Dot.NET Tutorial For Begginers ….

In the last part, you added some code to a button. All the code does is try and count how many times the letter “g” appeared in the word “Debugging”. We saw that the programme failed to do this, and came up with the answer zero. To help you find out what went wrong, there is a tool in VB .NET called a Breakpoint. Let’s see what this is, and how to use Breakpoints.

Breakpoints

A breakpoint is like a note to VB.NET to stop your programme at a particular place. You add one to your code by clicking in the margins. A brown circled then appears, indicating where the code will break. The following two images show how to add one:

Click in the margins to add a Breakpoint

When you click in the margins, to the left of a line of code, a brown circle appears:

A Breakpoint has been added

Notice that the line where you want VB.NET to break is highlighted brown.

Run your programme, and click the button. You are immediately returned to the coding window. The place where you put the Breakpoint will now have a yellow arrow on top of the brown circle. The brown highlighted line will now be yellow:

The Breakpoint is now highlighted in yellow

The yellow highlight indicates where in your code VB.NET is. To continue checking your code, press F10 on your keyboard (you can also press F11, but this will jump into any Subs or Functions you’ve set up.)

The next line in your code will be highlighted:

The programme has advanced to the next line

The yellow arrow, and the yellow highlight, jump down one line. Press the F10 key again. Then hold you mouse on the letter variable. The value this variable currently holds will be displayed:

The first time round the loop, the value in letter is “e” (The “c” next to it means that the variable type is Character).

If the “e” of “Debugging” is getting checked first, what happened to the “D”? Straight away, this indicates a problem. And the problem is that the Substring method starts counting from zero. So halt your programme by clicking “Debug > Stop Debugging“, or click the Stop icon on the toolbar. Change the line in question to this:

letter = strText.Substring(0)

Run your programme again, and click the button. When you are returned to your code, press the F10 key and check the value of the letter variable. It should now be this:

This time, the code is catching the first letter of the word when the loop begins, and not the second one.

Is that it? Have we found the cause of our problems? Stop your programme. Click on the brown circle to get rid of the Breakpoint. Run it again, and see what happens.

The number of G’s counted is still zero! So the logic error has not yet been tracked down. Create another Breakpoint at the same place, and try again.

You can continue pressing the F10 key until you’ve spotted the error. Or you can use another debugging tool – the Locals window.

While your programme is still in Debug mode (the yellow line will still be there, if it is), click Debug > Windows > Locals from the menu bar. You should see the following in the bottom left of your screen:

The locals window

Locals means “Local variables”. That is, variables declared in this section of the code. The variables i, letter and LetterCount are showing in the window. The value of these variables is also displayed: 0, Nothing and 0. Press F10 and these values will change. This is the Locals window after one go round the loop:

Press the F10 key to advance the programme

The variable i is now 2; letter is “D“, and LetterCount, is still 0. Keep pressing F10 and go round the loop a few times. What do you notice?

You should notice that the value in letter never moves on. It is “D” all the time. And that’s why LetterCount never gets beyond 0.

Exercise

Why does LetterCount never gets beyond 0? Correct the code so that your textbox displays the correct answer of 3 when the programme is run.

And that’s it for Error checking and debugging. It can often be an art form in itself. But one well worth persevering with: it could save you a lot of headaches!

In the next section, we’ll move on and have a look at what arrays are.

.Net Tuto Part 50 : Logic Errors in VB .NET

Posted on November 22, 2010 by jose4u under .net Tutorial

The third category of errors are the Logic errors. These can be thought of as coding errors. Your coding errors. They can be quite tricky to track down, and have you tearing your hair out with frustration. You will often hear yourself saying “But that should work! Why won’t it!”

Add another button to the form tou created in the first part and try this code as an example of a logic error:

Dim x As Integer
Dim y As Integer
Dim answer As Integer

x = 10.5
y = 3
answer = x * y
TextBox1.Text = answer

When you’ve added the code to your button, run your programme and test it out. Before you click the button, what answer did you expect to get?

