Property

Declares the name, arguments, and code that form the body of a Property procedure: a procedure that gets the value of a property, assigns a value to a property, or sets a reference to an object property.

A property is exposed to callers through up to three property procedures, all sharing the same name in the same module:

A Property Get procedure returns the property’s value (or object reference).
A Property Let procedure assigns a non-object value to the property.
A Property Set procedure assigns an object reference to the property.

Syntax:

[ attributes ]
[ Public | Private | Friend | Protected ] [ Static ] [ Overridable ] Property Get name [ ( Of typevars ) ] [ ( arglist ) ] [ As type ] [ binding-clause ]
     [ statements ] …
     [ [ Let ] name = expression ] …
     [ Set name = expression ] …
     [ Return expression ] …
     [ Exit Property ] …
     [ statements ] …
End Property
[ attributes ]
[ Public | Private | Friend | Protected ] [ Static ] [ Overridable ] Property Let name [ ( Of typevars ) ] ( [ arglist , ] value ) [ binding-clause ]
     [ statements ] …
     [ Exit Property ] …
     [ statements ] …
End Property
[ attributes ]
[ Public | Private | Friend | Protected ] [ Static ] [ Overridable ] Property Set name [ ( Of typevars ) ] ( [ arglist , ] reference ) [ binding-clause ]
     [ statements ] …
     [ Exit Property ] …
     [ statements ] …
End Property

attributes

optional One or more of the supported attributes for procedures.

Public

optional Indicates that the Property procedure is accessible to all other procedures in all modules. If used in a module that contains an Option Private statement, the procedure is not available outside the project.

Private

optional Indicates that the Property procedure is accessible only to other procedures in the module where it is declared.

Friend

optional Used only in a class module. Indicates that the Property procedure is visible throughout the project, but not visible to a controller of an instance of an object.

Protected

optional (twinBASIC) Used only in a class. Indicates that the Property procedure is accessible from inside the declaring class and from classes that derive from it via Inherits, but not from outside callers. All three accessor forms of the same property (Get, Let, Set) should agree on the access modifier.

Static

optional Indicates that the Property procedure’s local variables are preserved between calls. The Static attribute doesn’t affect variables that are declared outside the Property procedure, even if they are used in the procedure.

Overridable

optional (twinBASIC) Marks the Property as an inheritance hook that classes derived via Inherits may replace with an Overrides clause. Meaningful only on a member of a class that participates in an Inherits hierarchy.

name

Name of the Property procedure; follows standard variable naming conventions, except that the same name is shared by the matching Property Get, Property Let, and Property Set procedures in the same module.

Of typevars

optional One or more type variable names, following standard variable naming conventions. The names are separated by commas. Causes the procedure to be a generic Property procedure. The matching Property Get, Property Let, and Property Set procedures must declare the same generic parameters.

arglist

List of variables representing arguments that are passed to the Property procedure when it is called. Multiple arguments are separated by commas. The name and data type of each argument in a Property Let or Property Set procedure must be the same as the corresponding argument in the matching Property Get procedure. See arglist below for syntax. arglist is optional for Property Get; for Property Let and Property Set at least the value/reference parameter is required.

As type

optional Data type of the value returned by the Property Get procedure; may be Byte, Boolean, Integer, Long, Currency, Single, Double, Decimal, Date, String (except fixed length), Object, Variant, a user-defined type, or an array. The return type of a Property Get procedure must be the same data type as the value parameter of the corresponding Property Let procedure (if one exists), or compatible with the reference parameter of the corresponding Property Set procedure.

statements

optional Any group of statements to be executed within the body of the Property procedure.

expression

optional In Property Get, the value (or reference, when assigned with Set) returned by the procedure.

value

In Property Let, the variable that contains the value to be assigned to the property. When the procedure is called, this argument appears on the right side of the calling expression. The data type of value must be the same as the return type of the corresponding Property Get procedure. value cannot be Optional or a ParamArray.

reference

In Property Set, the variable containing the object reference used on the right side of the object reference assignment. reference cannot be Optional.

binding-clause

optional (twinBASIC) One of three trailing clauses that bind this accessor to a member declared elsewhere:

Handles object.event [ , object.event … ] — connects the property as a handler for the named event(s), replacing the traditional Object_Event naming convention. See Handles statement.
Implements iface.member [ , iface2.member2 … ] — provides the body for the named Interface (or Class) member, replacing the traditional Iface_Member naming convention. A comma-separated list permits one body to satisfy several interfaces’ members at once. See Implements statement.
Overrides base.member — supplies the body for an Overridable member inherited via Inherits. Combine with Overridable on the same header to allow further-derived classes to override again.

