Level: Intermediate to Object Oriented Programming; Beginner + with .Net and C#

 

    Design Patterns are a very useful programming concept that is often forgotten about in the heat of a programming project. Design Patterns are basically code design templates that have been perfected over the years by many different programmers and architects. They represent repetitive design concepts that don’t differ much.

 

 

            Frequently with applications many of the operations they perform are dynamic depending on several factors.  Think about a common scenario, sales tax.  Tax amounts are based off the place where you live.  There are many different tax rates in each country.  A good method of implementing dynamic patterns like taxes is needed.  The strategy pattern covers this gap for us.

 

            The Proxy design pattern shows a way to do just in time loading of objects that would consume too much memory to keep around, or takes a lot of time to load.  This can be a very useful pattern for many applications.  A good example of this pattern is in Microsoft Office.  When you open a large Word document that has lots of embedded pictures, Office doesn’t load them all at the time you open the document.  As you scroll down, Office will pull the pictures from the disk file and insert them into the document.  You can see this by scrolling very fast down the document.  It takes a second or so for the document to “catch up” to you and show the visible images.

 

            The factory design pattern is very simple.  Several other patterns build off of it though, so it is a common base pattern.  You use this pattern when one or more of the following are true:

 

            The command pattern is a very useful pattern when you want to give your users the ability to do certain things and undo them.  Typical examples are the undo and redo on many programs today.  This functionality is accomplished with the command pattern.  The GOF book says to use a command pattern when:

 

            The visitor design pattern is very useful in situations where normal polymorphism won’t work because we have fundamentally different objects, with different interfaces, that you want to work on your concrete main object.  The pattern is used to give us a way to do these different operations on the object.  According to the GOF book, you use the visitor pattern when: