Chapter 35: VB Forms

Section 35.1: Hello World in VB.NET Forms

To show a message box when the form has been shown:

Public Class Form1

Private Sub Form1_Shown(sender As Object, e As EventArgs) Handles MyBase.Shown MessageBox.Show("Hello, World!")

End Sub End Class

To show a message box before the form has been shown:

Public Class Form1

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

MessageBox.Show("Hello, World!")

End Sub

End Class

Load() will be called first, and only once, when the form first loads. Show() will be called every time the user launches the form. Activate() will be called every time the user makes the form active.

Load() will execute before Show() is called, but be warned: calling msgBox() in show can cause that msgBox() to execute before Load() is finished. It is generally a bad idea to depend on event ordering between Load(),

Show(), and similar.

Section 35.2: For Beginners

Some things all beginners should know / do that will help them have a good start with VB .Net:

Set the following Options:

'can be permanently set

' Tools / Options / Projects and Soluntions / VB Defaults

Option Strict On

Option Explicit On

Option Infer Off

Public Class Form1

End Class

Use &, not + for string concatenation. Strings should be studied in some detail as they are widely used.

Spend some time understanding Value and Reference Types.

Never use Application.DoEvents. Pay attention to the 'Caution'. When you reach a point where this seems like something you must use, ask.

The documentation is your friend.

Section 35.3: Forms Timer

The Windows.Forms.Timer component can be used to provide the user information that is not time critical. Create a form with one button, one label, and a Timer component.

For example it could be used to show the user the time of day periodically.

'can be permanently set

' Tools / Options / Projects and Soluntions / VB Defaults

Option Strict On

Option Explicit On

Option Infer Off

Public Class Form1

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click Button1.Enabled = False

Timer1.Interval = 60 * 1000 'one minute intervals 'start timer

Timer1.Start()

Label1.Text = DateTime.Now.ToLongTimeString

End Sub

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

Label1.Text = DateTime.Now.ToLongTimeString

End Sub

End Class

But this timer is not suited for timing. An example would be using it for a countdown. In this example we will simulate a countdown to three minutes. This may very well be one of the most boringly important examples here.

'can be permanently set

' Tools / Options / Projects and Soluntions / VB Defaults

Option Strict On

Option Explicit On

Option Infer Off

Public Class Form1

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click Button1.Enabled = False

ctSecs = 0 'clear count

Timer1.Interval = 1000 'one second in ms. 'start timers

stpw.Reset()

stpw.Start()

Timer1.Start()

End Sub

Dim stpw As New Stopwatch

Dim ctSecs As Integer

Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick ctSecs += 1

If ctSecs = 180 Then 'about 2.5 seconds off on my PC! 'stop timing

stpw.Stop() Timer1.Stop()

'show actual elapsed time 'Is it near 180?

Label1.Text = stpw.Elapsed.TotalSeconds.ToString("n1")

End If End Sub

End Class

After button1 is clicked, about three minutes pass and label1 shows the results. Does label1 show 180? Probably not. On my machine it showed 182.5!

The reason for the discrepancy is in the documentation, "The Windows Forms Timer component is single-threaded, and is limited to an accuracy of 55 milliseconds." This is why it shouldn't be used for timing.

By using the timer and stopwatch a little di erently we can obtain better results.

'can be permanently set

' Tools / Options / Projects and Soluntions / VB Defaults

Option Strict On

Option Explicit On

Option Infer Off

Public Class Form1

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click Button1.Enabled = False

Timer1.Interval = 100 'one tenth of a second in ms. 'start timers

stpw.Reset()

stpw.Start()

Timer1.Start()

End Sub

Dim stpw As New Stopwatch

Dim threeMinutes As TimeSpan = TimeSpan.FromMinutes(3)

Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick If stpw.Elapsed >= threeMinutes Then '0.1 off on my PC!

'stop timing stpw.Stop() Timer1.Stop()

'show actual elapsed time 'how close?

Label1.Text = stpw.Elapsed.TotalSeconds.ToString("n1")

End If End Sub

End Class

There are other timers that can be used as needed. This search should help in that regard.