Introduction to Interfaces in VB.NET
October 23rd, 2007
One problem some programmers new to VB.NET and object oriented programming in general have is understanding interfaces and how they related to classes. In this article, we’ll take an introductory look at interfaces and some of the ways you can use them in your VB.NET applications.
What is an Interface?
To put it in simple terms, an interface is a definition of methods and properties. Sometimes this is referred to as a ‘contract’. If a class agrees to implement an interface it must implement all of the properties and methods defined.
One difference you’ll see in an interface vs. a class is that there is no code, only the templates, aka contracts, for the properties and methods. You’ll also notice that there is no access level defined. It’s always considered public.
Here’s an example interface:
Public Interface IPerson Property ID() As Integer Property FirstName() As String Property LastName() As String End Interface
This means that if a class implements the IPerson interface it will have to implement these routines. Here’s an example of a class implementing this interface:
Public Class Customer Implements IPerson Protected _id As Integer Protected _firstName As String Protected _middleName As String Protected _lastName As String Public Property ID() As Integer Implements IPerson.ID Get Return _id End Get Set(ByVal value As Integer) _id = value End Set End Property Public Property FirstName() As String Implements IPerson.FirstName Get Return _firstName End Get Set(ByVal value As String) _firstName = value End Set End Property
Public Property MiddleName() As String Get Return _middleName End Get Set(ByVal value As String) _middleName = value End Set End Property Public Property LastName() As String Implements IPerson.LastName Get Return _lastName End Get Set(ByVal value As String) _lastName = value End Set End PropertyEnd Class
As you can see, each of the properties above, except for MiddleName, has an Implements statement that defines how it will fulfill the Interface contract. The rule is that you can add to the interface inside the implementing class but you can’t take away.
Remember that implementing an interface isn’t the same as creating an instance of a class or creating a subclass. All you’re doing is defining how the external interface should look, not what goes on inside. People do get confused over that point.
Coding Interfaces
Interfaces can be very simple or complex. Most current OOP thought suggests that interfaces be limited to a single method. For example, you might have an IUpdate interface that defines a single Update function and you might have another interface called ICreate that defines a single Create function. You would then combine them into classes that needed to implement them, as in this example:
Public Interface ICreate Function CreateNew(ByVal id As Integer) As Boolean End Interface Public Interface IUpdate Function Update(ByVal id As Integer) As Boolean End Interface
Public Class Customer Implements IPerson, IUpdate, ICreate
As you can see, a class can implement multiple interfaces. Whether it’s better to design with a lot of different interfaces or to group them into logical sections is really up to your own needs. My own take is a bit of a compromise in that I keep them simple with just a few tightly related methods or properties but I don’t religiously limit each interface to a single item.
Another use for interfaces is in defining a common interface for a plug-in architecture as I mentioned in this previous article: How To Create Application Plug-ins In VB.NET. In this case, you will want to have a more complex interface design in most cases since your main app will need a stronger connection to the plug-in.
Do you have any questions about using Interfaces in VB.NET? Please feel free to leave a comment and ask about it.
Entry Filed under: VB.NET Tutorials
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17 Comments Add your own
1. Khushal Patel | October 24th, 2007 at 9:54 pm
I think you should also give the diffrences between abstract Class Vs Interfaces. it will really helps newbies to understand about creating extensible appications. :-)
2. jfrankcarr | October 24th, 2007 at 10:07 pm
Thanks Khushal,
That’s a good idea. I’ll probably do an article on that at some point. I wrote this article mainly for someone who had looked at my article on creating application plug-ins and they were having trouble understanding what an Interface was.
3. Sangita Patra | November 18th, 2007 at 10:58 pm
can u please clear me that what is the need of interface in vb.net and the difference between a class and an interface
4. MARY | January 21st, 2008 at 2:19 pm
codes in visual basic for addin two numbers.
5. strangers | January 23rd, 2008 at 2:15 pm
Can you send me code for this:
I am developing common user control in one project where i have to develop a code for common user control.
So my problem how is that i am able to develop a code for the conneection of this form but i am not able to access the value.
More Clealy i would say it may any number of form how do i navigate a records with the same user control. I have used adodb connectivity.
6. Greg Frantsen | January 30th, 2008 at 10:59 am
I’m still a little foggy on the “why” of interfaces. Why would someone want to use them? What do they promote/protect against? Maybe an example of something with and w/o an interface?
Thanks.
7. Carlosfocker | May 5th, 2008 at 11:42 am
The biggest thing that interfaces protect you from is the dangers of multiple inheritance. I would suggest doing some research on the diamond of death issue that comes up with languages that allow multiple inheritance.
8. AMol | June 6th, 2008 at 8:32 am
This is a useful article.I got clear understanding of interfaces
9. Rakesh | July 11th, 2008 at 4:59 am
I do not understand that how Interface are subsitute of multiple inheritance in vb.net.Can we inherit System.Web.UI.Page and Syatem.Web.UI.MasterPage in a single class using Interface?
10. Rakesh | July 14th, 2008 at 1:57 am
I do not understand how Interface is subsitute of multiple inheritance in vb.net? Can we inherit “System.Web.UI.Page” and “System.Web.UI.MasterPage” in a single class?
11. Ankit | September 24th, 2008 at 12:49 am
You give a great focus on Interface. But i would like to say that if you give any real time example of Use of Interface which demostrate in which situation this interface very helpful to us then it will be great. I want to say give one demo application in which Progammer can understand the need of Interface
12. behzad_mn | September 28th, 2008 at 5:10 am
Hi,
I just dont undrestand why we need interface?
how it helps us?!
tank you for your article.
13. Faisal Zaman | November 5th, 2008 at 3:35 am
I am not understaning about very well because you should take a reall examale in this topic.
14. piyush | December 8th, 2008 at 3:19 am
i am searching for a project in vb.net and i want full information regarding about vb.net and its related topics
15. srivani | February 4th, 2009 at 3:13 am
I havae got just abasic idea about INTERFACES
but I have not under stand what r the advantages of Interfaces for technical or flexible or developer convinense or any other
ANY WAYS THank u very much for providing this
16. Will Cooper | March 26th, 2009 at 2:13 pm
This is a great article. I use interfaces but often have trouble trying to explain to others there benefit around structure and why use them at all. Many feel thye just add a layer of complexity without much benefit. If you don’t mind I’ll be using this article as it is pitched at the right level.
Many thanks
17. David | April 17th, 2009 at 10:16 am
Can you please describe how using an interface would help with a circular dependancy problem in VS?
e.g.
assembly 1 references assembly 2
assembly 1 references assembly 3
assembly 3 references assembly 2
I have read that i could use an interface to remove the problem between assembly 2 and 3. This is not a design issue as both 3 and 2 are class libraries which have to be seperated.
Thanks.
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