The first debugging tool we’ll look at is the Breakpoint. This
is where you tell C# to halt your code, so that you can examine what is in your
variables. They are easy enough to add.
To add a Breakpoint, all you need to do is to click in the margins to the left
of a line of code:
In the image above, we clicked in the margins, just to the left of line 21.
A reddish circle appears. Notice too that the code on the line itself gets highlighted.
To see what a breakpoint does, run your programme and then click your button.
C# will display your code:
There will be a yellow arrow on top of your red circle, and the line of code
will now be highlighted in yellow. (If you want to enable line numbers in your
own code, click Tools > Options from the C# menus at the top. On the
Options box, click the arrow symbol next to Text Editor, then C#. Click
on General. On the right hand side, check the box for Line Numbers, under
the Settings heading.)
Press F10 on your keyboard and the yellow arrow will jump down one line. Keep
pressing F10 until line 28 in your code is highlighted in yellow, as in the
Move your mouse pointer over the letter variable and C# will show you
what is currently in this variable:
Now hold your mouse over strText to see what is in this variable:
Although we haven’t yet mentioned anything about the Substring method, what
it does is to grab characters from text. The first 1 in between the round brackets
means start at letter 1 in the text; the second 1 means grab 1 character. Starting
at letter 1, and grabbing 1 character from the word Debugging, will get you
the letter “D”. At least, that’s what we hoped would happen!
Unfortunately, it’s grabbing the letter “e”, and not the letter “D”.
The problem is that the Substring method starts counting from zero, and not
Halt your programme and return to the code. Change your Substring line to this:
letter = strText.Substring(0, 1);
So type a zero as the first number of Substring instead of a 1. Now run your
This time, the correct letter is in the variable. Halt your programme again.
Click your Breakpoint and it will disappear. Run the programme once more and
it will run as it should, without breaking.
So have we solved the problem? Is the programme counting the letter g’s correctly?
No! The letter count is still zero! So where’s the error? To help you track
it down, there’s another tool you can use – the Locals Window.
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