Why Test Driven Development?

A popular buzzword is test driven development.  The thing that separates TDD from everything else is that you must write your unit tests first.  This confuses a lot of people.  They can't for the life of them understand the meaning behind this.  Many programmers simply believe that they are just too good for class.  In this case, TDD class.

These are the same individuals that say it's no problem to fix a bug in production.  They simply spend a few hours searching for the bug and then they spend another hour or two and fix the bug.  They hand this over to the QA team or someone who has to test the application and it's all done.  Just a few hours of waste.  It's no big deal, right?

Let's play with some numbers.  How much do you make an hour?  Do you think this is all that it's costing the company?  There are more costs associated with you than you think.  For instance, you have to have a desk, paper, printer, pens, markers, notepads, and obviously a computer.  If you work at a business that deals in other things besides software then you have to have the Microsoft Office Suite from like XP to the latest.  Visual Studio 2005, 2008, 2010, and 2012.  Maybe Team Foundation Server.  Then you have the test servers (they cost against you), and databases and etc.  This list can get quite long.

While many individuals cost roughly 150% of there salary, software developers are roughly around 250% or more.  So if you make $30 an hour, your actual cost is $75 an hour.  So if you waste 4 hours on a problem, you just cost the company $600.00.  How did I come up with that number?  $75.00 (per hour) * 4 hours = $300.00 + ($75.00 per hour * 4 hours you could have been working on something else) = $600.00.  After the four hours to fix the bug, then you have to submit it to QA or hand it around to people in the office to look at.  This takes some of there time and then you have to deploy the fix.  So by the time it's all said and done most are around the $5,000 to $10,000 range.  This number varies quite a bit and is really based on your company, so take those numbers with a grain of salt until you can figure it out.  I'm sure the price will be a lot higher than you expected, though.

The reason I came up with 4 hours for a minimum is because when communication occurs on a bug, your wasting twice as much time.  When the bug first goes off or is noticed, the individual must tell there manager what happened.  It may only take about 5 minutes, but it's 10 minutes total time wasted.  5 minutes of the individual and 5 minutes of the managers.  Then that manager will have to fill out a report or calls up support or someone in IT.  A 10 minute phone call wastes a total of 20 minutes.  Then the IT person notifies the manger and this just goes on and on until it finally comes to you.  It's so easy to jump to four hours even if the bug fix is a simple spelling correction.

If you have QA individuals who manually test your code then that cost rises astronomically.  Unfortunately, we don't have a QA where I work, so other developers and sometimes managers would have to help out with the manual testing.  They didn't think twice about the cost as it was something that just had to be done.  When I came along, no one ever thought twice about the cost of fixing all of those bugs.  However, when I tried to teach Test Driven Development to them, they saw the extra cost of <i>learning</i> within seconds.  "How much slower is development going to be?", they asked.  "So we have to write all of that extra code?  This is going to take a lot longer."

How do managers and developers not see the cost to fix the bugs after the deployment, but are so worried about the short additional time it takes to write unit tests?  I just don't understand it.  Do I need to scream at them?  I want to.

Now I'm done with that rant.  However, I never answered the question that I started with.  The whole reason to do test first is to make you see that your test fails.  This seems very trivial, but you would be amazed at how many times developers write a non-failing test.  Watching it turn green may also give you a deep warm fuzzy feeling.

The most important part is that you have to stop and think for a minute on what you want the production code to do.  You know that you have to keep your classes separated, so they can be easily testable.  It's so much harder to untangle the mess just to write a unit test.  It also becomes very boring and makes you lose your flow.

If you don't use TDD, you need to at least try it.  I don't mean a "check off the box" attempt at using test driven development.  You have to REALLY try it.  If you don't try it for at least a year then you didn't really try TDD.

CodeProject

Converted 4-Tier Architecture

This is a continuation article.  The original article can be found here.

The converted 4-tier architecture is about having flexible views and data access layers.  The view is simply a group of concrete classes that all implement something that is referenced in the presentation layer.  The presentation layer instantiates the concrete views (from a different assembly) by  using the IOC container.

In the presentation layer we'd create our IoC.  The presentation layer references the business logic layer, so it can see all of the interfaces.  So when our presentation layer needs an implementation of ICoupon, the IoC will retrieve the data access layers' implementation that is in a different assembly.

