CODEWITHSUNDEEP
vb.net
Shinobi1173
Shinobi1173

Reputation: 97

Conversion from string "" to type 'Double' is not valid exception

I keep getting this annoying message (even though it is my fault):

Conversion from string "" to type 'Double' is not valid.

I get this message when I am debugging in VB 2010 Express and I hit the "SLM" button. What the heck does this mean and what am I doing wrong?!?!

My code:

Public Class frmAssignment_5

    Private Sub btnSLM_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSLM.Click
        Dim Description As String
        Dim Year As String
        Dim Life As String
        Dim SLMDepreciation As String
        Dim SLMText As String = "straight-line"

        txtDescription.Clear()
        txtYear.Clear()
        txtCost.Clear()
        txtLife.Clear()
        lstResult.ClearSelected()

        Description = CStr(txtDescription.Text)
        Year = CStr(txtYear.Text)
        Life = CStr(txtLife.Text)
        SLMDepreciation = CStr(SLMD())

        lstResult.Items.Add("Description: " & Description)
        lstResult.Items.Add("Year of purchase: " & Year)
        lstResult.Items.Add("Cost: " & FormatCurrency(CStr(txtCost.Text), 2))
        lstResult.Items.Add("Estimated life: " & Life)
        lstResult.Items.Add("Method of depreciation: " & SLMText)


    End Sub

    Function SLMD() As Double
        Dim Salvage_Value As Integer = 0
        Dim Life As String = CStr(txtLife.Text)
        Dim Cost As String = CStr(txtCost.Text)

        SLMD = (CDbl(txtCost.Text) - Salvage_Value) / CDbl(Life)

        Return SLMD
    End Function
End Class

The error keeps happening at the "SLMD = (CDbl(txtCost.Text) - Salvage_Value) / CDbl(Life)" but when I change things around, it starts pointing at the things that I change with the same error.

If anyone can help me out, I will be in forever debt...

Upvotes: 0

Views: 11176

Answers (3)

SSS
SSS

Reputation: 5393

VB is complaining because you are trying to convert an empty String "" value to a number. If you put VAL() around each string, it will convert it to zero.

Salvage_Value is also declared as an Integer, but you are using it as a Double, so you need to convert that too when calculating SLMD:

SLMD = (Val(txtCost.Text) - CDbl(Salvage_Value)) / Val(Life)

You also have a risk of a division by zero error if "Life" is not set to a value, so you should add code to make sure a value is entered there.

Upvotes: 4

Mark Hall
Mark Hall

Reputation: 54532

This is very similar to a question I just answered. When you use CDbl to convert a string it will error out if the conversion is not possible, what I would do is use Double.TryParse and set the values individually, then if there was an error I would handle it.

Function SLMD() As Double
    Dim Salvage_Value As Integer = 0
    Dim Life As Double
    Dim Cost As Double
    If Not Double.TryParse(txtLife.Text, Life) Then
        'Do something to handle error
    End If
    If Not Double.TryParse(txtCost.Text, Cost) Then
        'Do something to handle error
    End If

    Return (Cost - Salvage_Value) / Life

End Function

Upvotes: 3

Zhihao
Zhihao

Reputation: 14747

It looks to me like you're changing trying to convert an empty String ("") to a Double at CDbl(txtCost.Text). This is not a valid conversion because "" is not a valid Double value.

I think you should be checking your txtCost.Text to see if it's actually a Double represented as a String before attempting the conversion. You could try to block invalid characters/values from the client or check it from the server after you receive it (but before you attempt the conversion). Another option would be to handle it in a try+catch+finally statement. This post might help as well with what you're encountering.

Upvotes: 0

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