[RESOLVED] Cast from String to type double is not valid

[sauce1979]

I am currently going through a book on beginning VB,NET and am trying to complete one of the exercises in the book. I have created a function that validates user entries to check that they are numeric and within certain a range.

If text value is entered into the textbox I get an exception error saying that

"Cast from String to type double is not valid"

The main code is as follows:

Private Sub btnCalculate_Click(ByVal sender As System.Object, _
            ByVal e As System.EventArgs) Handles btnCalculate.Click

        

        Dim dOrderTotal As Decimal
        Dim dDiscountPct As Decimal
        Dim dDiscountAmount As Decimal
        Dim dInvoiceTotal As Decimal



        If ValidEntry(txtOrderTotal.Text) = 0 Then
            dDiscountAmount = DiscAmount(dOrderTotal)
            dInvoiceTotal = dOrderTotal - dDiscountAmount
            lblDiscountAmount.Text = FormatNumber(dDiscountAmount)
            lblInvoiceTotal.Text = FormatNumber(dInvoiceTotal)
        ElseIf ValidEntry(txtOrderTotal.Text) = 1 Then
            ' Not IsNumeric
            MessageBox.Show("You must enter a numeric value.", "Entry error")
        ElseIf ValidEntry(txtOrderTotal.Text) = 2 Then
            ' Out of Range Value
            MessageBox.Show("Order total must be between 10 and 10000", "Entry error")
        End If

        txtOrderTotal.Focus()

    End Sub



Module CalcModule

    Const dMinOrderTotal As Decimal = 10
    Const dMaxOrderTotal As Decimal = 10000
    Public Function DiscAmount(ByVal Ordertot As Decimal) As Decimal

        Dim dDiscount As Decimal, dDisAmount As Decimal

        If Ordertot >= 500 Then
            dDiscount = 0.2
        ElseIf Ordertot >= 250 Then
            dDiscount = 0.15
        ElseIf Ordertot >= 100 Then
            dDiscount = 0.1
        Else ' < 100
            dDiscount = 0
        End If

        dDisAmount = dDiscount * Ordertot

        Return (dDisAmount)
    End Function
    Public Function ValidEntry(ByVal OrderTotal As String) As Integer

        Dim dValidType As Integer
        ' 0 = valid entry
        ' 1 = nonnumeric
        ' 2 = invalidrange

       If IsNumeric(OrderTotal) And OrderTotal >= dMinOrderTotal And OrderTotal <= dMaxOrderTotalIf IsNumeric(OrderTotal) And OrderTotal >= dMinOrderTotal And OrderTotal <= dMaxOrderTotal Then
            dValidType = 0
        ElseIf IsNumeric(OrderTotal) And (OrderTotal < dMinOrderTotal Or OrderTotal > dMaxOrderTotal) Then
            dValidType = 2
        Else
            dValidType = 1
        End If
        Return dValidType

    End Function
End Module

The highlighted code is where the exception is thrown.
Any help to solve this would be appreciated.

[cicatrix]

Use
Double.TryParse method.

' this will cast string to double:
Dim str As String = "3.14"
Dim d As Double = 0

If Not Double.TryParse(str, d) Then
   ' The string is in invalid format
End If

[sauce1979]

HI i am relatively new to VB.NET. I have tried to add TryParse to my code but get an error saying that:

Argument not specified for parameter 'result' pf [ublic Shared Function TryParse

my amended code is:

Dim dValidType As Integer
Dim d As Double = 0

If Double.TryParse(OrderTotal, d) And OrderTotal >= dMinOrderTotal And OrderTotal <= dMaxOrderTotal Then
            dValidType = 0
ElseIf Double.TryParse(OrderTotal, d) And (OrderTotal < dMinOrderTotal Or OrderTotal > dMaxOrderTotal) Then
         dValidType = 2
Else
         dValidType = 1

End If
Return dValidType

[Mur16]

Let the parameter in your function be of type Object. In your function do the IsNumeric check first. In the If statement each condition is evaluated even if the first fails.

[sauce1979]

I am relatively new vb.net Why should the parameter in the function be of type object?

[Shaggy Hiker]

Ok, there are a couple problems here. First off, use AndAlso and OrElse rather than And/Or. The reason for doing this is that And will evaluate all pieces of the If statement, while AndOr will stop evaluating once the overall answer is known. Therefore, if the first part (the TryParse) is False, then there is no point in even evaluating the rest. Using And, the rest will be evaluated anyways. Using AndAlso, the rest will not be evaluated.

The next point is that Option Strict should be ON. That makes the resulting code faster and avoids some simple bugs such as the one in that If statement. Using Option Strict, you will become a better coder, make less errors, and end up with faster code. It's just a good idea all around, and should be the set as a default for ALL of your projects (it can be set in the Project|Properties, or by code page, but the former is much better than the latter).

Lastly, while you do convert the string to a double with TryParse, you then promptly ignore the double (which is in d), and try to compare the string (orderTotal) to a double. The only way to do that would be for the compiler to implicitly convert orderTotal to a double and perform the comparison. You just did an explicit conversion with the TryParse statement, so why perform the slower implicit conversion in the very next part of the statement? Option Strict would prevent that. Using AndAlso would mean that the second comparison wouldn't even have happened if the string couldn't be converted to a double, and it's just a bad idea anyways. So those are the three points.

[sauce1979]

Hi

Thanks for the imformation. I took actually changed the code as following. The argument of my function was changed to control and put the isnumeric check first on its own.

Public Function ValidEntry(ByVal OrderTotal as Control) As integer

Dim dValidType As integer
Dim dOrdTot As Decimal


If Isnumeric(OrderTotal.Text) Then

  If OrderTotal.Text >= dMinOrderTotal And OrderTotal.Text <= dMaxOrderTotal Then
      dValidType = 0
  Else
     dValidType = 2
  End If
Else
    dValidType = 1
End If

Return dValidType

End Function

My reason for doing so was that i wanted to keep in line with the level I was at with the book I am reading. I will definitely takeon board what you have suggested for future reference. In the meantime I got the application to work.

Regards
Sauce1979

[Shaggy Hiker]

That certainly matters. I'm a bit surprised that a book on .NET would advocate IsNumeric, though.

[techgnome]

It shocks me even more that passing in a control like that would be "acceptable" ... but eh, you know, it seems like the academic world and increasingly the published world doesn't seem to want (or find it necessary) to teach real-world context or generally accepted best practices. Everything seems to be contrived these days.

-tg

[cicatrix]

That certainly matters. I'm a bit surprised that a book on .NET would advocate IsNumeric, though.

Any idiot can write a book, I noticed.
The code above makes me think that this style was suggested by a php coder. )