetPrice defined within the Structure. Using this code member, one can get UnitPrice of a product. You can also define Sub procedures, properties and events. You can define a property as a default property, but it must accept at least one argument. You can declare events using Shared Sub procedure. To cause an event to actually happen, you have to use RaiseEvent statement.

The default accessibility for a structure is Public. You can use Private, Protected and Friend keywords also. You must specify accessibility for every member you declare inside a Structure. If you declare them with Dim statement, they are considered as Public. Remember that you can not initialize a Structure's data members at the time of instantiation. You have to access and initialize them individually after creation of a Structure variable or through a Structure code member. If you assign one Structure variable to another, a new copy is created in the other variable. Hence the changes to the values of first variable does not happen to the second. On the other hand, if you assign a Class object to another, changes to the data of one object reflects in the other. This is obvious in the case of Classes because the two objects in the above situation share the same instance.

Conclusion

• Use Structures only if you want to use tiny pieces of related data as a single composite data type that resembles a built-in data type.
• Use Classes if you can't decide which one to use among Classes and Structures. Because, Classes are more flexible. Though there are storage and performance advantages with Structures, often they are negligible.