Wordpress

After a little bit or research I decided to move my blog over to wordpress.

http://vbguru.wordpress.com

Inheritance, and why it rocks!

Inheritance is something that is very foreign to a VB6 developer, however it is very, very cool. Think about this scenario lets say your are on a team of developers. You are working on a product intended to sell to a retailer who wants to process credit cards. But each retailer will use a different credit card processor so you need to support Costco, paypal, and Autorize.Net. Your job is to write the code that connects to the gateway, but there are other devlopers on your team that need to be able to use your code. How do you give them one interface to your code, but you want to execute the right routine based on the credit card processor? Well inheritance offers the perfect solution.

Here we create our 'base' class. We declare it as MustInhert so that we can use the MustOverride property on the ProcessCard Function

Public MustInherit Class CreditCard

  Public MustOverride Function ProcessCard(ByVal CardNumber As String) As Boolean

End Class

You'll notice that we created a function signature, but it has no code attached. We add the code later when we create the gateway's class.

When your create your class that inherits your base class you'll notice when you add the line 'Inherits CreditCard' that it will autofill the MustOverride Function.  Visual Studio makes it easy for you!

Public Class Costco

  Inherits CreditCard

  Public Overrides Function ProcessCard(ByVal CardNumber As String) As Boolean

    'code to process a costco card

  End Function

End Class

Public Class AutorizeNet

  Inherits CreditCard

  Public Overrides Function ProcessCard(ByVal CardNumber As String) As Boolean

    'code to process a Autorize.Net card

  End Function

End Class

Public Class PayPal

  Inherits CreditCard

  Public Overrides Function ProcessCard(ByVal CardNumber As String) As Boolean

    'code to process a PayPal.Net card

  End Function

End Class

Your other team members will love you if you now use a 'wrapper' to manage the gateway, check this code out:

Public Class MyCreditCard

  Private mGateWay As CreditCard

  Sub New()

    'ASK THE DATABASE OR INI OR SOMETHING

    'WHAT PROCESSOR TO USE and put it in i

    Dim i As Integer

    Select Case i

      Case 0

        mGateWay = New Costco

      Case 1

        mGateWay = New AutorizeNet

      Case 2

        mGateWay = New PayPal

    End Select

  End Sub

  Public Function ProcessCard(ByVal CardNumber As String) As Boolean

    mGateWay.ProcessCard(CardNumber)

  End Function

End Class

Now your UI guy just has to write something like this and he never has to worry about which credit card processing gateway you are using.

  Public Sub ProcessCC()

    Dim cc As MyCreditCard = New MyCreditCard

    cc.ProcessCard("12456789012345")

  End Sub

Using a One-Way Hash on Passwords

The best thing to do with a password is convert it into a one way Hash. This way no one knows what your user's passwords are, not even a hacker who got into your database. Here is a small class I wrote to Convert a password into a hash.

Public Class Hash

 

  Private Function ComputeHashValue(ByVal data() As Byte) As Byte()

    Dim hashAlg As System.Security.Cryptography.SHA1 = System.Security.Cryptography.SHA1.Create()

    Dim hashvalue() As Byte = hashAlg.ComputeHash(data)

    Return hashvalue

  End Function

 

  Public Function HashPassword(ByVal password As String) As String

    Dim encoder As System.Text.UTF8Encoding

    Dim b() As Byte

 

    encoder = New System.Text.UTF8Encoding

    b = encoder.GetBytes(password)

    b = ComputeHashValue(b)

 

    Return Convert.ToBase64String(b)

 

 

  End Function

 

End Class

Being Good With SQL, Using SQL Cursors

If you want to make any money programming in VB.NET then you need to be in the business of writing good SQL server code. VB, business programming and Databases all come together in the same package.  And so from time to time I will be giving out some tips about advanced SQL Server concepts.  I don't think of myself as a SQL Guru, but I do know a lot about it.  MS SQL is not the only database language you may need to learn, but the good news is that nearly all databases use the same T-SQL Language and so if you get good at one of them, it's generally pretty easy to use the others. Being good at designing table structure translates into EVERY database engine the same as well.


Today I wanted to talk for a moment about using SQL cursors, because they are very cool.  Have you ever wanted to do a loop inside a Stored procedure?  Well check out this code, you this will loop through all users that do not have a locker assigned (using a cool "NOT IN" SQL statement) and assign each of them the next available locker.