From Business Logic Layer:

Code Snippet

15.  
16. Public Interface ICoupon
17.     Inherits CouponService.IEditableObject
18.  
19.     ''' <summary>
20.     ''' Simple on and off states that allow simpler searches.
21.     ''' </summary>
22.     ''' <remarks>Instead of being required to validate every date in an open setting, this allows
23.     ''' the class itself to handle that interpretation.</remarks>
24.     Enum ExpirationState As Integer
25.         NotExpired = 0
26.         Expired = 1
27.     End Enum
28.  
29.     ''' <summary>
30.     ''' Gets the coupon id.
31.     ''' </summary>
32.     ''' <value>
33.     ''' The coupon id.
34.     ''' </value>
35.     ''' <remarks>This is primarily for the database.</remarks>
36.     ReadOnly Property CouponIdentification As Integer
37.     ''' <summary>
38.     ''' Gets or sets the item.
39.     ''' </summary>
40.     ''' <value>
41.     ''' The item.
42.     ''' </value>
43.     ''' <remarks>The item property is simply a name given by the person who has the coupon.  This
44.     ''' name doesn't have to be used and is purely for assistance in searching for specific coupons.</remarks>
45.     Property CouponName As String
46.     ''' <summary>
47.     ''' Gets or sets the expiration.
48.     ''' </summary>
49.     ''' <value>
50.     ''' The expiration.
51.     ''' </value>
52.     ''' <remarks>This is a set expiration date for the coupon.</remarks>
53.     Property ExpirationDate As DateTime
54.     ''' <summary>
55.     ''' Gets the type of the expiration.
56.     ''' </summary>
57.     ''' <value>
58.     ''' The type of the expiration.
59.     ''' </value>
60.     ''' <remarks>The expiration type has been added to allow a quick retrieval of getting a coupon
61.     ''' that has or has not been expired.</remarks>
62.     ReadOnly Property ExpirationType As ExpirationState
63.  
64.     ''' <summary>
65.     ''' Gets or sets the quantity products required.
66.     ''' </summary>
67.     ''' <value>
68.     ''' The quantity products required.
69.     ''' </value>
70.     ''' <remarks>This is the number of products that are required before the reduced price or the
71.     ''' reduced percentage can take into affect on the transaction.</remarks>
72.     Property QuantityOfProductsRequired As Integer
73.     ''' <summary>
74.     ''' Gets or sets the quantity products reduced.
75.     ''' </summary>
76.     ''' <value>
77.     ''' The quantity products reduced.
78.     ''' </value>
79.     ''' <remarks>The quantity of products that are allowed to be reduced in price.</remarks>
80.     Property QuantityOfProductsReduced As Integer
81.     ''' <summary>
82.     ''' Gets or sets at percentage.
83.     ''' </summary>
84.     ''' <value>
85.     ''' At percentage.
86.     ''' </value>
87.     ''' <remarks>The percentage of the reduction of price.</remarks>
88.     Property Percentage As Nullable(Of Integer)
89.     ''' <summary>
90.     ''' Gets or sets at amount.
91.     ''' </summary>
92.     ''' <value>
93.     ''' At amount.
94.     ''' </value>
95.     ''' <remarks>The amount of the reduction in price.</remarks>
96.     Property Amount As Nullable(Of Double)
97.  
98.     ''' <summary>
99.     ''' Gets or sets the products required.
100.     ''' </summary>
101.     ''' <value>
102.     ''' The products required.
103.     ''' </value>
104.     ''' <remarks>A list of products in which one or more may be required to have been bought in
105.     ''' order to reduce the price.</remarks>
106.     Property AvailableProductsRequired As IList(Of IProduct)
107.     ''' <summary>
108.     ''' Gets or sets the products reduced.
109.     ''' </summary>
110.     ''' <value>
111.     ''' The products reduced.
112.     ''' </value>
113.     ''' <remarks>A list of products in which the price may drop for.  The product has to be bought
114.     ''' for the coupon to take effect.</remarks>
115.     Property AvailableProductsReduced As IList(Of IProduct)
116.  
117.     ''' <summary>
118.     ''' Gets or sets the storage code.
119.     ''' </summary>
120.     ''' <value>
121.     ''' The storage code.
122.     ''' </value>
123.     ''' <remarks>This is a way of allowing the end users to write a storage code or number.</remarks>
124.     Property StorageInformation As String
125.     ''' <summary>
126.     ''' Gets or sets the required store.
127.     ''' </summary>
128.     ''' <value>
129.     ''' The required store.
130.     ''' </value>
131.     ''' <remarks>Some coupons work only at certain stores.  This may be a requirement</remarks>
132.     Property RequiredStore As IStore
133.  
134.     Sub GetCouponsByExpirationState(state As ExpirationState, callback As Action(Of IList(Of ICoupon)))
135. End Interface

