Chapter 8 Public Property

ChuckASP · March 6th, 2010, 11:02 AM

Imar,

I've been programming legacy systems for years and at times it is difficult to grasp all the oop concepts. I don't quite understand how to explain the VB.NET property code at the bottom of page 279. How can you define a Private variable called _displayDirection of type Direction and also have a Property called DisplayDirection as type Direction? I understand that Direction is a class or type that you create and then you define the variable as that type, but I don't get how it can be a property too.

Thanks,

Chuck

Imar · March 6th, 2010, 11:19 AM

Hi Chuck,

A Property is of a certain type. Consider this property to hold someone's first name:

Code:

 
Private _firstName As String = "Chuck"
 
Public Property FirstName() As String
  Get
    Return _firstName
  End Get
  Set(ByVal Value As String)
    _firstName = Value
  End Set
End Property

Here, the name of the property is FirstName. It's type is a String (or System.String to be exact). As the property's *backing store* the private variable _firstName is used. The underscore gives this variable a different name than the public property DisplayDirection. The type of that private variable is also a string. As a default value, it gets assigned the text "Chuck" (this is not required though).

When the property is asked for its value (when its Getter is accessed) it returns the value stored in _firstName. When the property gets assigned a value (its Setter is accessed) the value is stored in _firstName. In other words, properties don't contain their own state, but use a backing store (such as a private variable, but in later examples you see how to use ViewState for this purpose) to store their data.

The same principle is used for the DisplayDirection property:

Code:

 
Private _displayDirection As Direction = Direction.Vertical
 
Public Property DisplayDirection() As Direction
 Get
   Return _displayDirection
 End Get    Set(ByVal value As Direction)
   _displayDirection = value
 End Set
End Property

Here, the property is called DisplayDirection. Its type is Direction which is the Enum you defined earlier. As the backing store, the private variable _displayDirection is used. This variable gets the Enum value of Direction.Vertical assigned as its default value.

Does this clarify things? If not, feel free to post a follow message asking for clarification.

Cheers,

Imar

ChuckASP · March 6th, 2010, 11:46 AM

Yes, this makes sense now.
It's just odd, from the point of view of some one who has written in 3rd generation code, to go through all this just to set a variable to "Horizontal or "Vertical". I guess this has to be done to conform to all the oop concepts.

Thanks again,

Chuck

Imar · March 6th, 2010, 12:04 PM

You don't *have* to use a propeprty. This would work as well:

Code:

 
Public DisplayDirection As Direction = Direction.Vertical

However, with a public field you lose control over the values that are set. A property is about encapsulation and hiding the implementation. Your external code just accesses the property, and how it behaves internally is up to that property alone. This enables you to, for example, switch from using a private variable as the backing store to ViewState without modifying any external / calling code.

Hope this helps,

Imar