Class Statement (Visual Basic)

Declares the name of a class and introduces the definition of the variables, properties, events, and procedures that the class comprises.

[ <attributelist> ] [ accessmodifier ] [ Shadows ] [ MustInherit | NotInheritable ] [ Partial ] _
Class name [ ( Of typelist ) ]
    [ Inherits classname ]
    [ Implements interfacenames ]
    [ statements ]
End Class

Parts

Remarks

A Class statement defines a new data type. A class is a fundamental building block of object-oriented programming (OOP). For more information, see Objects and Classes in Visual Basic.

You can use Class only at namespace or module level. This means the declaration context for a class must be a source file, namespace, class, structure, module, or interface, and cannot be a procedure or block. For more information, see Declaration Contexts and Default Access Levels (Visual Basic).

Each instance of a class has a lifetime independent of all other instances. This lifetime begins when it is created by a New Operator (Visual Basic) clause or by a function such as CreateObject. It ends when all variables pointing to the instance have been set to Nothing (Visual Basic) or to instances of other classes.

Classes default to Friend (Visual Basic) access. You can adjust their access levels with the access modifiers. For more information, see Access Levels in Visual Basic.

Rules

• Nesting. You can define one class within another. The outer class is called the containing class, and the inner class is called a nested class.

• Inheritance. If the class uses the Inherits Statement, you can specify only one base class or interface. A class cannot inherit from more than one element.

  A class cannot inherit from another class with a more restrictive access level. For example, a Public class cannot inherit from a Friend class.

  A class cannot inherit from a class nested within it.

• Implementation. If the class uses the Implements Statement, you must implement every member defined by every interface you specify in interfacenames. An exception to this is reimplementation of a base class member. For more information, see "Reimplementation" in Implements Clause (Visual Basic).

• Default Property. A class can specify at most one property as its default property. For more information, see Default (Visual Basic).

Behavior

• Access Level. Within a class, you can declare each member with its own access level. Class members default to Public (Visual Basic) access, except variables and constants, which default to Private (Visual Basic) access. When a class has more restricted access than one of its members, the class access level takes precedence.

• Scope. A class is in scope throughout its containing namespace, class, structure, or module.

  The scope of every class member is the entire class.

  Lifetime. Visual Basic does not support static classes. The functional equivalent of a static class is provided by a module. For more information, see Module Statement.

  Class members have lifetimes depending on how and where they are declared. For more information, see Lifetime in Visual Basic.

• Qualification. Code outside a class must qualify a member's name with the name of that class.

  If code inside a nested class makes an unqualified reference to a programming element, Visual Basic searches for the element first in the nested class, then in its containing class, and so on out to the outermost containing element.

Classes and Modules

These elements have many similarities, but there are some important differences as well.

• Terminology. Previous versions of Visual Basic recognize two types of modules: class modules (.cls files) and standard modules (.bas files). The current version calls these classes and modules, respectively.

• Shared Members. You can control whether a member of a class is a shared or instance member.

• Object Orientation. Classes are object-oriented, but modules are not. You can create one or more instances of a class. For more information, see Objects and Classes in Visual Basic.

Example

The following example uses a Class statement to define a class and several members.

Class bankAccount
    Shared interestRate As Decimal 
    Private accountNumber As String 
    Private accountBalance As Decimal 
    Public holdOnAccount As Boolean = False 

    Public ReadOnly Property balance() As Decimal 
        Get 
            Return accountBalance
        End Get 
    End Property 

    Public Sub postInterest()
        accountBalance = accountBalance * (1 + interestRate)
    End Sub 

    Public Sub postDeposit(ByVal amountIn As Decimal)
        accountBalance = accountBalance + amountIn
    End Sub 

    Public Sub postWithdrawal(ByVal amountOut As Decimal)
        accountBalance = accountBalance - amountOut
    End Sub 
End Class

See Also

Tasks

How to: Use a Generic Class (Visual Basic)

Reference

Interface Statement (Visual Basic)

Module Statement

Property Statement

Concepts

Structures and Classes (Visual Basic)

Object Lifetime: How Objects Are Created and Destroyed (Visual Basic)

Generic Types in Visual Basic (Visual Basic)

Other Resources

Objects and Classes in Visual Basic