The data access layer:

Code Snippet

1. Imports CouponService
2. Imports CouponService.Naming
3.  
4. ''' <summary>
5. '''
6. ''' </summary>
7. Public Class Coupon
8.     Implements CouponService.ICoupon, CouponService.IEditableObject, System.ComponentModel.INotifyPropertyChanged, System.ComponentModel.INotifyPropertyChanging
9.  
10.     Public Event IsNowClean(sender As Object, e As System.EventArgs) Implements CouponService.IEditableObject.IsNowClean
11.     Public Event IsNowDirty(sender As Object, e As System.EventArgs) Implements CouponService.IEditableObject.IsNowDirty
12.     Public Event PropertyChanged(sender As Object, e As System.ComponentModel.PropertyChangedEventArgs) Implements System.ComponentModel.INotifyPropertyChanged.PropertyChanged
13.     Public Event PropertyChanging(sender As Object, e As System.ComponentModel.PropertyChangingEventArgs) Implements System.ComponentModel.INotifyPropertyChanging.PropertyChanging
14.  
15.     Private _IsDirty As Boolean
16.     Private _IsNew As Boolean
17.     Private _IsEditable As Boolean
18.  
19.     Private CouponContext As CouponContext
20.  
21.     Public Sub New(dataContext As Data.Linq.DataContext)
22.         CouponContext = dataContext
23.         _IsEditable = True
24.         _IsDirty = False
25.         _IsNew = True
26.     End Sub
27.  
28. #Region "IEditableObject Properties"
29.     Public ReadOnly Property IsDirty1 As Boolean Implements CouponService.IEditableObject.IsDirty
30.         Get
31.             Return _IsDirty
32.         End Get
33.     End Property
34.     Public Property IsDirty As Boolean
35.         Get
36.             Return _IsDirty
37.         End Get
38.         Set(value As Boolean)
39.             If _IsDirty <> value Then
40.                 _IsDirty = value
41.                 If value Then
42.                     RaiseEvent IsNowClean(Me, EventArgs.Empty)
43.                 Else
44.                     RaiseEvent IsNowDirty(Me, EventArgs.Empty)
45.                 End If
46.             End If
47.         End Set
48.     End Property
49.  
50.     Public ReadOnly Property IsEditable1 As Boolean Implements CouponService.IEditableObject.IsEditable
51.         Get
52.             Return _IsEditable
53.         End Get
54.     End Property
55.     Public Property IsEditable As Boolean
56.         Get
57.             Return _IsEditable
58.         End Get
59.         Set(value As Boolean)
60.             _IsEditable = value
61.         End Set
62.     End Property
63.  
64.     Public ReadOnly Property IsNew1 As Boolean Implements CouponService.IEditableObject.IsNew
65.         Get
66.             Return _IsNew
67.         End Get
68.     End Property
69.     Public Property IsNew As Boolean
70.         Get
71.             Return _IsNew
72.         End Get
73.         Set(value As Boolean)
74.             _IsNew = value
75.         End Set
76.     End Property
77. #End Region
78.  
79.     Public ReadOnly Property CouponIdentification As Integer Implements CouponService.ICoupon.CouponIdentification
80.         Get
81.             Return Me.CouponID
82.         End Get
83.     End Property
84.  
85.     Public Property ExpirationDate As Date Implements CouponService.ICoupon.ExpirationDate
86.         Get
87.             Return Me.Expiration
88.         End Get
89.         Set(value As Date)
90.             If value <> Me.ExpirationDate AndAlso IsEditable Then
91.                 IsChanging(GetPropertyName(Function() ExpirationDate))
92.                 Me.Expiration = value
93.                 HasChanged(GetPropertyName(Function() ExpirationDate))
94.             End If
95.         End Set
96.     End Property
97.  
98.     Public ReadOnly Property ExpirationType As CouponService.ICoupon.ExpirationState Implements CouponService.ICoupon.ExpirationType
99.         Get
100.             If Me.ExpirationDate < Now Then
101.                 Return CouponService.ICoupon.ExpirationState.NotExpired
102.             Else
103.                 Return CouponService.ICoupon.ExpirationState.Expired