Exit Property

optional Immediately returns from the Property procedure without setting a return value. Valid in Property Get, Property Let, and Property Set.

Return expression

optional Valid only in a Property Get procedure. Immediately returns from the procedure with expression as the property’s value. The expression is required in this form; a bare Return is reserved for the GoSub…Return construct and does not exit a Property procedure.

arglist

Syntax: One or more of
[ Optional ] [ ByVal | ByRef ] [ ParamArray ] varname [ () ] [ As type ] [ = defaultvalue ]

Optional: optional Indicates that an argument is not required. If used, all subsequent arguments in arglist must also be optional and declared by using the Optional keyword. The right side of a Property Let or Property Set call (the value or reference parameter) cannot be Optional.
ByVal: optional Indicates that the argument is passed by value.
ByRef: optional Indicates that the argument is passed by reference. ByRef is the default unlike in Visual Basic .NET.
ParamArray: optional Indicates that the argument is an Optional array of Variant elements. The ParamArray keyword permits passing an arbitrary number of arguments. It may not be used with ByVal, ByRef, or Optional, and it may not be the value/reference parameter of a Property Let or Property Set procedure.
varname: Name of the variable representing the argument; follows standard variable naming conventions.
type: optional Data type of the argument passed to the procedure; may be Byte, Boolean, Integer, Long, Currency, Single, Double, Decimal, Date, String (variable length only), Object, Variant, a specific object type, or the name of a generic type argument. If the parameter is not Optional, a user-defined type may also be specified.
If the name of a generic type parameter is used, it becomes bound to the concrete type of the argument passed to the procedure. The name binding has the scope of the body of the procedure.
defaultvalue: optional Any constant or constant expression. Valid for Optional parameters only. If the type is an Object, an explicit default value can only be Nothing.

If not explicitly specified by using Public, Private, or Friend, Property procedures are public by default. If Static isn’t used, the value of local variables is not preserved between calls.

The Friend keyword can only be used in class modules. However, Friend procedures can be accessed by procedures in any module of a project. A Friend procedure doesn’t appear in the type library of its parent class, nor can a Friend procedure be late bound.

All executable code must be in procedures. A Property procedure cannot be defined inside another Property, Sub, or Function procedure.

The Exit Property statement, and the Return expression statement (in Property Get only), cause an immediate exit from a Property procedure. Program execution continues with the statement following the statement that called the Property procedure. Any number of these statements can appear anywhere in a Property procedure.

Like Sub and Function procedures, a Property procedure is a separate procedure that can take arguments, perform a series of statements, and change the values of its arguments. A Property Get procedure can be used on the right side of an expression in the same way as a Function or a property name. A Property Let procedure can only be used on the left side of a property assignment expression or Let statement. A Property Set procedure can only be used on the left side of an object reference assignment or Set statement.

Example

This example uses the Property statements to define a PenColor property: a Property Let that accepts a color name as a string and stores a numeric code, and a Property Get that returns the color name from the stored numeric code.

Dim CurrentColor As Integer
Const BLACK = 0, RED = 1, GREEN = 2, BLUE = 3

' Set the pen color property for a Drawing package.
' The module-level variable CurrentColor is set to
' a numeric value that identifies the color used for drawing.
Property Let PenColor(ColorName As String)
    Select Case ColorName ' Check color name string.
        Case "Red"
            CurrentColor = RED ' Assign value for Red.
        Case "Green"
            CurrentColor = GREEN ' Assign value for Green.
        Case "Blue"
            CurrentColor = BLUE ' Assign value for Blue.
        Case Else
            CurrentColor = BLACK ' Assign default value.
    End Select
End Property

' Returns the current color of the pen as a string.
Property Get PenColor() As String
    Select Case CurrentColor
        Case RED:   PenColor = "Red"
        Case GREEN: PenColor = "Green"
        Case BLUE:  PenColor = "Blue"
    End Select
End Property

' Calling code:
PenColor = "Red"        ' Calls Property Let.
ColorName = PenColor    ' Calls Property Get.

A Property Set procedure assigns an object reference, in much the same way as Property Let assigns a value:

' The Pen property may be set to different Pen implementations.
Property Set Pen(P As Object)
    Set CurrentPen = P ' Assign Pen to object.
End Property

Property

arglist

Example

See Also