You’d think that 10.5 multiplied by 3 would give you the answer 31.5. Click your button. The answer that appears in your textbox is 30!

This is a logic error: when you don’t get the answer you thought you’d get. The problem, if you were paying attention during the variable types sections, is that we are trying to put floating point numbers into an Integer variable type. The Integer variable only works with whole numbers. When you assign 10.5 to the variable x, the point 5 on the end gets chopped off. So only the 10 gets stored in x. 10 times 3 is thirty, and this is the answer that appears in the textbox.

But the point is that VB.NET did not raise a Design-time error. Nor did it raise a Runtime error. The programme executed, and did not “bug out” on us. It just didn’t give you the answer you expected – it was a logic error.

Logic errors can be fairly simple to track down and solve. (The problem above can be solved by changing the variable types from Integer to Single or Double.) But they can also be quite difficult to track down. Especially as your code gets longer and longer. Here’s another example of a logic error.

Erase the code you have for button2, and add the following instead:

Dim i As Integer
Dim LetterCount As Integer
Dim strText As String
Dim letter As Char

strText = “Debugging”

For i = 1 To strText.Length – 1
letter = strText.Substring(1)

If letter = “g” Then
LetterCount = LetterCount + 1
End If
Next

TextBox1.Text = “G appears ” & LetterCount & ” times”

All the code does is to try and count how many times the letter “g” appears in the word “Debugging”. We’re using a For loop, and Substring to get one letter at a time. This single letter is then placed inside the variable called letter. An If Statement is used to check if the letter is a “g”. If it is, we increment the LetterCount variable. The answer we’re expecting in the textbox is 3. Except, we don’t get 3. We get zero:

There were no wiggly lines and therefore no Build errors. When the button was clicked, a Runtime exception did not crash the programme. So that leaves a logic error. But where is it?

In the next part, you’ll learn how to use VB .NET’s inbuilt tools to help you track down logic errors.

.Net Tuto Part 49 : Try … Catch in VB .NET

Posted on November 22, 2010 by jose4u under .net Tutorial

VB.NET has a inbuilt class that deals with errors. The Class is called Exception. When an exception error is found, an Exception object is created. The coding structure VB.NET uses to deal with such Exceptions is called the Try … Catch structure.

In the coding area for your button, type the word Try. Then hit the return key on your keyboard. VB.NET completes the rest of the structure for you:

Try

Catch ex As Exception

End Try

The Try word means “Try to execute this code”. The Catch word means “Catch any errors here”. The ex is a variable, and the type of variable it is is an Exception object.

Move your line of code from the last section to the Try part:

Try

rt1.LoadFile(“C:\test10.txt”, RichTextBoxStreamType.PlainText)

Catch ex As Exception

End Try

When you run your programme, VB will Try to execute any code in the Try part. If everything goes well, then it skips the Catch part. However, if an error occurs, VB.NET jumps straight to Catch. Add the following to your Catch part:

MsgBox(ex.Message)

Your coding window should look like this:

Your coding window

Because ex is an object variable, it now has its own Properties and methods. One of these is the Message property. Run your programme and test it out. Click your button. You should see the following error message:

File Not Found Error

The message is coming from the “additional Information” section of the error message we saw earlier, the one we didn’t handle. But the point about this new message box is that it will not crash your programme. You have handled the Exception, and displayed an appropriate message for the user.

If you know the kind of error that a programme might throw, you can get what Type it is from the Error message box you saw earlier. This one:

The File Not Found error

Click the View Details links under Actions to see the following:

Details about the error thrown by VB NET

The first line tells us the Type of Exception it is:

System.IO.FileNotFoundException

You can add this directly to the catch part. Previously, you were just catching any error that might be thrown:

Catch ex As Exception

But if you know a “file not found” error might be thrown, you can add that to the Catch line, instead of Exception:

Catch ex As System.IO.FileNotFoundException

You can keep the Exception line as well. (You can have as many Catch parts as you want.) This will Catch any other errors that may occur:

Try

rt1.LoadFile(“C:\test10.txt”, RichTextBoxStreamType.PlainText)