104.             End If
105.         End Get
106.     End Property
107.  
108.     Public Property CouponName As String Implements CouponService.ICoupon.CouponName
109.         Get
110.             Return Me.Item
111.         End Get
112.         Set(value As String)
113.             If value <> Me.Item AndAlso IsEditable Then
114.                 IsChanging(GetPropertyName(Function() CouponName))
115.                 Me.Item = value
116.                 HasChanged(GetPropertyName(Function() CouponName))
117.             End If
118.         End Set
119.     End Property
120.  
121.     Public Property StorageInformation As String Implements CouponService.ICoupon.StorageInformation
122.         Get
123.             Return Me.StorageCode
124.         End Get
125.         Set(value As String)
126.             If value <> Me.StorageCode AndAlso IsEditable Then
127.                 IsChanging(GetPropertyName(Function() StorageInformation))
128.                 Me.StorageCode = value
129.                 HasChanged(GetPropertyName(Function() StorageInformation))
130.             End If
131.         End Set
132.     End Property
133.  
134.     Public Property RequiredStore As CouponService.IStore Implements CouponService.ICoupon.RequiredStore
135.         Get
136.             Return Me.Store
137.         End Get
138.         Set(value As CouponService.IStore)
139.             If value.StoreId <> Me.Store.StoreID AndAlso IsEditable Then
140.                 IsChanging(GetPropertyName(Function() RequiredStore))
141.                 Me.Store = value
142.                 HasChanged(GetPropertyName(Function() RequiredStore))
143.             End If
144.         End Set
145.     End Property
146.  
147.     Public Property Amount As Double? Implements CouponService.ICoupon.Amount
148.         Get
149.             Return Me.AtAmount
150.         End Get
151.         Set(value As Double?)
152.             If value <> Me.AtAmount AndAlso IsEditable Then
153.                 IsChanging(GetPropertyName(Function() Amount))
154.                 Me.AtAmount = value
155.                 HasChanged(GetPropertyName(Function() Amount))
156.             End If
157.         End Set
158.     End Property
159.  
160.     Public Property Percentage As Integer? Implements CouponService.ICoupon.Percentage
161.         Get
162.             Return Me.AtPercentage
163.         End Get
164.         Set(value As Integer?)
165.             If value <> Me.AtPercentage AndAlso IsEditable Then
166.                 IsChanging(GetPropertyName(Function() Percentage))
167.                 Me.AtPercentage = value
168.                 HasChanged(GetPropertyName(Function() Percentage))
169.             End If
170.         End Set
171.     End Property
172.  
173.     Public Property AvailableProductsReduced As System.Collections.Generic.IList(Of CouponService.IProduct) Implements CouponService.ICoupon.AvailableProductsReduced
174.         Get
175.             Return Me.ProductsReduced.ToList
176.         End Get
177.         Set(value As System.Collections.Generic.IList(Of CouponService.IProduct))
178.             If IsEditable Then
179.                 IsChanging(GetPropertyName(Function() AvailableProductsReduced))
180.                 Me.ProductsReduced.Clear()
181.  
182.                 For Each prod As CouponService.IProduct In value
183.                     Me.ProductsReduced.Add(prod)
184.                 Next
185.                 HasChanged(GetPropertyName(Function() AvailableProductsReduced))
186.             End If
187.         End Set
188.     End Property
189.  
190.     Public Property AvailableProductsRequired As System.Collections.Generic.IList(Of CouponService.IProduct) Implements CouponService.ICoupon.AvailableProductsRequired
191.         Get
192.             Dim ProductsToReturn As IEnumerable(Of IProduct) = From p In Me.ProductsRequired Select p.Product
193.             Return ProductsToReturn.ToList
194.         End Get
195.         Set(value As System.Collections.Generic.IList(Of CouponService.IProduct))
