HELP please.

[theBadKid]

Hey guys,

I'm sure this is in the wrong section, I don't know where to put it, I'm sorry.

So I'm making program that calculates the cost of a call based on data inputted, but I am have a major problem, I keep getting an error saying "Conversion from string "" to type 'Double' is not valid."

I have been searching for days and I understand the problem, that I cannot convert an empty string to a double, but what do I do?

Here is the code:

Public Class Form1

    Private Sub Label1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lblAccountLabel.Click

    End Sub

    Private Sub Label1_Click_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lblTelephoneLabel.Click

    End Sub

    Private Sub Label5_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lblBasic.Click

    End Sub

    Private Sub btnCalculate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCalculate.Click
        Dim iAccount As Integer
        Dim sTelephone As String
        Dim sPlan As String
        Dim iDuration As Integer
        Dim dDate As Date
        Dim dCountry As Double
        Dim iTime As Integer
        Dim bEvening As Boolean
        Dim dFlagfall As Double
        Dim dBasic As Double
        Dim dPlanDisc As Double
        Dim dLoyalty As Double
        Dim dTotal As Double
        Dim dGST As Double

        'Determines Flagfall by inputted plan
        sPlan = txtPlan.Text
        
        dFlagfall = CDbl(lblFlagfall.Text) <-- IT POINTS TO THIS WITH THE ERROR

        If sPlan = "No Plan" Then
            dFlagfall = 2.0
        ElseIf sPlan = "Bronze" Then
            dFlagfall = 1.5
        ElseIf sPlan = "Silver" Then
            dFlagfall = 0.75
        ElseIf sPlan = "Gold" Then
            dFlagfall = 0.25
        Else
            MsgBox("Unknown Plan")
        End If

        'Calculates rate per minute by inputted country
        dCountry = CDbl(lblCountry.Text)

        If txtCountry.Text = "Japan" Then
            dCountry = 0.34
        ElseIf txtCountry.Text = "China" Then
            dCountry = 0.86
        ElseIf txtCountry.Text = "India" Then
            dCountry = 0.78
        ElseIf txtCountry.Text = "UK" Then
            dCountry = 0.37
        ElseIf txtCountry.Text = "USA" Then
            dCountry = 0.42
        ElseIf txtCountry.Text = "Canada" Then
            dCountry = 0.56
        ElseIf txtCountry.Text = "Indonesia" Then
            dCountry = 0.38
        ElseIf txtCountry.Text = "New Zealand" Then
            dCountry = 0.22
        ElseIf txtCountry.Text = "Other" Then
            dCountry = 1.1
        End If

        'Displays inputted plan for summary reasons
        lblPlan.Text = sPlan

        'Calculates basic cost then displays
        txtDuration.Text = lblDuration.Text
        iDuration = lblDuration.Text
        dBasic = lblBasic.Text

        dBasic = dFlagfall + (iDuration * dCountry)

        'Displays Plan Discount
        dPlanDisc = CDbl(lblPlanDiscount.Text)
        If sPlan = "No Plan" Then
            dPlanDisc = 0
        ElseIf sPlan = "Bronze" Then
            dPlanDisc = 0.05
        ElseIf sPlan = "Silver" Then
            dPlanDisc = 0.1
        ElseIf sPlan = "Gold" Then
            dPlanDisc = 0.15
        Else
            MsgBox("Unknown Plan")

        End If

        'Calculates and displays Loyalty discount
        dLoyalty = CDbl(lblLoyalty.Text)

        dLoyalty = iTime * 0.02

        'Calculates and displays total cost




    End Sub

    Private Sub lblFlagfall_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lblFlagfall.Click

    End Sub
End Class

Any help would be so greatly appreciated and hopefully mods move this to it's correct location.

Thank you.

[jmcilhinney]

That means that you haven't entered anything into the TextBox, so it's Text property is an empty string. Obviously an empty string isn't a number, so the conversion to a number fails. You should be validating the input to make sure it is a number before using it. Your best bet is to use Double.TryParse, which will return True or False to indicate whether the input was valid, plus pass back the converted value if it was valid. There are lots of examples around.

By the way, please provide meaningful thread titles in future. The title is supposed to9 tell us what the thread is about so that we don;t have to open every single one to find those that are relevant.

[theBadKid]

I am terribly sorry for the thread heading.

Thank you for the help though, it got me further but I am still having trouble.

Should the code look like this?

        sPlan = txtPlan.Text
        dFlagfall = lblFlagfall.Text

        If Not Double.TryParse(lblFlagfall.Text, dFlagfall) Then
            MsgBox("Error")
        ElseIf sPlan = "No Plan" Then
            dFlagfall = 2.0
        ElseIf sPlan = "Bronze" Then
            lblFlagfall.Text = CDbl(1.5)
        ElseIf sPlan = "Silver" Then
            lblFlagfall.Text = CDbl(0.75)
        ElseIf sPlan = "Gold" Then
            lblFlagfall.Text = CDbl(0.25)
        Else
            MsgBox("Unknown Plan")
        End If

Thanks again.

[jmcilhinney]

The point of TryParse is to try to parse the user input and, if successful, give you the result. Where is the user input? Presumably it's not in lblFlagfall because the user can't enter data into a Label.

Presumably the user is entering the data into a TextBox, so it should be the Text of that TextBox you pass to TryParse. You should also be passing a Double variable that you want to contain the result of the conversion. If the conversion is successful then TryParse returns True and that variable contains the result, so you can use that variable. If the conversion fails then TryParse returns False and the variable contains zero, so you should not use the variable unless you specifically want to use zero when invalid data is entered.

Here's an example of how you might use Double.TryParse:

Const HOURLY_RATE As Double = 10.0
Dim hoursWorked As Double
 
If Double.TryParse(hoursWorkedTextBox.Text, hoursWorked) Then
    Dim totalPay As Double = hoursWorked * HOURLY_RATE
 
    MessageBox.Show(totalPay.ToString("c"), "Total Pay")
Else
    MessageBox.Show("Please enter a valid number of hours.")
End If