Question About Error I am receiving When Calling A Function

Hello, I am writing an application to gain an understanding of how to use Functions and Sub procedures. I am receiving the following error when I am trying to call my functions. If anybody can offer any feedback it would be appreciated. Please view my code below.

Here is the error I am receiving:
An unhandled exception of type 'System.InvalidCastException' occurred in microsoft.visualbasic.dll

Additional information: Cast from string "" to type 'Decimal' is not valid.

The program '[2456] Function_Problem2.exe' has exited with code 0 (0x0).

Here is the code to Call The Functions
'Declaring Variables'
Dim decSubtotal As Decimal ' Holds Services & Labor'
Dim decPart As Decimal ' Holds parts from other services'
Dim decTax As Decimal ' Holds tax on parts'
Dim decTotalFees As Decimal ' Holds the total of everything'

'The breakpoint at RunTime is going to this First line of code'
decSubtotal = OilLubeCharges(txtSerLabor.Text) + FlushCharges(txtSerLabor.Text) + MiscCharges(txtSerLabor.Text) + OtherCharges(txtSerLabor.Text)
decPart = OtherCharges(txtPart.Text)
decTax = TaxCharges(txtTax.Text)
decTotalFees = TotalCharges(txtTotalFees.Text)

Here are the Functions I wrote in my form
Function OilLubeCharges(ByRef pdecOil As Decimal) As Decimal
Dim decoilChangeCost As Decimal
If chkOilChange.Checked = True Then
decoilChangeCost = " $26.00 "
Else
decoilChangeCost = " $18.00 "
End If

Return decoilChangeCost
End Function

Function FlushCharges(ByRef pdecFlushes As Decimal) As Decimal
Dim decFlushes As Decimal
If chkRadFlush.Checked = True Then
decFlushes = " $30.00 "
Else
decFlushes = " $80.00 "
End If

Return decFlushes
End Function

Function MiscCharges(ByRef pdecMisc As Decimal) As Decimal
Dim decMisc As Decimal
If chkInspection.Checked = True Then
decMisc = " $15.00 "
ElseIf chkMuffler.Checked = True Then
decMisc = " $100.00 "

Else
decMisc = " $20.00 "
End If

Return decMisc
End Function

Function OtherCharges(ByRef pdecOther As Decimal) As Decimal
Dim decPart As Decimal
Dim decLabor As Decimal
Dim decTotal As Decimal

decPart = Val(txtPart.Text)
decLabor = Val(txtLabor.Text)

txtPart.Text = decPart
txtLabor.Text = decLabor
decTotal = decPart + decLabor

Return decTotal
End Function

Function TaxCharges(ByVal amount As Decimal) As Decimal
Dim decSalesTax As Decimal
decSalesTax = amount * 6%
Return decSalesTax
End Function

Function TotalCharges(ByVal total As Decimal) As Decimal
Dim decTotal As Decimal
decTotal = txtSerLabor.Text + txtParts.Text + txtTax.Text + txtTotalFees.Text
Return decTotal
End Function

It appears that you have Option Strict ON, which is just as it should be, however, it also means that implicit conversions don't work. That's fine, they are slow anyways.

The problem is that your functions are expecting a decimal value, while the textbox text property returns a string. If you can be certain that what is in the textbox is a valid number, then you can simply call the function like this:


OilLubeCharges(CDec(txtSerLabor.Text))

CDec() takes a string (the txtSerLabor.Text in this case), and turns it into a decimal value. The only drawback to this function is that if there are any invalid characters in the textbox (a letter, for instance), CDex will throw an exception. There are a couple of ways to get around that. The worst is to wrap the call in a Try...Catch block. This is bad because the error is preventable, and wrapping in a block like that won't really allow you to fix the problem, it will just be a particularly slow way to identify the problem.

A better solution would be to use Decimal.TryParse(), which returns True if the string can be converted, and false if it cannot. Unfortunately, in your case, it wouldn't be all that good, because the converted number is returned in one of the arguments to the function, so you can't use it as an argument to a function (at least not in a nice fashion).

The best solution would be to restrict the textboxes to only allow valid characters, or validate them (perhaps using TryParse()) before you use the data. If you want to restrict the numbers, there are lots of threads in this forum that cover that. On the other hand, if you are just trying things out, you can simply ignore that issue for now and just use CDec.

Thank you, very much for your help and explanation. It was very helpful.

Hey, another quick question. I took your advice and did the following.

decSubtotal = OilLubeCharges(CDec(txtSerLabor.Text) + FlushCharges(CDec(txtSerLabor.Text) _
+ MiscCharges(CDec(txtSerLabor.Text) + OtherCharges(txtSerLabor.Text)

But, I keep getting an error after OtherCharges(txtSerLabor.Text) telling me ')' expected. What am I missing here to keep getting this error. Thank you, for your help again.

it looks like you are missing some closing parens " ) ". you need to add then in what looks like two places.

:)

tehehehe.... it's kinda like Where's Waldo ... I found three

decSubtotal = OilLubeCharges(CDec(txtSerLabor.Text)) + FlushCharges(CDec(txtSerLabor.Text)) _
+ MiscCharges(CDec(txtSerLabor.Text)) + OtherCharges(txtSerLabor.Text)

-tg

ooopsss....I think I should get my eyes checked ;)

There's actually a hidden fourth one, too, because the CDec is missing for the call to OtherCharges(), which would cause the same error that started this thread.