196.             If IsEditable Then
197.                 IsChanging(GetPropertyName(Function() AvailableProductsRequired))
198.                 Me.ProductsRequired.Clear()
199.  
200.                 For Each prod As CouponService.IProduct In value
201.                     Me.ProductsRequired.Add(prod)
202.                 Next
203.                 HasChanged(GetPropertyName(Function() AvailableProductsRequired))
204.             End If
205.         End Set
206.     End Property
207.  
208.     Public Property QuantityOfProductsReduced As Integer Implements CouponService.ICoupon.QuantityOfProductsReduced
209.         Get
210.             Return Me.QuantityProductsReduced
211.         End Get
212.         Set(value As Integer)
213.             If value <> Me.QuantityProductsReduced AndAlso IsEditable Then
214.                 IsChanging(GetPropertyName(Function() QuantityOfProductsReduced))
215.                 Me.QuantityProductsReduced = value
216.                 HasChanged(GetPropertyName(Function() QuantityOfProductsReduced))
217.             End If
218.         End Set
219.     End Property
220.  
221.     Public Property QuantityOfProductsRequired As Integer Implements CouponService.ICoupon.QuantityOfProductsRequired
222.         Get
223.             Return Me.QuantityProductsRequired
224.         End Get
225.         Set(value As Integer)
226.             If value <> Me.QuantityProductsRequired AndAlso IsEditable Then
227.                 IsChanging(GetPropertyName(Function() QuantityOfProductsRequired))
228.                 Me.QuantityProductsRequired = value
229.                 HasChanged(GetPropertyName(Function() QuantityOfProductsRequired))
230.             End If
231.         End Set
232.     End Property
233.  
234.     Private Sub IsChanging(propertyName As String)
235.         RaiseEvent PropertyChanging(Me, New System.ComponentModel.PropertyChangingEventArgs(propertyName))
236.     End Sub
237.     Private Sub HasChanged(propertyName As String)
238.         If IsEditable Then
239.             IsDirty = True
240.         End If
241.  
242.         RaiseEvent PropertyChanged(Me, New System.ComponentModel.PropertyChangedEventArgs(propertyName))
243.     End Sub
244.  
245.     Public Sub GetCouponsByExpirationState(state As CouponService.ICoupon.ExpirationState, callback As Action(Of IList(Of ICoupon))) Implements CouponService.ICoupon.GetCouponsByExpirationState
246.         If state = ICoupon.ExpirationState.Expired Then
247.             callback.Invoke(GetExpiredCoupons)
248.         Else
249.             callback.Invoke(GetNonExpiredCoupons)
250.         End If
251.     End Sub
252.     Friend Function GetExpiredCoupons() As IList(Of ICoupon)
253.         Dim CouponsToReturn As IEnumerable(Of Coupon) = From CurrentCoupon As Coupon In CouponContext.GetTable(Of Coupon)() _
254.                                                         Where CurrentCoupon.Expiration < Now _
255.                                                         Select CurrentCoupon
256.  
257.         Return MarkAsEditable(CouponsToReturn).ToList
258.     End Function
259.     Friend Function GetNonExpiredCoupons() As IList(Of ICoupon)
260.         Dim CouponsToReturn As IEnumerable(Of Coupon) = From CurrentCoupon As Coupon In CouponContext.GetTable(Of Coupon)() _
261.                                                         Where CurrentCoupon.Expiration >= Now _
262.                                                         Select CurrentCoupon
263.  
264.         Return MarkAsEditable(CouponsToReturn).ToList
265.     End Function
266.  
267.     Private Function MarkAsEditable(ByVal CouponsToMark As IEnumerable(Of ICoupon)) As IEnumerable(Of ICoupon)
268.         For Each CurrentCoupon As Coupon In CouponsToMark
269.             CurrentCoupon.IsNew = False
270.             CurrentCoupon.IsEditable = True
271.             CurrentCoupon.IsDirty = False
272.         Next
273.  
274.         Return CouponsToMark
275.     End Function
276. End Class