CREATE PROCEDURE [dbo].[spDEMO]

AS

BEGIN

    SET NOCOUNT ON;

DECLARE @idUser as int

DECLARE @idLocker as int

DECLARE UserLocker_cursor CURSOR FOR

        --Get all the users that don't have lockers

        SELECT idUser From tblUser where idUser NOT IN

            (

                SELECT idOwner from tblLocker where idOwner is not null

            )

        OPEN UserLocker_cursor

        FETCH NEXT FROM UserLocker_cursor

        INTO @idUser

        WHILE @@FETCH_STATUS = 0

        BEGIN

            SET ROWCOUNT 1 --Return only one record

            SELECT @idLocker=idLocker From tblLocker where idLocker is null

            SET ROWCOUNT 0 --Return all records (important to reset this setting!)

            --Now Assign a locker to each user           

            Update tblLocker set idOwner=@idUser Where idLocker = @idLocker

        FETCH NEXT FROM UserLocker_cursor

        INTO @idUser

        END

    CLOSE UserLocker_cursor

    DEALLOCATE UserLocker_cursor

END

Threading

When VB.NET first came out I had many thoughts of going back to VB6. They seemed to have changed everything, and the VB6 to .NET converter always failed to convert my code. But then there was two things that I discovered that sold me on VB.NET.

#1) Threading
#2) Registry Access

Back in my VB6 days my team tried many times to get things to work muti-threaded, there were magazine articles at the time that claimed to be able to do it using windows API hacks, and MDI forms, but we were never able to get it to work. In .NET you just have to call .Start on a thread object is just Amazing.

Registry access in VB6 was crazy. You had to write pages and pages of code using a bunch of Win32 API Calls. In .NET I changed my pages of code into just a few lines. Once I wrote my first .NET Registry access routine I never wanted to write in VB6 again.


Here is some code that I modified that does both Registry Access and threading. I'm not sure who wrote this code originally, but I modified it so it would be thread safe. Your form needs to have the following controls:

A timer named Timer1, set to go off every 5 seconds (5000 ms)
A datagrid control named DataGridView1
Two buttons Button1 and Button2

If your are using this on a VISTA box you will need to set your UAC Settings, see this post for more information. http://vb-daryl.blogspot.com/2009/11/uac-administrator-access.html

Imports System.Threading.Thread

Imports Microsoft.Win32

Public Class Form1

  Dim dtResults As DataTable

  Dim dtFound As DataTable

  Public Shared ProcessThread As System.Threading.Thread

  Dim Path As String = "SYSTEM\\"

  Dim SearchStr As String = "USERNAME"

  Dim WaitRequest As Boolean

  Dim TS As ThreadStatus

  Enum ThreadStatus

    Running

    Wating

  End Enum

  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    dtResults = New DataTable

    dtResults.Columns.Add("Name", GetType(String))

    dtResults.Columns.Add("Value", GetType(String))

    dtFound = New DataTable

    dtFound.Columns.Add("Name", GetType(String))

    dtFound.Columns.Add("Value", GetType(String))

    Timer1.Enabled = True

    TS = ThreadStatus.Running

    ProcessThread = New System.Threading.Thread(AddressOf SearchThread)

    ProcessThread.Start()

    '    SearchThread()

  End Sub

  Sub SearchThread()

    Search(Path, SearchStr)

    MsgBox("Search Complete")

  End Sub

  Public Sub Search(ByVal Path As String, ByVal SearchStr As String)

    If WaitRequest = True Then

      WaitRequest = False

      TS = ThreadStatus.Wating

      While TS = ThreadStatus.Wating

        System.Threading.Thread.Sleep(10)

      End While

    End If

    Dim dRow As DataRow

    Dim ParentKey As RegistryKey = Registry.LocalMachine.OpenSubKey(Path, True)

    ' Loop through values in the subkey

    For Each valueName As String In ParentKey.GetValueNames()

      On Error Resume Next

      'Create String to Compare against

      Dim CurStr As String = valueName + " = " + ParentKey.GetValue(valueName)

      Dim CurValue As String = ParentKey.GetValue(valueName)

      dRow = dtResults.NewRow

      dRow("Name") = valueName

      dRow("Value") = ParentKey.GetValue(valueName)

      dtResults.Rows.Add(dRow)

      'ListBox1.Items.Insert(0, valueName + " = " + ParentKey.GetValue(valueName))