Catch ex As System.IO.FileNotFoundException

MsgBox(ex.Message)

Catch ex As Exception

MsgBox(ex.Message)

End Try

There is one last part of the Try … Catch Statement that VB.NET doesn’t add for you – Finally:

Try

Catch ex As Exception

Finally

End Try

The Finally part is always executed, whether an error occurs or not. You typically add a Finally part to perform any cleanup operations that are needed. For example, you may have opened a file before going into a Try … Catch Statement. If an error occurs, the file will still be open. Whether an error occurs or not, you still need to close the file. You can do that in the Finally part.

But Microsoft advise that you always use Try … Catch Statements in your code. However, throughout the rest of this course, for convenience sake, we won’t be using them much. Even when we should be.But that’s no excuse for you not to use them!

In the next part, we’ll take a look at Logic Errors.

.Net Tuto Part 48 :Runtime errors in VB .NET

Posted on November 22, 2010 by jose4u under .net Tutorial

Runtime errors are a lot harder than Design Time errors to track down. As their name suggests, these errors occur when the programme is running. Runtime errors are the ones that crash your programme. A simple way to crash a programme is to divided by zero. Change the code for your button to this, and try it out:

Dim Num1 As Integer
Dim Num2 As Integer

Num1 = 10
Num2 = 0

TextBox1.Text = CInt(Num1 / Num2)

The CInt( ) part means Convert to an Integer. We’re just making sure to convert the answer to the sum into a number. But run your programme and test it out. Click your button and see what happens.

What happens is that you’ll get the following error message popping up:

The Overflow Error

Click the Break button, and then stop your programme from running.

When you try to divide by zero, VB.NET throws up the Overflow error message – there would be just too many zeros to go into the Integer variable type. Even if you change the Type into a Single or a Double, you’d still get the same error message. Programming environments just don’t like you dividing a number by zero. If this were in a real programme, chances are it would crash, or “bug out”. And you’ll get the blame!

If you think the answer to a calculation could result in zero, you should check for this. We’ll see how to write code to trap Runtime errors in a moment. But here’s another example of one.

From the controls toolbox, add a RichTextBox control to your form. Change the Name property of your RichTextBox to rt1. A RichTextBox is just like a normal textbox but with more functionality. One of these extra functions is the ability to load a file directly. Delete or comment out any code you have for your button, and add the following line:

rt1.LoadFile(“C:\test10.txt”, RichTextBoxStreamType.PlainText)

However, if you are using Vista or windows 7 then change the file name above to this:

“C:\Users\Owner\Documents\test10.txt”

All the line does is to load (or try to) the text file called “test10.txt” into the RichTextBox. The second argument just specifies that the type of file we want to load is a Plain Text file.

Run your programme, and then click the button. If you don’t have a text file called “test10.txt” in the root folder of your C drive, you’ll get the following Runtime error message:

The File Not Found error

The additional information is quite useful this time. It’s saying that the file “C:\test10.txt” could not be found. If the error occurred in a normal programme, it would shut down. Not something you want a programme to do in mid stream! But let’s see how to deal with it.

In the next part we take a look at Try … Catch in order to trap any errors in your code.

.Net Tuto Part 47 :Design Time Errors in VB .NET

Posted on November 22, 2010 by jose4u under .net Tutorial

Design time errors, remember, are syntax errors. It’s when VB .NET hasn’t understood what you have typed. If that isn’t clear, a few examples should explain it beter. So do the following:

Create a new Windows project
Add a button and textbox to your form
Leave the Name properties on the defaults of Button1 and Textbox1
Double click your button to access its code, and type the following:

Textbox2.Text = “Debug”

When you finish typing the line, VB.NET puts a blue wiggly line under Textbox2:

Notice the blue wiggly line

If you hold your mouse over Textbox2, you’ll see a yellow tool tip appear, like the following:

