For an intro to VB project I have to write a program which takes an
amount of Miles, Yards, and Inches.....and converts it to metric (KM,
M, and CM) when all values are entered to the input text boxes it works
fine, but if there is a number missing I get the following error:

An unhandled exception of type 'System.InvalidCastException' occurred
in microsoft.visualbasic.dll

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

i have already tried a

If txtmiles.Text = "" Then
txmiles.Text = "0"
End If and still didnt work

here is my code if anyone can help thatd be great

Private Sub btnCompute_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btnCompute.Click

        'This string takes the inputted numbers and converts them all
to inches.
        Dim inches As String
        inches = (txtMiles.Text) * 63360 + (txtYards.Text) * 36 +
(txtFeet.Text) * 12 + (txtInches.Text)

        'This part converts CStr(inches) which was calculated above
into Meters
        Dim meters As String
        meters = CStr(inches) / 39.37

        'This calculates the calculates the kilometers, meters, and
centimeters.
        Dim Kilometers As String
        Kilometers = CStr(meters) / 1000
        Int(CStr(Kilometers))

        Dim OutputMeters As String
        OutputMeters = ((CStr(Kilometers) - Int(CStr(Kilometers))) * 10
^ 3)
        Int(CStr(OutputMeters))

        Dim Centimeters As String
        Centimeters = ((CStr(OutputMeters) - Int(CStr(OutputMeters))) *
10 ^ 2)

        'This posts the results to the results boxes
        txtKilometers.Text = Int(CStr(Kilometers))
        txtmeters.Text = Int(CStr(OutputMeters))
        'This will round the centimeters to the decimal place and post
it
        Dim n As Double = CStr(Centimeters)
        txtCentimeters.Text = (Math.Round(n, 1))

        'This will disable the txt box and enable the results
        txtMiles.ReadOnly = True
        txtYards.ReadOnly = True
        txtFeet.ReadOnly = True
        txtInches.ReadOnly = True
        txtmeters.ReadOnly = False
        txtCentimeters.ReadOnly = False
        txtKilometers.ReadOnly = False

    End Sub

    Private Sub btnReset_Click(ByVal sender As System.Object, ByVal e
As System.EventArgs) Handles btnReset.Click
        'This button will reset and clear all the text boxes
        txtMiles.Clear()
        txtYards.Clear()
        txtFeet.Clear()
        txtInches.Clear()
        txtKilometers.Clear()
        txtmeters.Clear()
        txtCentimeters.Clear()
        'This will enable the txt box and disable the results
        txtMiles.ReadOnly = False
        txtYards.ReadOnly = False
        txtFeet.ReadOnly = False
        txtInches.ReadOnly = False
        txtKilometers.ReadOnly = True
        txtmeters.ReadOnly = True
        txtCentimeters.ReadOnly = True

    End Sub
End Class

DC15,

I can see that you don't have in top of your program Option Strict On, I
think that surely for this program you need that.

Net is calculating with most operators with doubles.

Therefore does it look for me better to take the double as the internal
value in this situation.

You can convert a string to double with simple CDbl(String)

I hope this helps,

Cor

Show quoteHide quote

Turn on 'Option Strict'.

Thanks for all the help, i'd always thought Option Strict On was a
default setting but I guess  I was wrong. I solved the first problem by
using an If Statement, but now im running into a whole new problem.
When I enabled option strict I got an error "

"Option Strict On disallows implicit conversions from 'Double' to
'String'."
on the part posting the output to the text boxes.... here is my code

ption Strict On

Public Class Form1
    Inherits System.Windows.Forms.Form




" Windows Form Designer generated code "

    Private Sub btnCompute_Click(ByVal sender As System.Object, ByVal e
As System.EventArgs) Handles btnCompute.Click

        'These IF statements will prevent an error from occuring if
there is no value in the text boxes
        If txtMiles.Text.Trim() = "" Then
            txtMiles.Text = "0"
        End If
        If txtYards.Text.Trim() = "" Then
            txtYards.Text = "0"
        End If
        If txtInches.Text.Trim() = "" Then
            txtInches.Text = "0"
        End If
        If txtFeet.Text.Trim() = "" Then
            txtFeet.Text = "0"
        End If

        'This Double takes the inputted numbers and converts them all
to inches.
        Dim inches As Double
        inches = CDbl(txtMiles.Text) * 63360 + CDbl(txtYards.Text) * 36
+ CDbl(txtFeet.Text) * 12 + CDbl(txtInches.Text)


        'This part converts Cdbl(inches) which was calculated above
into Meters
        Dim meters As Double
        meters = CDbl(inches) / 39.37

        'This calculates the calculates the kilometers, meters, and
centimeters.
        Dim Kilometers As Double
        Kilometers = CDbl(meters) / 1000
        Int(CDbl(Kilometers))

        Dim OutputMeters As Double
        OutputMeters = ((CDbl(Kilometers) - Int(CDbl(Kilometers))) * 10
^ 3)
        Int(CDbl(OutputMeters))

        Dim Centimeters As Double
        Centimeters = ((CDbl(OutputMeters) - Int(CDbl(OutputMeters))) *
10 ^ 2)

        'This posts the results to the results boxes
        txtKilometers.Text = Int(CDbl(Kilometers))

        txtmeters.Text = Int(CDbl(OutputMeters))


        'This will round the centimeters to the decimal place and post
it
        Dim n As Double = CDbl(Centimeters)
        n = CDbl(Centimeters)
        txtCentimeters.Text = Math.Round(n, 1)

        'This will disable the txt box and enable the results
        txtMiles.ReadOnly = True
        txtYards.ReadOnly = True
        txtFeet.ReadOnly = True
        txtInches.ReadOnly = True
        txtmeters.ReadOnly = False
        txtCentimeters.ReadOnly = False
        txtKilometers.ReadOnly = False

        If txtMiles.Text = "" Then
            txtMiles.Text = "0"
        End If
    End Sub


    Private Sub btnReset_Click(ByVal sender As System.Object, ByVal e
As System.EventArgs) Handles btnReset.Click
        'This button will reset and clear all the text boxes
        txtMiles.Clear()
        txtYards.Clear()
        txtFeet.Clear()
        txtInches.Clear()
        txtKilometers.Clear()
        txtmeters.Clear()
        txtCentimeters.Clear()
        'This will enable the txt box and disable the results
        txtMiles.ReadOnly = False
        txtYards.ReadOnly = False
        txtFeet.ReadOnly = False
        txtInches.ReadOnly = False
        txtKilometers.ReadOnly = True
        txtmeters.ReadOnly = True
        txtCentimeters.ReadOnly = True

    End Sub
End Class


I've tried everything I can, and this is really annoying me. Does
anyone have any ideas on how i can fix this? itd be greatly appreciated

Hello dc15,

<SNIP>
How about txtCentimeters.Text=n.ToString("n1"). Likewise you could use txtCentimeters.Text=Math.Round(n,1).ToString.
(Beware you will get rounding differences between these options. Math.Round
uses financial rounding whereas the n.ToString("n1") always rounds .5 up.

Additionally, you may want to work more on your type checking. What happens
in your solution when you type "a" in one of the text boxes? You might want
to look into Decimal.TryParse if you are using 2005.

Jim Wooley

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