Now this coupon class is a partial class that was generated using Linq2Sql.  In other words, my presentation layer is referencing the business logic layers' ICoupon interface which is implemented in the data access layer which is the same as my Linq2Sql entity.  I pass in a data context to the constructor.  Now this data context concrete class is also in the Data access layer and it implements the interface IDataAccess which is in my original article.

Here is the concrete Data Access class.

Code Snippet

1.  
2. Public Class DataAccess
3.     Implements CouponService.IDataAccess
4.  
5.     Private _DataContext As CouponContext
6.  
7.     Public Sub New(connectionString As String)
8.         _DataContext = New CouponContext(connectionString)
9.     End Sub
10.  
11.     Public ReadOnly Property GetDataContext As System.Data.Linq.DataContext Implements CouponService.IDataAccess.GetDataContext
12.         Get
13.             Return _DataContext
14.         End Get
15.     End Property
16.     Public Property Timeout As Integer Implements CouponService.IDataAccess.Timeout
17.         Get
18.             Return _DataContext.CommandTimeout
19.         End Get
20.         Set(value As Integer)
21.             _DataContext.CommandTimeout = value
22.         End Set
23.     End Property
24.  
25.     Public Function Commit() As Boolean Implements CouponService.IDataAccess.Commit
26.         Try
27.             _DataContext.SubmitChanges()
28.             Return True
29.         Catch ex As Exception
30.             Return False
31.         End Try
32.     End Function
33.     Public Function Commit(mode As System.Data.Linq.ConflictMode) As Boolean Implements CouponService.IDataAccess.Commit
34.         Try
35.             _DataContext.SubmitChanges()
36.             Return True
37.         Catch ex As Exception
38.             Return False
39.         End Try
40.     End Function
41.  
42.     Public Function ConnectionState() As System.Data.ConnectionState Implements CouponService.IDataAccess.ConnectionState
43.         Return _DataContext.Connection.State
44.     End Function
45.     Public Function OpenConnection() As Boolean Implements CouponService.IDataAccess.OpenConnection
46.         _DataContext.Connection.Open()
47.     End Function
48.  
49.     Public Sub Rollback() Implements CouponService.IDataAccess.Rollback
50.         For Each change As Object In _DataContext.GetChangeSet.Updates
51.             _DataContext.Refresh(Data.Linq.RefreshMode.OverwriteCurrentValues, change)
52.         Next
53.         For Each change As Object In _DataContext.GetChangeSet.Inserts
54.             _DataContext.Refresh(Data.Linq.RefreshMode.OverwriteCurrentValues, change)
55.         Next
56.         For Each change As Object In _DataContext.GetChangeSet.Deletes
57.             _DataContext.Refresh(Data.Linq.RefreshMode.OverwriteCurrentValues, change)
58.         Next
59.     End Sub
60.  
61. #Region "IDisposable Support"
62.     Private disposedValue As Boolean ' To detect redundant calls
63.  
64.     ' IDisposable
65.     Protected Overridable Sub Dispose(disposing As Boolean)
66.         If Not Me.disposedValue Then
67.             If disposing Then
68.                 ' TODO: dispose managed state (managed objects).
69.             End If
70.  
71.             ' TODO: free unmanaged resources (unmanaged objects) and override Finalize() below.
72.             ' TODO: set large fields to null.
73.         End If
74.         Me.disposedValue = True
75.     End Sub
76.  
77.     ' TODO: override Finalize() only if Dispose(ByVal disposing As Boolean) above has code to free unmanaged resources.
78.     'Protected Overrides Sub Finalize()
79.     '    ' Do not change this code.  Put cleanup code in Dispose(ByVal disposing As Boolean) above.
80.     '    Dispose(False)
81.     '    MyBase.Finalize()
82.     'End Sub
83.  
84.     ' This code added by Visual Basic to correctly implement the disposable pattern.
85.     Public Sub Dispose() Implements IDisposable.Dispose
86.         ' Do not change this code.  Put cleanup code in Dispose(ByVal disposing As Boolean) above.
87.         Dispose(True)
88.         GC.SuppressFinalize(Me)
89.     End Sub
90. #End Region
91. End Class

Eventually, I will be putting the whole application online when it's completed.  If you have any questions please feel free to ask.  I have a total of 6 assemblies.  2 of them are for automated tests.  I have 1 that is testing the presentation layer that mocks the views and the DAL.  My other one tests the concrete DAL classes.

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4 Tier IOC Application (Converted Tier Architecture)

Have you ever noticed that if the business logic entities adds a member then both the database and the views will be updated?  If the business logic entities adds functionality then just the view might need updating.  There are plenty of times when no functionality changes at all, but our view needs to be adjusted to enhance the user experience.  Doing an update to the view requires a complete update.  What if you want the same functionality, but you also have users who like the old archaic looking application instead?  It'd be like creating different skins depending on the user.

