Try, Catch

Kevin7777 · March 10th, 2009, 07:36 PM

Hi Imar, I'm working on page 169 and would like to add a Try / Catch if the user enters a letter instead of a number, this is for my own knowledge since it's been quite a while since I programmed.

I came up with this, but it doesn't work:

Code:

 
If txtValue1.Text.Length > 0 AndAlso txtValue2.Text.Length > 0 Then
Dim result As Double = 0
Try
Dim value1 As Double = Convert.ToDouble(txtValue1.Text)
Dim value2 As Double = Convert.ToDouble(txtValue2.Text)
Dim myCalculator As New Calculator()
Select Case lstOperator.SelectedValue
Case "+"
result = myCalculator.Add(value1, value2)
Case "-"
result = myCalculator.Subtract(value1, value2)
Case "*"
result = myCalculator.Multiply(value1, value2)
Case "/"
result = myCalculator.Divide(value1, value2)
End Select
Catch ex As Exception
lblResult.Text = "Please enter a number"
End Try
lblResult.Text = result.ToString()
Else
lblResult.Text = String.Empty
End If

Thanks if you can steer me in the right direction...

Kevin

dparsons · March 10th, 2009, 09:50 PM

I think a more elegant solution might be something like:

vb Code:

Dim value1, value2 as Double
 
If (Double.TryParse(txtValue1.Text, value1)) AndAlso (Double.TryParse(txtValue2.Text, value2)) Then
 'Being Case statement 
Else
    lblResult.Text = String.Empty
End If

TryParse does two things:

-It returns a bool value if the first parameter (in your case txtValue1 or 2) can be converted to a double
-It returns a double via an output parameter (the second parameter) if the Parse completes successfully.

Obviously, the TryParse will return false if a string is passed in.

Imar might have other/additional suggestions.

hth.
-Doug

Kevin7777 · March 10th, 2009, 10:24 PM

Thanks Doug.

Well as you can see I'm still learning. Still trying to grasp vb, thank you, i need all the help i can get.

Better to use TryParse?

Quote:

It returns a double via an output parameter (the second parameter) if the Parse completes successfully.

Not quite sure what you meen by (the second parameter)

Kevin

dparsons · March 10th, 2009, 10:43 PM

No problem Kevin.

TryParse, in this case, takes two parameters:

TryParse(valueToParse, returnValue)

In this case you would pass either the value of either txtValue1 or txtValue2 in as the first parameter and then you would supply either value1 or value2 (the variables that have been declared as doubles) for the second parameter. What this does is that if TryParse returns True the variable you passed in as the second parameter will contain the double value.

To help explain think of this example:

Suppose that a user entered the value 3.14 into txtValue 1 and 0 into txtValue2. Then suppose you have the following code in the click event handler of a button:

vb Code:

Dim value1, value2 as Double 
If (Double.TryParse(txtValue1.Text, value1)) AndAlso (Double.TryParse(txtValue2.Text, value2)) Then 
Response.Write(value1.ToString())
Response.Write("<br />")
Response.Write(value2.ToString())
Else 
lblResult.Text = "Both values must be numeric!"
End If

The output to the browser window would be:
3.14
0

So all an output parameter is is a variable you provide as one of the parameters in the method signature (in your case either value1 or value2) which will then contain a value after the function or method has finished executing. In this case, provided TryParse returns true, the output parameter assumes the value of whatever was passed in as the first parameter; if TryParse were to fail in this case value1 or value2 would contain the value 0 (the default value for non-nullable doubles).

The only rules for output parameters, off the top of my head, is that they must be the last parameter of the signature and you can only have 1 output parameter pre method/function.

Does this make more sense?

hth.
-Doug

Kevin7777 · March 11th, 2009, 09:42 AM

Thanks Doug.

Well, it's starting to sink in. I get confused with parameters flying everywhere(newbie syndrone).

I just wanted to catch the error if someone entered a letter in context to the example on page 169, to see the code in action would probably clear things up for me.

Kevin

dparsons · March 11th, 2009, 09:57 AM

Don't misunderstand me, what you have will work just fine with one minor modification. Move this line:

lblResult.Text = result.ToString()

inside of your Try. Otherwise, regardless of what happens in your Try Catch, lblResult will always be set to the value of result.

Now here is the obvious problem with the code you have provided (don't take this as nitpicking just trying to help you out). If I were to enter the value 3.14 for value1 and the value 0 for value2 and then chose / as my operator the exception that is going to be thrown is a System.DivideByZeroException because, obviously, you can't divide by zero. So your Catch statement is going to catch this exception and then populate the label and tell me to provide a number. Well I provided two valid numbers so the message you display is a bit misleading to a user in that case.

IMHO you should try and code in a proactive way (e.g. validating the data you have coming in and make sure that it is what you expect otherwise kick it back out to the user and inform them as such) rather than reactive where you assume that the data coming in is good and have a myriad of Try Catches in place to handle user error. Again, don't misunderstand me, Try Catches are an essential part of programming and you should continue to use them but they should be there in a situation where the data you have coming in is valid but something happened during the execution that caused an exception to be thrown.

hth.
-Doug

Kevin7777 · March 12th, 2009, 07:51 PM

Thanks Doug for the insight, like I said, I need all the help I can get.

I did try to move:

lblResult.Text = result.ToString()

into my Try Catch, but it didn't work.

I have moved on in the book, but would like to see how to fix my Try Catch, can you help me?

Kevin

Imar · March 13th, 2009, 04:28 PM

Can you show your final code?

Imar

Kevin7777 · March 14th, 2009, 09:15 AM

Thanks Imar.

Code:

 
ProtectedSub btnCalculate_Click(ByVal sender AsObject, _
ByVal e As System.EventArgs) Handles btnCalculate.Click
If txtValue1.Text.Length > 0 AndAlso txtValue2.Text.Length > 0 Then
Dim result AsDouble = 0
Try
Dim value1 AsDouble = Convert.ToDouble(txtValue1.Text)
Dim value2 AsDouble = Convert.ToDouble(txtValue2.Text)
Dim myCalculator AsNew Calculator()
SelectCase lstOperator.SelectedValue
Case"+"
result = myCalculator.Add(value1, value2)
Case"-"
result = myCalculator.Subtract(value1, value2)
Case"*"
result = myCalculator.Multiply(value1, value2)
Case"/"
result = myCalculator.Divide(value1, value2)
EndSelect
Catch ex As Exception
lblResult.Text = "Please enter a number"
lblResult.Text = result.ToString()
EndTry
Else
lblResult.Text = String.Empty
EndIf
EndSub

This is for my own learning, just curious on how to use Try Catch for this example, in beginner terms

Kevin

Imar · March 14th, 2009, 09:23 AM

As doug suggested, move the line that assigns the result to the Label to inside the Try block. Currently you have it in the Catch block:

Code:

  Try
    ...
    result = myCalculator.Divide(value1, value2)
    ...
    lblResult.Text = result.ToString()
  Catch ex As Exception
    lblResult.Text = "Please enter a number"
    lblResult.Text = result.ToString()
  End Try
Else
  lblResult.Text = String.Empty
End If

Hope this helps,

Imar

---------------------------------------------------------------------------------------------------------------------
Imar Spaanjaars
http://Imar.Spaanjaars.Com
Everyone is unique, except for me.
Author of Beginning ASP.NET 3.5 : in C# and VB, ASP.NET 2.0 Instant Results and Dreamweaver MX 2004
While typing this post, I was listening to: Future Proof by Massive Attack (Track 1 from the album: 100th Window) What's This?