Property Statement

Language Items List

Definition:

Used to define a new procedural property. A "procedural" property is similar to a regular property in that it associates a name with access to a piece of data of a certain type. The method for setting and retrieving this data, however, is unique.

Syntax:

Property<PropertyName>Get<GetFunctionName>[Set<SetSubName>]As<type>

Syntax Description

PropertyName The name of a procedural property.

GetFunctionName The name of the function to run to retrieve the current value

SetSubName The name of the Sub to run to set the current value.

type The type of the property, such as Integer or Object. Any type that it is valid to specify for a Function is valid here.

The syntax for GetFunctionName is as follows:

Function GetFunctionName() As type
GetFunctionName = <current value>
End Function

Sub SetSubName(newValue as type)
<current value> = newValue
End Sub

The function must take no arguments and must be defined to return the same type declared with the procedural property. The sub must take a single argument of the same type as the procedural property.

Details:

Procedural properties are useful when:

images/Phx90000.gif the data requires special validation rules.

images/Phx90000.gif the representation of the data is externally held. (e.g. Visibility of Window).

images/Phx90000.gif the setting of the data requires side affects.

Using Procedural properties can reduce the housekeeping overhead that is usually associated with trying to maintain two versions of the same thing. Like trying to keep track of whether a window is visible or not. This is error-prone if done with anything except a procedural property that is really a transparent mechanism for getting and setting the "real" state of the data.

To set the value of the property the "SetSubName" Sub is called. To retrieve the current value of the property the "GetFunctionName" is called. Hence the name "procedural." Procedures are run to access the data. This allows for a great degree of flexibility in how the data is represented.

Note: Since the Set syntax is optional, it is possible to make a property that can have its value queried, but not set. This corresponds to a ReadOnly property.

When calling SetSubName, it is not required to accept the given value. Just like any fundamental data type, value validation can be performed and certain sets can be ignored. If they are ignored, they can be ignored silently, or they can throw an interpretive exception, as shown in the following:

Sub SetSubName(newValue as Integer)
If (newValue <= 0) Then Throw IllegalSetValue(newValue)
...
End Sub

This type of validation could correspond to a procedural property that is defined to take only positive non-zero values.