The error is occurring because you don’t have a textbox called Textbox2. You’ll also see this same “Not declared” error if you try to set up a variable on the fly (which you were allowed to do in previous version of VB.) As an example, change your code to this:

strText = “Debug”
TextBox1.Text = strText

Here, we’re trying to put the word “Debug” into a variable called strText. We then want to assign this variable to the Text property of Textbox1. However, VB.NET protests about this, and puts a wiggly line under all occurrences of strText:

Hold your mouse over the variable strText and you’ll see the “not declared” tip again:

The problem this time is that we haven’t declared the variable strText. Change the code to this:

Dim strText As String

strText = “Debug”
TextBox1.Text = strText

Now that we have declared a variable, the wiggly lines will go away. If we added the variable declaration in the wrong place, however, the wiggly lines would come back. Change you code to this:

strText = “Debug”
TextBox1.Text = strText

Dim strText As String

The wiggly lines will be back. That’s because the declaration comes on the third line. When VB.NET meets the first two lines, it knows nothing about the strText variable.

If you have the Error List window open, you’ll see a report of your error (If you can’t see the Task List window, from the menu bars click View > Other Windows > ErrorList):

The Task List Window

The description of the error is “Local variable ‘strText’ cannot be referred to before it is declared“. If you double click the icons on the left, VB.NET will highlight the error in your code.

Move the “Dim … ” Line back to the top, and not only do the blue wiggly lines go away, but the Task List will be erased.

Design-time errors like the one above can be quite straightforward to correct. Others can be quite difficult, and you’ll see the blue wiggly line but not understand how to correct the error. The Task List should be your first port of call when faced with such an error.

In the next part, we’ll take a look at Runtime errors.

.Net Tuto Part 46 :Error Handling and Debugging in VB .NET

Posted on November 22, 2010 by jose4u under .net Tutorial

Debugging your code is something you will need to do. Unless you write perfect code every time, there’s no getting away from it. In this section, we’ll take a look at ways you can track down errors using VB.NET.

Types of Error

Programming errors are generally broken down into three types: Design-time, Runtime, and Logic errors.

A Design-time error is also known as a syntax error. These occur when the environment you’re programming in doesn’t understand your code. These are easy to track down in VB.NET, because you get a blue wiggly line pointing them out. If you try to run the programme, you’ll get a dialogue box popping up telling you that there were Build errors.

Runtime errors are a lot harder to track down. As their name suggests, these errors occur when the programme is running. They happen when your programme tries to do something it shouldn’t be doing. An example is trying to access a file that doesn’t exist. Runtime errors usually cause your programme to crash. If and when that happens, you get the blame. After all, you’re the programmer, and you should write code to trap runtime errors. If you’re trying to open a database in a specific location, and the database has been moved, a Runtime error will occur. It’s your job to predict a thing like this, and code accordingly.

Logic errors also occur when the programme is running. They happen when your code doesn’t quite behave the way you thought it would. A classic example is creating an infinite loop of the type “Do While x is greater than 10”. If x is always going to be greater than 10, then the loop has no way to exit, and just keeps going round and round. Logic errors tend not to crash your programme. But they will ensure that it doesn’t work properly.

In the next few pages, we’ll take a closer look at all three types of error.

.Net Tuto Part 45 : Add Option Buttons to a VB .NET form

Posted on November 22, 2010 by jose4u under .net Tutorial

Radio Buttons, sometimes called Option Buttons, are used when you want to restrict a user’s choice to one, Male/Female, for example. A Checkbox would be no good here, because a user could tick both boxes. You want to force your users to pick only one from your list of options.

Adding Radio Buttons to a Form is exactly the same process as adding a Checkbox. Again, we’ll add some Radio Buttons to a Group Box, and write code to extract what the user has chosen.

Add a Group Box to your Form.
Set the Text Property of the Group Box to “Best Sit Com of all time”
Set the Font options to anything you like
Place five Radio Buttons into your Group Box (By default, they’ll be called “Option1”, “Option2”, “Option3”, etc
Set the Text Property of the Five Radio Buttons to Only Fools and Horses, Dad’s Army, Blackadder, Fawlty Towers, Vicar of Dibley
Your Form should now look something like this:

Radio Button Form in VB NET

Run your programme and test to see if you can indeed only select one item from the list.

