VBA 2012 - Need to return an error when a user enters text instead of a numbers - vba

Question

I am taking a VBA class and am completely stuck on this problem. We CANNOT use the masked text box, which would solve this problem. Instead the professor actually wants me to learn the code, can you imagine?
All kidding aside, the user needs to enter a gas price into a text box, then hit calculate to receive the total cost of the trip. There is much more to the interface but will spare you the details. If a user enters anything else number than a positive number with one decimal place, it should return an error. I have figured out 0 or 0000 as well as a negative number such as -3.45. Now I have to get any text or special characters to give me an error as well as something like 34.56.12.45. You never know, a user may feel the need to type in their IP address. The key to the assignment is that I catch all probable user errors.
Here is what I've written for the calculation as well as catch the errors. I have tried the Try/Catch statements as well. Nothing worked but I got the first two parts of the IF statement to work yet always failing on the last IF part until it gets to the calculation.
Private Sub btnCalc_Click(sender As Object, e As EventArgs) Handles btnCalc.Click
Dim Mileage As Decimal
Dim Miles As Decimal
Dim GasPrice As Decimal
Dim Cost As Decimal
If CDec(txtbxGasPrice.Text) = 0 Then
MessageBox.Show("Please enter a positive dollar amount")
txtbxGasPrice.Text = String.Empty
End If
If CDec(txtbxGasPrice.Text) < 0 Then
MessageBox.Show("Please enter a positive dollar amount")
txtbxGasPrice.Text = String.Empty
End If
If Cost = CDec((Miles / Mileage) * GasPrice) Then
Miles = CDec(lblTMiles.Text)
Mileage = CDec(lblMileage.Text)
GasPrice = CDec(txtbxGasPrice.Text)
lblTotalCost.Text = Cost.ToString("C2")
End If
If CBool(txtbxGasPrice.Text = "") Then
MsgBox("You must enter a dollar amount")
End If
*If Not IsNumeric(txtbxGasPrice.Text) Then
MessageBox.Show("Please enter a positive dollar amount")
txtbxGasPrice.Text = String.Empty*
End If
End Sub
'I have placed this at the top, in the middle, at the bottom but no luck. What am I missing?
Appreciate your thoughts - Lauren

Answer 1

This one seems to meet your criteria and pass David's tests:
Function IsValid(txt As String) As Boolean
If Not IsNumeric(txt) Then
Exit Function
End If
If Len(txt) < 2 Then
Exit Function
End If
If Not Mid(txt, Len(txt) - 1, 1) = "." Then
Exit Function
End If
If Not txt > 0 Then
Exit Function
End If
IsValid = True
End Function

Answer 2

This seems like a perfect application of regular expressions, but could be out of scope for this problem, maybe even better though vb has a Decimal.TryParse(or parse) that will take a string and try to parse it to a decimal.
http://msdn.microsoft.com/en-us/library/system.decimal.tryparse.aspx
on a side not I'm not 100% sure how it acts with xx.xx.xx but I'm betting it will fail and help your problem

Answer 3

I'm going to expand on CSgoose's idea to use RegEx, it seems a lot more reliable. I am very, very green when it comes to using RegEx but I try to work on Q's here at SO, so this may not be the optimal pattern to match, but a function like this seems to do the trick when I test a few value.
0/0.0/0.00 = False
-1.5 = False
1.5 = True
5.45 = False
Steve = False
Steve.6 = False
Steve6.58 = False
6.574.2 = False
A value that evaluates to 0 will return false. Negative values return false. Value must have a decimal component, be comprised of any # of digits (this can be tweaked if you want to limit it, eg., to ##.# format, etc.). Matches full text only, so things like IP addresses won't return true, etc.
NOTE This is VBA, but should be easily adaptable for your purposes)
Sub YourSub()
If Not IsMatch(CStr(txtbxGasPrice.Text)) Then
MsgBox "Please ensure that the value you enter is a positive dollar amount, to 1 decimal place!", vbCritical, "Invalid Gas Price Value!"
End IF
End Sub
This function requires enabling reference to Microsoft VBScript Regular Expressions 5.5, or you could use late-binding.
Function IsMatch(str As String) As Boolean
'Tests for a positive numeric value, formatted 0.0 with a mandatory decimal component
' exact match only
Dim re As RegExp
Dim allMatches As MatchCollection
Dim retVal As Boolean
retVal = False 'by default
If Not IsNumeric(str) Then GoTo EarlyExit 'ignore any non-numeric value
Set re = New RegExp
re.Pattern = "\d*\.[0-9]"
Set allMatches = re.Execute(str)
If allMatches.Count = 1 Then
'If there are multiple matches, then I think safe to say it's not a match,
' make sure it's a full string match
If str > 0 Then
retVal = (allMatches(0) = str)
End If
End If
EarlyExit:
Set re = Nothing
IsMatch = retVal
End Function
Update to force, for example, ##.# format, you could do
re.Pattern = "[1-9]?\d\.\d"