      If CurValue = SearchStr Then

        Dim dRowF As DataRow = dtFound.NewRow()

        dRowF("Name") = valueName

        dRowF("Value") = CurValue

        dtFound.Rows.Add(dRowF)

        CurStr = ""

        CurValue = ""

      End If

    Next

    'if there are sub keys loop through and be recursive

    If ParentKey.SubKeyCount > 0 Then

      For Each subKeyName As String In ParentKey.GetSubKeyNames()

        dRow = dtResults.NewRow

        dRow("Name") = Path + subKeyName

        dRow("Value") = ""

        dtResults.Rows.Add(dRow)

        'ListBox1.Items.Insert(0, "")

        'ListBox1.Items.Insert(0, Path + subKeyName) ' Writing the Keyname

        'This is what makes me recursive!

        Dim Thispath As String = Path + subKeyName + "\\"

        Search(Thispath, SearchStr)

      Next

    End If

  End Sub

  Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick

    WaitRequest = True

    Timer1.Stop()

    While TS = ThreadStatus.Running

      System.Threading.Thread.Sleep(10)

      Application.DoEvents()

    End While

    Me.Label1.Text = "Checked " & dtResults.Rows.Count & " Rows"

    Dim dtTemp As DataTable = dtResults.Copy

    Dim dtTempF As DataTable = dtFound.Copy

    Me.DataGridView1.DataSource = dtTemp

    Me.DataGridView2.DataSource = dtTempF

    TS = ThreadStatus.Running

    Timer1.Start()

    Dim x As DataView = New DataView(dtTemp)

    x.Sort = "idOrder, Name"

  End Sub

  Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click

    ProcessThread.Abort()

    ProcessThread = Nothing

  End Sub

End Class

This code does have a bug that I didn't take the time to figure out, after it searches 200,000 or so keys it starts crashing. I'm not sure why, my guess is it is trying to access some System Protected Key, but this code works well enough that I am not going to worry about that.

Now that you've seen that I would like to discuss some of the concepts that you need to be aware of in Threading.

Mainly "Thread Safety" and "Thread Management".

"Thread Safety"

Most people new to threading do not think much about this, you spawn some thread and want to post it's results back to the UI.  But you will find when you code tries to access a datagrid, or listbox that is on the UI thread that it will throw an error as a background thread will not be allowed to mess with the UI thread. There are a number of solutions to this, I generally use the Singleton pattern to create a buffer to hold data in.  I'll talk more about Singletons in later posts.  For this example I decided to use the timer control to pause the background thread, and update the UI with it's current results.

"Thread Management"

This concept is important as well. If you do not stop your other threads when your application is shut down they will keep running forever.  You also need to be concerned about how many threads you create. The first time I wrote a threading application I wrote a program which searched all files and folders on a hard drive. The main thread searched for new folders, and it spawned another thread to search the files. As it ran it just kept spawning thread after thread until it hit around 32,000 threads at which time my computer, without giving an error message, turned itself off.  In this example we only create one thread so you need not worry about it crashing you computer.

I generally create my own threading class which spawns a set number of threads (usually in it's constructor) and manages them.  It keeps a Queue of new things to spawn threads for, and then checks the status of each thread, once the thread is done with it's task the threading class records the results, disposes the thread and creates a new thread with the next task in it's Queue.  I believe this is called "Thread Polling" but I don't know I created this method on my own without reading about it.

i += 1

Here is just a little trick that if you are an old VB6 programmer you might not realize is available in VB.NET. Do you still use i = i + 1 when you need to increment your i counter? well there is a shorter way, just do i += 1 this will increase i by 1. I don't know why they don't let us use i++ it seems this would be pretty easy for the .net framework, but maybe they are trying to not make VB look too much like C#.

Create a variable without Dim

So this is a cool feature of .NET, I might be wrong but I think this is a feature that didn't come out until after the 2005 edition.

Do you ever Dim i as Int and use it over and over again, just to realize that you forgot to set i = 0 before one of your loops? Well fear no more, you can create a variable named i without diming it and it only lives inside a narrow scope. Check this out:

    For i As Integer = 0 To 10

 

    Next

You can even Dim a variable inside a IF Block (or any other block) and it is only available in the scope of that block.

    If True = True Then

      Dim i As Integer = 0

      i = i + 1

    End If

 

    If True = True Then

      Dim i As Integer = 0

      i = i + 1

    End If