The reason you can only select one is that all the radio buttons are placed in the same group box. You can place another set of radio buttons in a second group box, and these would work independently of the set of radio buttons in the first group box.

To test which Sit Com was chosen, you can use an If … Elseif Statement. You can do this because only one of the radio buttons will be True if selected: all the others will then have a value of False.

So place a Button on your form. Set the Text property to something appropriate. Then double click your new button to open up the code window. Type the following code (Notice that the Property is now Checked, and not CheckState):

Dim ChosenSitCom As String

If RadioButton1.Checked = True Then
ChosenSitCom = RadioButton1.Text
ElseIf RadioButton2.Checked = True Then
ChosenSitCom = RadioButton2.Text
ElseIf RadioButton3.Checked = True Then
ChosenSitCom = RadioButton3.Text
ElseIf RadioButton4.Checked = True Then
ChosenSitCom = RadioButton4.Text
ElseIf RadioButton5.Checked = True Then
ChosenSitCom = RadioButton5.Text
End If
MsgBox(“You voted for ” & ChosenSitCom)

By using If … ElseIf we can check which radio button a user selected. The Text property from the chosen radio button is then placed in a String variable called ChosenSitCom. At the end, we then display the selected radio button in a message box.

Run your programme and test it out. Select a Sit Com, and then click your Button. You should see the item you selected displayed:

Exercise

Add a Textbox to your Form. Write code to transfer a chosen Sit Com to the Textbox when the button is clicked. Add a label next to the Textbox with the Caption “You Voted For. . . ”

And that’s all there is to adding Option Buttons to your VB .NET forms. In the next section of the course, we’ll take a look at error checking.

.Net Tuto Part 44 : Check Boxes in VB .NET

Posted on November 22, 2010 by jose4u under .net Tutorial

Two more useful controls in the Visual Basic toolbox are the Check box and the Option Button. You use these when you want to give your users a choice of options. We’ll add both of these to a new Form, and then combine them with a Select Case statement to read what the user has chosen.

Check Boxes

So start a new project. Locate the Checkbox control in the toolbox. Double click the control and a Checkbox appears on your new Form

You’ll see that the Checkbox has the Text property of CheckBox1 by default, and a Name of CheckBox1. If you were to double click again on the Checkbox icon in the toolbox, the new control would be called CheckBox2.

The problem with this approach is that by double clicking each Checkbox, you have several individual Checkboxes. And if you wanted to move them around you’d have to move each Checkbox separately. There is a way to group all your Check Boxes together, and move them around as one – by using a Group Box. (You can use a Panel control as well, but we’ll stick with the Group Box.)

So, click on your Checkbox with the right mouse button. From the menu that pops up, select delete to get rid of it.

Now locate the Group Box control in the toolbox:

It’s better to draw this one on the form, rather than dragging and dropping. When you’ve added one, the only thing you should have on your Form is a Group Box.

We’re not going to be using many of the Properties in the Group Box Property box. But click on your Group Box to select it, and change to the Text Property to “Soaps”. Change the Font Property to anything you like. You should now have a Form like this one

A Group Box on a VB .NET form

The Group Box we just added will hold our Checkboxes. It acts as a container for the controls. To move the Checkboxes about, we can just click on the Group Box to select it, and drag and drop the Group Box somewhere else. The Checkboxes will all move with the Group Box. Let’s add some Checkboxes to it.

You CAN’T double click a checkbox and add it to a Group Box. The only way to add a control to a Group Box is to draw one on the Group Box.

Click once with your left mouse button on the Checkbox icon in the VB toolbox
Move your mouse pointer over to the inside of the Group Box. The mouse pointer will change to a cross
Hold down you left mouse button inside the Group Box. Keep the button held down, and drag outwards. Release the left button when you’re happy with the size. You can always resize it later.
Add 5 Checkboxes to your Group Box
Change the Text property of each of your Checkboxes to any five Soap Operas. Your Form should now look something like the one below:

What your form should look like now

Run your programme to test it out. Click inside a Checkbox to select an item. Click again to deselect it. When you’ve finished, return to the Design Environment and click on the Group Box itself to select it. Make sure the Group Box IS selected, and not one of your Checkboxes. You can now drag the Group Box around your Form and all the Checkboxes will move with it.

