Question

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…

Answer 1

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.