Confusing with Approaching If statements and Debugging - visual-studio-2010

Question

I'm extremely new to Visual Studio 2010.
For my class, I need to make a calculator. The requirement for this problem is that if A or B is zero, then for the program to not actually calculate A or B; instead, the program assigns the value of the variable that is not zero to eh result, and if they're both zero, then just make the answer zero.
Here is my code:
Public Class Form1
Private Sub AddBox_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles AddButton.Click
Dim A As Decimal
Dim B As Decimal
Dim Result As Decimal
A = ABox.Text
B = BBox.Text
A = Decimal.Parse(ABox.Text)
B = Decimal.Parse(BBox.Text)
If (ABox.Text = 0) Then
ResultLabel.Text = BBox.Text
End If
If (BBox.Text = 0) Then
ResultLabel.Text = ABox.Text
End If
If (BBox.Text = 0 And ABox.Text = 0) Then
ResultLabel.Text = 0
End If
Result = A + B
ResultLabel.Text = Result.ToString("N2")
End Sub
My questions are as follows:
Are if statements good for this, or would Try/Catch be better?
Since the answer will automatically be right, even if the code is incorrect (e.g. 9+0 will be nine regardless, whether or not the If or Try/Catch actually works), they key is proper step by step debugging. What's the optimal way to do this? I had the one menu displaying everything step by step with how the program functioned before, but for some reason, I can't seem to find the window after I closed it. I want to see it step by step as I debug with break points.
Any other tips on good syntax for these type of conditional operators?
Sorry for sounding stupid; I couldn't find the topics on here that would cover this type of problem.

Answer 1

OK...first off, you don't really need the Ifs at all. (Identity Property of Addition: For any x, x + 0 = x.) Any error that's going to happen is going to happen before you even get to them, so the only thing you've done is say that if ABox parses to zero and BBox's value is something like "5.000000", all those zeros will get copied into the result. Likewise if the boxes are reversed.
Also, if you use Decimal.TryParse instead of Decimal.Parse, un-number-like stuff in the input boxes won't kill your program. They'll just get turned into 0.
You'd get similar results (minus the extra zeros) with some code like
Private Sub AddBox_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles AddButton.Click
Dim A as Decimal, B as Decimal
Decimal.TryParse(ABox.Text, A)
Decimal.TryParse(BBox.Text, B)
ResultBox.Text = (A + B).ToString("N2")
End Sub
If for some stupid reason you really, really, really need all that If crap, you will want to use ElseIf to ensure that the default case doesn't run.
Private Sub AddBox_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles AddButton.Click
Dim A as Decimal
Dim B as Decimal
Decimal.TryParse(ABox.Text, A)
Decimal.TryParse(BBox.Text, B)
If A = 0 then
ResultBox.Text = BBox.Text
ElseIf B = 0 then
ResultBox.Text = ABox.Text
Else
Dim result as Decimal = A + B
ResultBox.Text = result.ToString("N2")
End If
End Sub
But it's a waste -- and what's worse, if the point is to add two numbers, it's incorrect. Someone could put "I Like Cheese" in the first box and "YAAAAAAAAY!!!" in the other, and the result would be "YAAAAAAAAY!!!". Clearly not what should happen.
Private Sub AddBox_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) _
Handles AddButton.Click
Dim A as Decimal
Dim B as Decimal
Try
A = Decimal.Parse(ABox.Text)
B = Decimal.Parse(BBox.Text)
ResultBox.Text = (A + B).ToString("N2")
Catch ex as FormatException
MessageBox.Show("Exactly two numbers (one per box), please!")
End Try
End Sub
But i detest doing that for user input. You should expect users to be malicious and/or idiotic, and count on them doing everything they can to mess up your app. It'll save you lots of debugging later on. But if we assume the worst, is malformed input really an exceptional condition? I say no. (Others may disagree. But they're wrong. :) )
If you want to notify the user of an input error, Decimal.TryParse also returns a boolean (true/false) saying whether it succeeded. If it failed, you can show a message telling the user to quit being an idiot. (Slightly more diplomatically, if you want.)
Secondly...turn Option Strict on. Please. All this implicit casting gives me the willies. Note that that will probably add like a dozen errors to your list...but that's a good thing. One should be aware when they're trying to put a number in a string variable and so on, cause lots of magic happens there that (in my opinion) people need to be more aware of.
K, now, as for debugging...if you've followed my advice up to this point, you shouldn't really need to debug. The exceptions are gone, and the code's going to be more correct before you can even compile it. But if you still have to, one way to go step by step through the program is to set a breakpoint on either the Function line itself, or the first line that isn't a declaration. (Click the left margin next to the line. A red ball should appear in the margin about where you clicked. That lets you know there's a breakpoint there.) You can probably try to set a breakpoint on the declarations too, but i seem to remember that causing the breakpoint to be either on the next line that actually does something, or on the beginning of the function. I forget which, and don't have VS on this machine to check.)
When you run the program from VS, the program will start, and when it hits the breakpoint, VS will pop back into the foreground with a yellow arrow where the red ball is. That yellow arrow points at the next line to be executed. From there, you can use the debug toolbar (look in your toolbars; you should have extra buttons, blue i think, that look like play/stop/etc buttons) to step through the code. You should also see windows labeled 'Locals', 'Watches', and other windows you'd expect to see in a worthwhile debugger. :)
Just be aware that while the program's stopped at a breakpoint, or stepping through the code, the app will appear frozen. UI updates probably will not appear right away, and consequently, you won't be able to type anything in the input boxes or hit the button til you resume (hit the "play' button).