The point about having Checkboxes is to offer your users multiple choices. We’ll now write some code to get the choices made by the user. All the Checkboxes with ticks inside them will have their Text displayed in a Message Box.

We’ll do that in the next part of this tutorial.

.Net Tuto Part 43 : The View Images menu Item

Posted on November 22, 2010 by jose4u under .net Tutorial

This tutorial is part of an ongoing lesson. Please do if you haven’t yet created a menu.

It’s easy to add an image to your form with VB.Net. To insert an image, locate the Picture control in the toolbox. Either double click the control, or hold down your mouse on the form and draw one out. You should see something like this:

The Picture Box control

Change the Height and Width properties of the Picture Box to 100, 100. You’ll have a small square. To make it stand out more, locate the BorderStyle property. Change the value to Fixed3D. Your Picture Box will then look like this:

A fixed3d picture box

To add a picture at design time, locate the Image property in the properties box:

The Image property of the Picture Box

Download and unzip the image at the top of the page. Then click the button with the three dots on it. A dialogue box appears. Locate an image. Select it, and then click Open in the dialogue box. The image will appear in your Picture Box:

An image in a picture box

If you select an image that is too big for the picture box, only part if it will be visible. The Picture Box control does not resize your image.

You can, however, set another property of the picture box – the SizeMode property. Set this to AutoSize and your picture box will resize to the size of your image.

Insert an Image with your View Menu

You can use your open file dialogue box again to specify an image for the user to select. We’ll do this from the View Images menu item.

Highlight you code for the mnuOpen item. Copy the first five lines, these lines:

Dim strFileName As String

openFD.InitialDirectory = “C:\”

openFD.Title = “Open an Text File”
openFD.Filter = “Text Files|*.txt”
Dim DidWork As Integer = openFD.ShowDialog()

Paste them to your mnuViewImages menu item code. Change the Title property to this:

openFD.Title = “Open an Image”

And change the Filter property to this:

openFD.Filter = “jpegs|*.jpg|gifs|*.gif|Bitmaps|*.bmp”

Run your code and click your View Images menu item. You should see the Open dialogue box appear. If you look at the “Files of type” box, you should see this:

Available Image Types

You should now be able to see only the three image formats we’ve specified.

To insert an image into your Picture Box, some new code is needed. Again though, we’ll wrap it up in an If Statement.

Add the following code below the lines you’ve just added:

If DidWork <> DialogResult.Cancel Then
strFileName = openFD.FileName
PictureBox1.Image = Image.FromFile(strFileName)
openFD.Reset()
End If

There’s only two lines you haven’t met yet. The first is this line:

PictureBox1.Image = Image.FromFile(strFileName)

Previously, you were loading the image into the Image property of PictureBox1 directly from the Properties Box (by clicking the grey button with the three dots in it). Here, we’re loading an image into the Image property using code. The way you do it is with the FromFile method of the Image Class.

Although that might be a bit baffling at this stage of your programming career, all it means is that there is some in-built code that allows you to load images from a file. In between round brackets, you type the name and path of the file you’re trying to load. Since our file name has been placed inside of the strFileName variable, we can just use this. You can then assign this to the Image property of a Picture Box.

The last line, openFD.Reset(), will reset the initial directory of the open file dialogue box. To see what this does, comment out the line (put a single quote at the start of the line). Run your programme and Click View > View Images. Insert an image and then click File > Open. You’ll notice that the files displayed in your dialogue are from the last directory you opened, rather than the one you set with “InitialDirectory = “C:\”. By resetting the open dialogue box control, you’re fixing this problem.

OK, that concludes our look at menus. We’ll create a new programme now, and explore checkboxes, radio buttons and Group Boxes.