In my previous article (part 1), I shared what an n-Tier architected application is and why you should familiarize yourself with the concepts behind them.  In this article, I will take this a step further and show you how you can create a simple n-Tier architected ASP.NET application using Visual Studio 2010.  I won’t create all the CRUD (create, read, update, delete) operations for the business objects, but it should give you a strong start to creating a simple multi-tiered ASP.NET application.

When we are finished, your Visual Studio solution will have the following layers:
1) SalesSystem.Web - ASP.NET website
2) SalesSystem.Presentation - Presentation Controllers
3) SalesSystem.BusinessObjects – Business Logic Layer (Objects)
4) SalesSystem.Core – Core Utility Objects

We will start by creating an empty Visual Studio solution and then adding projects to it.  Each project acts as a logical layer (tier).  Projects can reference each other via the use of project references.  When you build the Visual Studio solution, it will build each project for you automatically.  Before I show how to create an empty solution and add projects (logical layers/tiers) to it, here’s a brief explanation of what each project will do.

SalesSystem.Core – I used this project to contain generic utilities, functions, extensions, helpers, etc which can be used across any new application you may develop.  Think big.  You don’t want to reinvent the wheel if you don’t have to.  I thought I may be developing more web applications in the future, so a common task is reading connection strings from the web.config.  So I wrote a utility object for the web.config and put a base class to be inherited on all web pages in there too.  This project/assembly is used by each of our other tiers.

SalesSystem.BusinessObjects – I made this project to house all of my business objects.  I didn’t take the time to create an official DataAccess Layer… but business objects often are made to interact with one particular data source type.  In this case I am interacting with a SQL Server and the connection string to run the queries on is passed in via a connection string to still maintain a dynamic and flexible approach.  Regardless of merging the business objects in with data access routines, I am able to use these business objects going forward easily as long as the application (whatever type it is) supports CRUD operations on a SQL Server.

SalesSystem.Presentation – Okay, this project may be a bit confusing for most.  I made another tier/layer to act as the liason between the UI and business objects.   I have a couple of reasons for this.  First, it makes calls simpler from ASP.NET page code behinds.  An ASP.NET page can have one controller which does a myriad of tasks (page centric) on many types of objects.  Second, it will help with automated unit testing in the future if we decided to take it to that level.  Your presentation layer should essentially be presenting the UI and capturing data from the user.  The less “clutter” you can put in the presentation the better.  So using presentation controllers can assist here.

SalesSystem.Web – This project is the presentation layer and we are using an ASP.NET website to do this.  Our objective in this layer is to simplify the logic down to presenting and capturing data from the user.  Any business object acquisition or saving should be done through the controller.  This makes the UI code much cleaner and simpler.

Here’s what the final solution will look like:

Step 1: Create an Empty Solution – First we want to create an empty Visual Studio solution so we can add our projects (layers/tiers) to it.  Below is a screenshot on how to do this.

Step 2: Add Projects to the Solution – Next we want to add projects for each tier we want to our solution.  We have 4 layers that we want to develop, so we’ll add 4 projects to the solution which will facilitate logical layers.  Each layer may be a different project type.  For instance, the presentation tier (SalesSystem.Web) will be an ASP.NET web project.  However our core, presentation controller, and business projects will all be windows class libraries as they simply have .NET classes in them.  Below are screenshots to help facilitate how to add the projects to our empty solution.

Step 3: Add Project References – The final step to getting your ASP.NET n-Tier architected solution to work is to setup project references to each tier can communicate, consume, and/or interact with each other.  Think of project references as “inheritance”.  If you need to use a tier’s (project’s) functions or data types, you need to add a project reference (or inherit) from it.

SalesSystem.Web References - We know that SalesSystem.Web will need to make calls to the SalesSystem.Presentation controllers which will return objects from SalesSystem.BusinessObjects.  So the SalesSystem.Web will need project references to both projects.  It will also need a project references to SalesSystem.Core because that is our core project which we have our core objects & utilities that we can use everywhere.

SalesSystem.Presentation References – This tier doesn’t care about the UI.  It makes calls to the business object layer.  Therefore it only needs a reference to SalesSystem.BusinessObjects.  Again, add a reference to SalesSystem.Core as well.

SalesSystem.BusinessObjects References – This tier is only concerned with its own business objects and interacting with the database.  It doesn’t care about what presentation layer is employed to use them… so the only references you need is to SalesSystem.Core.

Here is how you add a project reference:

Step 4: Code – I’m not going to go into detail on how to code the tiers.  Instead I am attaching the solution which includes the code in each tier to acquire customer sales from the AdventureWorks database using our n-Tiered solution.  The end deliverable is an ASP.NET website which a page’s codebehind class makes a call to our presentation controller tier which makes calls to our business objects and back out again.

Summary - Imagine taking this application to the next level and offering a Windows WPF UI option.  We could reuse the same presentation controllers, core, and business object tiers.  We would simply replace the ASP.NET web presentation tier and most of our work would already be done.  If a call to the database was broke, we’d fix it in one business object and be done.  One place, one time.  We wouldn’t have to be searching through all code-behind files trying to locate every instance of the problem.  If you had a UI designer/coder, they could be designing the look of the web pages while you focused on the business objects and back-end tiers (parallelism).  There are many reasons why applications are constructed like this.

Hopefully this helps you get a good start to understanding n-Tier architected applications and how you can accomplish this in Visual Studio using logical layers with VS projects and project references.

n-Tier Architecture – Part 1
n-Tier Architecture – Part 2

SalesSystem_Soln.zip (792.10 kb)