What if the database needs to be switched, but no other functionality should be changed?  What if you want to only move a few people (like a specific branch) over at a time?

A traditional n-tier application looks like this:

The presentation layer references the business logic layer.  The business logic layer references the data access layer and the data access layer connects to the database.  I am calling this the Converted Tier Architecture until someone notifies me that I'm wrong.  Now let me try and show you what I am trying to explain:

Here the views references the presentation layer (which I usually call the application layer).  The presentation layer references the business logic layer.  However, instead of the business logic layer referencing the data access layer, I have the data access layer reference the business logic layer.  Then the data access layer still connects to the database.

How this works in more detail is that the views are really just concrete classes that implement interfaces that are found in the presentation layer.  My IOC container will connect these two assemblies together.  I can have one or more view assemblies with different "skins" and they can be switched by a simple change in a configuration file.  I can mix and match multiple view assemblies and really have a lot of flexibility when it comes to determining the best user experience and can really personalize what the end user is viewing.  Now, of course you have to create multiple assemblies and I don't recommend making "skins" just to make a "skin", but it has advantages in work applications.  I can go to someone who uses my application and change their config, drop in the assembly and see how they react.  When I'm done I can either switch it back to the original assembly or leave it as is.

The presentation layer still references the business logic layer like usual.  The business logic layer no longer references the data access layer.  The data access layer references the business logic layer.  I use an interface that defines the connection (in this example it's a simple Linq2Sql data context).  My data layer class just needs to implement the below interface and the IOC container hooks up the business logic layer with the concrete data access layer class.

Code Snippet

11.  
12. Public Interface IDataAccess
13.     Inherits IDisposable
14.  
15. #Region "Properties"
16.     ''' <summary>
17.     ''' Gets the get data context.
18.     ''' </summary>
19.     ''' <value>
20.     ''' The get data context.
21.     ''' </value>
22.     ReadOnly Property GetDataContext As Data.Linq.DataContext
23.     ''' <summary>
24.     ''' Gets or sets the timeout.
25.     ''' </summary>
26.     ''' <value>
27.     ''' The timeout.
28.     ''' </value>
29.     Property Timeout As Integer
30. #End Region
31. #Region "Functions"
32.     ''' <summary>
33.     ''' Gets the connection state of the underlying database connection.
34.     ''' </summary>
35.     ''' <returns>The <see cref="System.Data.ConnectionState" /> value of the underlying database connection.</returns>
36.     Function ConnectionState() As System.Data.ConnectionState
37.     ''' <summary>
38.     ''' Opens the connection.
39.     ''' </summary>
40.     ''' <remarks>This only returns true if the function was able to connect.  If the connection was already established
41.     ''' then <c>false</c> would be returned since the function did not connect when called.</remarks>
42.     ''' <returns><c>true</c> if this functionality was able to connect; otherwise <c>false</c>.</returns>
43.     Function OpenConnection() As Boolean
44.     ''' <summary>
45.     ''' Commits to the database using the specified mode.
46.     ''' </summary>
47.     ''' <returns><c>true</c> if successful; otherwise <c>false</c>.</returns>
48.     Function Commit() As Boolean
49.     ''' <summary>
50.     ''' Commits to the database using the specified mode.
51.     ''' </summary>
52.     ''' <param name="mode">The <see cref="System.Data.Linq.ConflictMode" /> that the datacontext should follow.</param>
53.     ''' <returns><c>true</c> if successful; otherwise <c>false</c>.</returns>
54.     Function Commit(mode As System.Data.Linq.ConflictMode) As Boolean
55.     ''' <summary>
56.     ''' Removes any pending updates/inserts/deletes from the underlying data connection.
57.     ''' </summary>
58.     ''' <remarks>This is a rollback of any "Transactional" data on the underlying data context.
59.     ''' This is not meant to be used on a multi-DataContext transaction.</remarks>
60.     Sub Rollback()
61. #End Region
62. End Interface

Linq2Sql entities are created with a partial class declaration.  I create a new class with the same name (so they become the same concrete class) and I implement the interfaces found in the business logic layer.  My application layer which only knows of the business logic layer interface entities will end up getting a concrete class from the data access layer that it never needs to reference.

I will post a complete application with notes on using this tiered architecture at another time.  I'm still making it for my wife, but will create a trimmed down version for you to view soon.
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