Introduction
Classes are a very powerful tool in intermediate to advanced level VBA programming. This page is an introduction to what a class and an object are and will hopefully get you started working with classes. This is by no means a comprehensive guide. Entire books have been written about Object Oriented Programming, of which classes are an essential component.
In VBA, a class is defined in class module and serves as a template for an object. The term object is deliberately vague. An object can be defined to represent whatever you want. Anything that you can describe conceptually can be represented by a class. The difference between a class and an object is that a class does nothing and consumes no memory. It is like a blueprint. When you have a variable of that class type and create instance of that class with the New keyword, a process called instantiating, it becomes an object and consumes memory and can carry out actions. A class is defined by its properties, which describe attributes of the class, and its methods (sub and function procedures), which carry out actions in the object. If a class is analogous to a noun, a property is like an adjective — it describes the object. A method is like a verb — it carries out an action.
You must instantiate a class into an object in order to do anything with it. There is nothing you can do with a class module beyond creating an object from it. An example of instantiation is shown below:
Dim C As Class1 Set C = New Class1
where Class1 is the name of the class module. Unlike other languages, VB/VBA allows for only one class in a class module, and the name of the class is the name of the module. You can now work with the properties and methods defined in Class1 in the C object variable.
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Before getting in to classes and objects, it will prove useful to examine briefly a class’s logical ancestor, the Type declaration. A Type is made up of other basic variable types. You may be familiar with Types from other programming languages, in which they are called a struct, structure, or record. For example, we could declare a Type that describes an employee:
Type Employee Name As String Address As String Salary As DoubleEnd Type
This defines a single type named Employee which has three elements: Name, Address, and Salary. You can then create variables of the Employee type and give values to the elements. For example,
Dim Manager As Employee Manager.Name = “Joe Smith” Manager.Address = “123 Main Street” Manager.Salary = 40000
Types are quite useful, but have three shortcomings. First, you can’t declare new instances of a Type. You must declare all the variables you’ll need at design time or you need a dynamic array that is resized with Redim Preserve, an awkward and expensive operation. The second shortcoming of a Type is that you have no control over what values are assigned to the elements of aType. For example, there is nothing to prevent the assignment of a negative value to the Salary element. Finally, a Type can’t do anything. It cannot carry out actions; it is simply a static data structure.
While Types have their place (they are used extensively in Windows API functions), a class module is often a better solution. New instances of a class may be created with the New keyword and stored in a Collection or Dictionary object. Next, the properties of a class can be set or retrieved with Property Let and Property Get procedures, which can contain executable code. Thus, code could be written to raise an error or take other appropriate action if an invalid value is used to set a property value, such as a negative value for a Salary. Finally, classes have methods (sub and function procedures) which can carry out actions. In the example of an employee, there might be a method to print a paycheck for the employee.
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Class Basics
For illustration, let’s adapt the Employee Type described above into a class. First, insert a class module into your VBProject (from the Insert menu in the VBA editor). Name the class CEmployee (it is common practice to use a ‘C’ as the first letter of a class). There are three properties to create: Name, Address, and Salary. These values will be stored in private variables within the class. Since they are declared Private, they cannot be accessed outside the class module.
Private pName As String Private pAddress As String Private pSalary As Double
Next, we need to declare Property procedures to allow these variables to be read from and written to. This is done with Property Get and Property Let functions (or Property Set for object type variables).
””””””””””” ‘ Name property ””””””””””” Public Property Get Name() As String Name = pName End Property Public Property Let Name(Value As String) pName = Value End Property ””””””””””” ‘ Address property ””””””””””” Public Property Get Address() As String Address = pAddress End Property Public Property Let Address(Value As String) pAddress = Value End Property””””””””””” ‘ Salary property ””””””””””” Public Property Get Salary() As Double Salary = pSalary End Property Public Property Let Salary(Value As Double) pSalary = Value End Property
The Get procedure is used to return a value out of the class, and the Let procedureis to put a value into the class. Note that the return data type of the Get property procedure must be the same data type as the (last) parameter to theLet property procedure. Otherwise, you’ll get a compiler error.
Because Property procedures can contain any code you like, the Let Salary procedurecan be written to exclude non-positive values.
Public Property Let Salary(Value As Double) If Value > 0 Then pSalary = Value Else ‘ appropriate error code here End If End Property
A property can be made read-only simply by omitting the Let procedure. For example,a read-only property might be withholding tax, which is calculated when it is called.E.g.,
Property Get WithholdingTax() As Double WithholdingTax = calculated value End Property
Finally, the class can contain methods, such as a PrintPaycheck procedure.
Public Sub PrintPaycheck() ‘ actual code to print check End Sub
Now that we have defined the class, we can create objects based on the class. Ina standard code module, declare a variable of type CEmployee.
Dim Emp As CEmployee
Then, Set that variable to a new instance of the class and assign some property values.
Set Emp = New CEmployee Emp.Name = “Joe Smith” Emp.Address = “123 Main Street” Emp.Salary = 40000
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Storing Multiple Objects In A Collection
If you need to store multiple instances of a class, such as for a group of employees, you can create mutliple objects from theclass and store them in a Collection or Dictionary object, as shown below.
Dim Employees As Collection Dim Emp As CEmployeeSet Employees = New CollectionFor Each Item In SomeList Set Emp = New CEmployee ‘ set properties for Emp Employees.Add Emp Next Item
Now, you can use a simple For Each loop to loop through the collection and iteratethrough the collection and access each instance of CEmployee sequentailly:
Dim Emp As CEmployee For Each Emp In Employees Debug.Print Emp.Name Next Emp
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The Instancing Property Of A Class
The Instancing property of a class controls where that classmay be used. The default value is Private, which means that the class can be usedonly in the project in which the class is defined. Youcan set the instancing property to PublicNotCreatable, which allows a variable tobe declared as that class type in projects that have a reference to the projectcontaining the class. The second class may declare a variable of the class type,but cannot create an instance of the class with the New keyword. See the next sectionfor more details.
Using Classes In Multiple Projects
If the Instancing property of the class is PublicNotCreatable a variable of that class type may be declared in other projects, but cannot be created in that project. You can use a function in the project containing the class to return a new instance to the caller. First, change the name of the project containing the class from the default value of VBProject to something meaningful like projSourceProject. Then, in the class that will use the class, set a reference to projSourceProject. Back in the project containing the class, create a procedure that will create and return a new instance of the class:
Public Function GetClass() As CEmployee Set GetClass = New CEmployee End Function
Then call this function in the project that will use the class:
Dim NewEmp As projSourceProject.CEmployee Set NewEmp = projSourceProject.GetClass()
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Self-Referencing An Instance Of A Class
Code within a Property or method of a class can refer to its own instance by using the Me reference. For example,
Private pName As String Property Let Name(S As String) pName = S End Property Public Sub SomeMethod() '''''''''''''' ' some code '''''''''''''' Me.Name = "ABCD" End Sub
This refers to the Name property of the instance of the class from which it is executed. Other programming languages, such as C++, C# and Java, use the keyword this to refer to the instance of a class.
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Setting The Default Value Of A Class
You can specify a property to be the default property of a class. When you do this, you can omit that property name and the compiler will use the default property. For example if you made Name the default property, the following lines of code are functionally equivalent: Emp.Name = “Joe Smith” Emp = “Joe Smith”
See Default Property Of A Class for information and examples of creating a default property.
This page just scratches the surface of what you can do with classes. Consult a goodbook on VBA or VB6 programming for a more in depth treatment of classes.
This page last updated: 26-Jan-2012
待译。
引自： http://www.cpearson.com/excel/classes.aspx
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